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MANAGEMENT
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Introduction Managing the environment is important to increase the efficacy
of natural enemies, which depend on production technologies such as varietal
development, cropping systems, tillage practices and chemical inputs. Late
trends in agriculture have, nevertheless, been toward decreasing
environmental heterogeneity, increasing fertilizer and pesticide input,
increasing mechanization and decreasing genetic diversity (USDA 1973,
Bottrell 1980, Whitham 1983, Altieri & Anderson 1986, Altieri & Letourneau
1999). Such creates agricultural environments that impede pest population
regulation by natural enemies. The current emphasis on IPM, the increasing
restrictions on various pesticides and growing public concern about pesticide
contamination, as well as increased production costs, justify increased
research efforts for long term alternatives to the current trends. Although
agroecosystems devoted food and fiber production have been stressed, these
same systems frequently generate pests that are of human and veterinary
health concern, such as mosquito, gnat and fly outbreaks. Numerous research
has been conducted to document the importance of manipulating environmental
properties of crop fields to make them more favorable to natural enemies and
less amenable for insect pests, since van den Bosch & Telford (1964)
presented their classical chapter that encouraged biological control in
agroecosystems (van Emden & Williams 1974, Perrin 1980, Cromartie 1981,
Thresh 1981, Altieri & Letourneau 1982, 1984; Price & Waldbauer 1982,
Risch et al. 1983, Herzog & Funderburk 1985). For pests of medical and
veterinary importance environmental management is essential to the maximized
performance of parasitoids and predators (Please refer to Selected
Reviews &
Detailed Research
) Since the mid
1970's most effort has been directed to analyzing the effects of reduced
tillage and vegetational diversification of agroecosystems. Research on other
types of cultural manipulation such as strip-harvesting, trap cropping, use
of nests or artificial shelter, etc., has been scarce, except for the use of
food sprays (Hagen 1986) and kairomones (Lewis et al. 1976, Nordlund et al.
1981a,b, 1987) that enhance the activity of specific natural enemies. There has been much research on multiple
cropping systems and their effect on insect dynamics (Root 1973,
Litsinger & Moody 1976, Perrin 1977, Altieri et al. 1978, Perrin & Phillips
1978, Bach 1980a,b; Risch 1980, 1981; Andow 1983b, Letourneau & Altieri
1983, Altieri & Liebman 1986). These studies provide a basis for
designing crop systems with vegetational attributes that enhance
reproduction, survival and efficacy of natural enemies. However, because
agricultural land use is driven principally by economic forces, pest control
plans are seldom made on the basis of habitat management. In developed
countries farmers reduce unit production costs by increasing farm size and
becoming more specialized, with the consequence that environmental
manipulation strategies with demonstrated effectiveness under experimental
conditions, such as cotton/alfalfa strip cropping for Lygus management in cotton (Stern et al. 1964), or the use
of Rubus plantings around
vineyards for conservation of grape leafhopper parasitoids (Doutt &
Nakata 1973), have not been adopted on a regional scale. The political and
economic context of modern farming does not support the maintenance of
landscape diversity, which is one of the main obstacles to the implementation
of many of the alternative strategies to pesticides. The effective environment of an organism has been
characterized by Rabb et al (1976) as weather, food, habitat (shelter, nests)
and other organisms. Environmental management for biological control is
concerned with the functional environment, i.e., the physical and biotic
elements that directly or indirectly impact survival, migration,
reproduction, feeding and the behaviors associated with these life processes.
Although pest populations can be controlled directly through cultural control
methods that modify the habitat, the main thrust of this section is
conservation (maintenance of natural enemy abundance and diversity) and
enhancement (increased immigration, tenure time, longevity, fertility and
efficiency) strategies that can be used to manipulate natural enemies in
agroecosystems. Habitat management is directed at (1) enhancing habitat
suitability for immigration and host finding, (2) providing alternative
prey/hosts during times when pests are scarce, (3) providing supplementary
food (food sprays, nectar and pollen for predators/parasitoids), (4)
maintenance of noneconomic levels of the pest or alternative hosts over long
periods to ensure continued survival of natural enemies and (5) providing
refugia for mating or overwintering. Cropping techniques that enhance
parasitoids through these five processes have been reviewed by Powell (1986)
and shown in table form by Altieri & Letourneau (1999). Approaches to manipulating natural enemies include several
levels, from agroecosystem processes to eco-physiological features of
individual organisms. The number of elements that can be manipulated and
their degree of flexibility depend on characteristics of the agroecosystem.
The role, methods and future directions of environmental management as a
preventative control strategy are detailed after Vandermeer & Andow
(1986) in the following sections. A unique set of agroecosystems are found in different regions,
which result from local climate, topography, soil, economic relations, social
structure and history. A number of farming features can be modified and some
can impact the dynamics of insect populations. The agroecosystems of a region
often include both commercial and local use agricultures, which rely on
technology to a different extent depending on the availability of land,
capital and labor. Some technologies in modern systems aim at efficient land
use, such as reliance on biochemical inputs, while others reduce labor or
mechanical inputs. On the other hand, resource poor farmers usually adopt low
technology, labor intensive practices that optimize production efficiency and
recycle scarce resources (Mattson et al. 1984). Area wide environmental
management techniques are difficult to design and implement because of
differences in climate, agricultural products and economic and political
structure of each agricultural system. Many farming systems are in transition,
with changes forced by shifting resource needs, unequal resource
availability, environmental degradation, economic growth or stagnation,
political change, etc. Strategies amenable to labor intensive operations will
be radically different from those designed for mechanized, large scale
operations. Specialization and concentration of crops are the most important
factors limiting the application of many environmental management options for
a particular region. Farms may be classified by type of agriculture or
agroecosystem even though there are many individual differences among farms
in a region. Functional grouping is essential for devising areawide habitat
management strategies. Norman (1979) listed five criteria that can be used to
classify agroecosystems in a region: (1) the types of crop and livestock, (2)
the methods used to grow the crops and produce the stock, (3) the relative
intensity of use of labor, capital and organization, and the resulting output
of product, (4) the disposal of the products for consumption (whether used
for subsistence or supplement on the farm or sold for cash or other goods),
and (5) the structures used to facilitate farming operations. Using these
criteria Giggs (1974) recognized seven main types of agricultural systems in
the world: (1) shifting cultivation systems, (2) semi-permanent rain-fed
cropping systems, (3) permanent raid-fed cropping systems, (4) arable
irrigation systems, (5) perennial crop systems, (6) grazing systems, and (7)
systems with regulated farming (alternating arable cropping and sown
pasture). Systems 4 and 5 evolved into habitats which are much simpler in
form and poorer in species than the others, which can be considered more
diversified, permanent and less disturbed and consequently inherently containing
elements of natural pest control. It is obvious that modern systems require
more radical modifications of their structure to approach a more diversified,
less disturbed state. If it is argued that such modifications are not
possible in large scale agriculture due to technical or economic factors,
then there is a strong conservative argument in favor of small, multiple use
farms. Types of
Environmental Management An obvious form of environmental management concerns
vegetational designs across appropriate levels of scale. AT the regional
level landscape vegetation mosaics influence the distribution of food and
shelter resources and consequently, colonization patterns of insects (Andow
1983b). At a smaller scale, herbivores and their natural enemies respond to
localized patterns of plant spacing, plant structure and plant species (or
varietal) diversity. Environmental components and their management in
agroecosystems have three main dimensions: temporal, spatial and biological.
Other means of biotic management through inundative releases and classical
biological control are considered in other sections. Mechanical modes of
environmental management, such as cultivating, mowing and harvesting affect
the structure and permanence of the habitat and thus the life processes of
insects in agroecosystems. Chemical inputs, such as the periodic application,
water, fertilizers, behavior modifying agents and the pesticides affect the
rates of growth and survival of pests and natural enemies. Biotic,
physical/chemical and mechanical manipulations are imposed upon
agroecosystems often as means to achieve objectives unrelated to insect pest
management, but the possible range of environmental manipulations designed
for higher yields can be broad enough to incorporate tactics which
simultaneously improve pest control. Management of
Vegetation.--Monocultures
which are frequently disturbed often favor the rapid colonization and growth
of herbivore populations. Initial conditions of natural enemy-free space and
high abundance of pests further reduces the ability of natural enemies to
regulate them (Price 1981). These negative factors can be minimized or
eliminated by providing continuity of vegetation (and the associated food and
shelter) in time and space, thereby aiding natural enemies. Studies
documenting direct behavioral and physiological effects of plants on natural
enemies are numerous (e.g., van Emden 1965, Leius 1967, Campbell & Duffey
1979, Nettles 1979, Altieri et al. 1981, Letourneau & Altieri 1983,
Boethel & Eikenbary 1986, Letourneau 1987). Entomophages are sometimes
more abundant in the presence of certain plants, even in the absence of hosts
or prey, or they are attracted or arrested by chemicals released by the
herbivore's host plant or other associated plants. Some parasitoids prefer
particular plants over others (Monteith 1960, Shahjahan 1974, Nettles 1979).
Other authors recognized that parasitism of a pest was higher on some crops
than on others (Read et al. 1970, Martin et al. 1976, Nordlund et al. 1985,
Johnson & Hara 1987). Noncrop plants within and around fields can also benefit
biological control agents (Altieri & Whitcomb 1979a,b; Barney et al.
1984, Norris 1986). Rapidly colonizing, fast growing plants offer many
important requisites for natural enemies such as alternate prey or hosts,
pollen or nectar, and microhabitats which are not available in weed free
monocultures (van Emden 1965, Doutt & Nakata 1973) but these interactions
can be difficult to define and to implement in control programs (Flaherty et al.
1985). Outbreaks of some kinds of crop pests are more apt to occur in weed
free fields than in weed diversified crop systems (Dempster 1969, Flaherty
1969, Root 1973, Smith 1976a, Altieri et al. 1977). Crop fields with dense
weed cover and high diversity usually have more predaceous arthropods than do
weed free fields (Pimentel 1961, Dempster 1969, Flaherty 1969, Pollard 1971,
Root 1973, Smith 1976b, Speight & Lawton 1976). Carabids (Dempster 1969,
Speight & Lawton 1976, Thiele 1977), syrphids (Pollard 1971, Smith
1976b), and coccinellids (Bombosch 1966, Perrin 1975) are abundant in weed
diversified systems. Relevant examples of cropping systems in which the
presence of specific weeds has enhanced the biological control of particular
pests are numerous. The potential for managing weeds as useful components of
agroecosystems is great, but not all weeds promote biological control (see
Powell et al. 1986). Leius (1967) found that the presence of wild flowers in apple
orchards resulted in an 18X increase in parasitism of tent caterpillar pupae
over nonweedy orchards; parasitism of tent caterpillar eggs increased 4X, and
parasitism of codling moth larvae increased 5X. A cover crop of bell beans, Vicia faba L. in rain fed apple orchards in northern California
decreased infestations by codling moth. This lower moth infestation was
correlated significantly with increased numbers of predators in the Aranae,
Coccinellidae, Syrphidae and Chrysopidae, which were present on the trees
(Altieri & Schmidt 1985). Similar observations were made by Dickler
(1978) in Germany. In New Jersey peach orchards, control of the oriental
fruit moth increased in the presence of ragweed, Ambrosia sp., smart weed, Polygonum sp., lambsquarter, Chenopodium album
L., and goldenrod, Solidago
sp. Such weeds provided alternate hosts for the parasitoid Macrocentrus ancylivorus Rohwer (Bobb 1939).
O'Connor (1950) suggested the use of a cover crop in coconut groves in the
Solomon Islands to improve the biological control of coreid pests by an ant, Oecophylla smaragdina subnitida
Emery. In Ghana, coconut served this purpose by providing sufficient shade
for cocoa to support high populations of Oecophylla
longinoda Latreille which maintained
the cocoa crop free of cocoa caspids (Leston 1973). Annual crops diversified
with cover crops also suffer less damage. Brust et al. (1986) reported
dramatically higher predation rates of Lepidoptera larvae (black cutworms, Agrotis ipsilon (Hufnagel), armyworms, Pseudaletia unipunctata
Haworth, stalk borers, Papaipema
nebris (Guenée) and European
corn borers, Ostrinia nubilalis (Hübner) tethered to
corn sown into a grass/legume mixture than to corn in monoculture. Carabid
beetles were more abundant to the living mulch system and were among the
larval predators in both systems. Because farming in a region differs in energy inputs, levels
of crop diversity and successional stages, variations in insect dynamics may
occur that are difficult to predict. However, low pest potentials may be
expected in agroecosystems that show traits as follows: (1) high crop
diversity through mixtures in time and space (Cromartie 1981, Altieri &
Letourneau 1982, Risch et al. 1983, Andow & Risch 1985, Nafus &
Schreiner 1986). (2) Discontinuity of monoculture in time through rotations,
use of short maturing varieties, use of crop-free or host free periods, etc.
(Stern 1981, Lashomb & Ng 1984). (3) Small scattered fields creating a
structural mosaic of adjoining crops, and uncultivated land which potentially
provide shelter and alternative food for natural enemies (van Emden 1965,
Altieri & Letourneau 1982). Pests also may proliferate in these
environments depending on plant species composition (Altieri & Letourneau
1984, Collins & Johnson 1985, Levine 1985, Slosser et al. 1985, Lasack
& Pedigo 1986). But the presence of low levels of pest populations and/or
alternate hosts may be necessary to maintain natural enemies in the area. (4)
Farms with a dominant perennial crop component. Orchards are considered to be
more stable as permanent ecosystems than are annual crop systems. Because
orchards suffer less disturbance and are characterized by greater structural
diversity, possibilities for the establishment of biological control agents
are generally higher, especially if floral undergrowth diversity is
encouraged (Huffaker & Messenger 1976, Altieri & Schmidt 1985).
Sometimes orchard sanitation practices may interfere with the performance of
natural enemies, as is the case with sanitation to remove mummied almond
fruit from almond and walnut trees that serve as overwintering reservoirs for
parasitized hosts (Legner 1983a). (5) High crop
densities and the presence of tolerable levels of weeds (Shahjahan &
Streams 1973, Altieri et al. 1977, Sprenkel et al. 1979, Mayse 1983, Andow
1983a, Buschman et al. 1984, Ali & Reagan 1985). (6) High genetic
diversity resulting from the use of variety mixtures or several lines of the
same crop (Perrin 1977, Whitham 1983, Gould 1986, Altieri & Schmidt
1987). The above generalizations can serve in the planning of a
vegetation management strategy in agroecosystems; but they must take into
account local variations in climate, geography, crops, local vegetation,
inputs, pest complexes, etc., which might cause increases of decreases in the
potential for pest development under some conditions. The selection of
component plant species also can be critical. Systematics studies on the
quality of plant diversification with respect to the abundance and efficiency
of natural enemies are needed. While 59% of the 116 species of entomophages
in documented studies reviewed by Andow (1986) exhibited increased abundance
when plant species were added to the system, 10% decreased in abundance and
20% were variable, sometimes increasing and other times decreasing. Nafus
& Schreiner (1986) found lower parasitism rates in intercropped corn. The
addition to squash decreases the abundance of Coleomegilla maculata
(DeGeer) on squash because of a nonuniform distribution of prey (Andow &
Risch 1985). However, Orius tristicolor (White), a
generalist predator, is more abundant on squash when corn is interplanted,
and plant architecture and the nonuniform distribution of prey are beneficial
(Letourneau 1988). Plant density and diversity may interact negatively to
determine ground beetle emigration rates (Perfecto et al. 1986). Mechanistic
studies to determine the underlying elements of plant mixtures that enhance
or disrupt colonization and population growth of natural enemies allow a more
precise planning of cropping schemes and increase the chances of a desired
effect beyond the current levels. Management of
Crops With Mechanical Devices.--Manipulating the
environment with mechanical devices may disturb the system depending on its
severity and frequency. Low input, perennial systems would present an extreme
contrast to mechanized annual crop production systems, for example. But
slight modifications in cultural practices for sowing, maintaining and harvesting
annual crops can effect substantial changes in natural enemy populations
which bring them nearer to those observed in less disturbed perennial
counterparts (Arkin & Taylor 1981, Barfield & Gerber 1979, Blumberg
& Crossley 1983, Herzog & Funderburk 1985). Cultivation &
Habitat Disturbance.--Modern tillage practices reflect attempts to limit
mechanical disturbance of the soil; and there is an emphasis on surface
tillage and no tillage as alternative to plow tillage in order to control
soil erosion, enhance crop performance, use energy more efficiently (Sprague
1986) and reduce soil breeding chloropid eye gnats (Legner 1970 ). Minimum
tillage systems can conserve and enhance natural enemies of important pests
(Legner 1970 , House & All 1981, Luff 1982, Blumberg & Crossley
1983, All & Musick 1986), altho each case must be considered
independently. Plowing, disking and other manipulations of the soil or
breeding habitat can affect ground or waste-dwelling arthropods, whether they
inhabit the soil consistently or intermittently (Legner 1970 , Legner et al. 1973-1980). The extent of
direct mortality depends on their distribution with respect to soil depth and
their phenologies. Less directly put potentially as important effects are
caused by the removal of resources and natural enemies associated with living
undergrowth and plant residues. The impact of natural enemies on crop pests
in such systems, and the casual links between tillage practices, numbers of
natural enemies, and level of biological control has been shown in only a few
cases (Risch et al. 1983, Letourneau 1987). Significantly higher densities of carabids, including Amara spp., Pterostichus spp. and Amphasia spp occurred in no
tillage systems and were the major factor reducing black cutworm damage below
that achieved in conventional corn systems (Brust et al. 1985). Other studies
show that herbivore damage is reduced in no tillage fields despite similar
predator abundance in tilled and nontilled fields. For example, reduced
rootworm, Diabrotica spp.,
damage to corn in nontilled fields compared to plowed fields reflected lower
herbivore densities (Stinner et al. 1986). Although spider density was
highest in nontilled systems, predators in general did not exhibit higher
densities. Probably efficiency rather than abundance of predators/parasitoids
are enhanced and the vegetative component may be important by providing
alternative resources to entomophages. Foster & Ruesink (1984) showed
that the flowering weeds associated with reduced tillage in corn are
important nectar sources that increase survival and fecundity of Meteorus rubens (Nees) an important parasitoid of the black
cutworm. Ants are generalist
predators sensitive to tillage practices in agroecosystems (Risch &
Carroll 1982). Altieri & Schmidt (1984) reported greater species richness,
abundance and predation pressure in uncultivated orchard systems than in
those cultivated twice in six weeks. Both lack of nest disturbance and
habitat suitability due to vegetational cover may be important causes of
greater ant abundance. Similar results were predicted for a highly effective
predator of bollworm, Iridomyrmex
pruinois (Rogers) in
Arkansas cotton fields (Kirkton 1970) based on field observations. Carroll
& Risch (1983) and Letourneau (1983) sampled ant activity in lowland
tropical Mexico where farming practices are in transition between
slash-and-burn and mechanized cropping practices. The number of ant species
at tuna baits in maize fields was similar whether they had been plowed or
sown into slash (20-23 spp.). But in central Texas, spring plowing decreased
ant species richness from 12 species to 2 species. Among the species that
were no longer present at baits after plowing were those that prey on Solenopsis invicta Buren queens. Pests can be suppressed directly by plowing the soil (Watson
& Larsen 1968) and burying stubble (Holmes 1982). Talkington & Berry
(1986) significantly reduced the adult emergence of the pyralid moth pest Fumibotys fumalis (Guenée), in peppermint fields by burying the
prepupae into the soil; tillage depth was directly correlated with control.
In locations where natural enemies are not effective, deep burial of infested
stubble may be necessary (Umeozor et al. 1985). However, a study by Telenga
& Zhigaev (1959) on the beet weevil, Bothynoderes
punctiventris Germer, shows
how differential effects on pests and their natural enemies can be achieved
through carefully planned tillage practices. Although >90% of the weevil
eggs were destroyed by deep plowing, surface tillage with a disk increased
the survival of a parasitoid on the eggs, which caused a greater level of
pest control. Nilsson (1985) found that an average of 4X as many parasitoids
of Meligethus sp. pollen
beetles emerged from fallow fields or from plots of rape that had direct
drilling of winter wheat than emerged from disk harrowed or plowed plots.
Although the effect of these practices on parasitization were not studied, a
regional use of direct-drilling was recommended. Studies in northern Florida
by Altieri & Whitcomb (1979a,b) have shown that weed species composition
changes markedly according to the date of plowing. Early winter plowing
stimulated populations of golden rod, Solidago
altissima L. and 58 predator
species which feed on the aphids, Uroleucon
spp, and other herbivores associated with this weed. However, plowing in
mid-autumn caused camphor weed populations to be enhanced along with the 30
predator species associated with herbivores of this weed. Mowing, Harvesting
& Weed Control.--When crops are pruned or mowed, arthropods may move from the
cut plant material and there will be a period of new growth. These can have
important consequences on the performance and synchrony of natural enemies.
Weeding can also stimulate crop colonization by associated arthropods, the
extent to which movement will occur depending on distance and arthropod
mobility, but some weeding operations leave associated arthropods intact and
promote such movement. When patches of stinging nettle, Urtica dioica
L., are cut in late spring, predators are forced to move into crop fields
(Perrin 1975). Also Coccinellidae have been observed to move to orchard trees
in southeastern Slovakia when grass weed cover was cut (Hodek 1973). Alfalfa strip-cutting systems typically illustrate how natural enemy
movement prompted by vegetation cutting can occur. Van den Bosch & Stern
(1969) compared densities of several predators, including Geocoris pallens Stal, Nabis
americoferus Caryon, Orius tristicolor, Chrysoperla
carnea (Stephens), and Hippodamia spp. in strip-cut
and solid cut fields. Movement out of the field was uncommon even for these
mobile predators in strip-cut fields; most moved onto adjacent plants so that
on a field wide basis these predators were conserved. Strip cutting also
reduced mortality of Aphidius
smithi Sharman & Rao by
providing shelter from adverse physical conditions and host scarcity. Host
availability for the parasitoid, Cotesia
medicaginis (Muesebeck) in
alfalfa was altered through a different mechanism, however. Oviposition rates
of the alfalfa butterfly, Colias
philodice eurytheme Godart, peak on new
growth following harvest, which causes periodicity in the availability of
early instar larvae. Strip cropping can spread the vulnerable stages more
evenly over time and thus favor the maintenance of A. medicaginis
populations over the season. When fire is used to prepare land for cropping by the "slash and burn"
practice or to reduce crop residue, the affects on resident natural enemies
and incoming colonists can be serious. Burning of old fallow vegetation in a
tropical slash and burn system decreased ant abundance and foraging activity
for more than four months (Saks & Carrol 1980). Although fire has been
used as a tool for direct control of pests (Komareck 1970), generalizations
on its effect on natural enemies are not possible. An isolated study showed
that controlled burning increased spider and ant densities and biomass due to
increased food supply for herbivores in the form of succulent plant growth
after the burn (Hurst 1970). Chemical Usage.--Although the influence of water and fertilizer applications
on herbivores is complex (Scriber 1984, Louda 1986), fertilizer and herbivory
levels may be causally related through changes in plant quality or phenology
that affect the dynamics of predator/prey and host/parasitoid interactions.
However, pesticides have direct detrimental effects on natural enemies and
their use in environmental management must be limited to situations where
they are timed carefully or selectively applied. Perhaps the use of behavior
modifying chemicals (Lewis & Nordlund 1985) will provide new tools for
the manipulation of biological control agents, but to date practical
deployment has not resulted (Chiri & Legner 1983, 1986). Fertilizer.--Changes on the physiological conditions of crops caused by
soil amendments may have consequences for pest management, which depend on
soil variability, the growth, developmental and biochemical responses of the
plant, the direct effects of such changes on herbivores and the secondary
impact on natural enemies. Much work has been done on herbivore response to
fertilizers that increase nitrogen levels in plants. Mattson (1980) believed
that foliage N-level is a major regulator of herbivory rate. Although insects
often improve their survival, fecundity and growth rates when plant quality
is increased (higher N), general statements on the direct responses of
herbivores to nitrogen fertilizer are not possible because of the array of responses
by different species (Scriber 1984). Experiments on links between soil
amendments and pest management relate to the effects on the pest via their
response to resistant and susceptible varieties under conditions of different
sources or levels of Ca, Mg, N, P, K or S (Kindler & Staples 1970,
Culliney & Pimentel 1986, Shaw et al. 1986, Manuwoto & Scriber 1984).
Thus the natural enemy's environment is affected by soil amendments through
changes in plant quality as well as by the concomitant changes in the
herbivores. The direct effects of fertilizer on biological control are not
well known. Many herbivores exhibit marked increases in population growth on
nitrogen enriched hosts. There is an obvious concern for the ability of
natural enemies to track their prey/hosts under conditions. There were no
differences in biological control of mites detected on apple trees treated
with three levels of nitrogen fertilizer (Huffaker et al. 1970). Although the
fecundity of Panonychus ulmi (Koch) increased with the
nitrogen level up to a 4X increase, when Amblyseius
potentillae (Garman)
predators were not present, the predators were able to compensate for most of
the increased prey density. However, fertilized cotton plots exhibited higher
levels of Heliothis zea (Boddie) than did controls
despite significantly higher population densities of Hippodamia convergens
Guerin-Meneville, Coleomegilla
maculata langi Timberlake and Orius insidiosus (Say) in fertilized cotton (Adkisson 1958). Chiang
(1970) showed that fertilized corn fields (50 tons manure/acre) had
significantly fewer (ca. 1/2) corn rootworms than did unfertilized controls.
Although ground beetles and spiders were not affected, the populations of
phytophagous and predaceous mites were 3X higher in manure treatment plots.
Through three seasons of field and laboratory experiments Chiang (1970)
concluded that mit predation accounted for 20% control of corn rootworm under
natural field conditions and 63% control when manure was applied. Other
effects of fertilizers on natural enemies may be predicted based on the
combined information of relevant studies. For example it is known that the
parasitoid Diaretiella rapae (McIntosh) attacks the
green peach aphid Myzus persicae (Sulzer) more readily
when the aphid is associated with Brassica
spp. (Read et al. 1970), the mustard oils in crucifers serving as
attractants. It has also been shown that some glucosinolates are inversely
related to nitrogen level (Wolfson 1980), and thus soil fertility may have
profound effects on pest control by limiting the production of semiochemicals
that play an important role in mediating interactions between plants,
herbivores and natural enemies. The frequency and levels of fertilizer applications can modify
the synchrony of predators with their prey. Low nutritive quality of host
plants may cause immature herbivores to develop more slowly, and thus
increase their availability to natural enemies (Feeny 1976, Moran &
Hamilton 1980, Price et al. 1980). A predaceous pentatomid was found to
regulate more efficiently the Mexican bean beetles on nutritionally poor
plants than on highly fertilized ones (Price 1986). Host plant phenology can
also be driven by fertilizer inputs, and Hogg (1986) suggested that the timing
of square availability was one factor influencing predation and parasitism
rates of H. zea in cotton. Changes in nutritive quality of host plants as influenced by
fertilizer may indirectly affect the survival and reproduction of natural
enemies by determining prey quality. Although direct examples of fertilizer
effects have not been demonstrated, nitrogen content is known to be an
important aspect of prey quality. Nitrogen content may be responsible for
higher egg production by H. convergens when fed apterate
instead of alate green peach aphids (Wipperfürth et al. 1987). Analagous
effects may occur in the case of prey of different quality due to host plant
conditions. Zhody (1976) observed that size, fecundity and longevity of Aphelinus asychis (Walker) was dependent on the food composition of
the host Myzus persicae. But low quality food
can also impair the ability of a host to encapsulate a parasitoid (El-Shazley
1972a,b). Nutrients in the host plant can also modify toxic effects to
parasitoids (Duffey & Bloem 1986) and influence their sex ratio
(Greenblatt & Barbosa 1981). Host size is often an important determinant
of egg fertilization by ovipositing females (Charnov 1982). Although studies
on direct effects of nitrogen on crop architecture and subsequent effects on
searching efficiency are not available, some studies indicate that these
interactions can occur. The sex ratio of Diadegma
reared from larvae of Plutella
xylostella L. from field
plots over a wide range of nitrogen fertilizer inputs showed a significant
trend for female bias in heavy fertilized plots. Soil nutrient levels are known to influence plant size, leaf
area, canopy closure and crop architecture, and these conditions define
searching area for natural enemies (Kemp & Moody 1984). Predator/prey or
parasitoid/host contact rates are a function of habitat preference, searching
area, prey density and dispersion patterns. Fye & Larsen (1969) found
that the searching efficiency of Trichogramma
spp. was dependent on structural complexity. Hutchison & Pitre (1983) did
not find this effect with Geocoris
punctipes (Say) on H. zea, however. Shady conditions resulting from overgrowing
vegetation reduce parasitism levels of Pieris
spp. (= Artogeia
spp.) by Cotesia glomerata (L.) (Sato & Ohsaki 1987) by deterring the parasitoid. The levels of key chemical constituents in the soil can
indirectly affect natural enemies by influencing weed composition in a field.
In Alabama fields with low soil potassium were dominated by buckhorn
plantain, Plantiago lanceolata L. and curly dock, Rumex crispus L., while fields with low soil phosphorus were
dominated by showy crotalaria, Crotalaria
spectabilis Roth, morning
glory, Ipomoea purpurea Roth, sicklepod, Cassia obtusifolia L., Geranium
carolinianum L. and coffee senns,
Cassia occidentalis L. (Hoveland et al. 1976). Soil pH can
influence the growth of weeds, e.g., weeds of the genus Pteridium occur on acid soils while Cressa sp. inhabits only alkaline soils. Other species of
Compositae and Polygonaceae are found growing in saline soils (Anon. 1969). Water.--Plant quality and RH at the field level can be influenced by
flooding fields, draining land and furrow, drip or sprinkler irrigation. The
desert valleys of southeastern California are suitable habitat for the predaceous
earwig Labidura riparia (Pallas) due to
favorable conditions produced by irrigation (van den Bosch & Telford
1964). Much of the experimental work on the effects of plant stress from
water conditions has targeted herbivores (Miles et al. 1982, Bernays &
Lewis 1986, Louda 1986). Water availability can affect palatability, feeding
duration, developmental time, migration, survival and fecundity of
plantfeeders. Therefore, many important effects of water conditions on
natural enemies are indirect and are mediated through changes in host/prey
abundance and dispersion or through qualitative changes. For example, rape
plants under drought conditions had increased proline levels and an
associated shift in the balance of free amino acids (Miles et al. 1982). Cabbage
aphids reached adulthood faster on stressed plants, and availability of
suitable hosts for parasitoids might thus be decreased both by the duration
of vulnerable stages and if the parasitoids require slower development than
the host, if plants are water stressed. The direct effects of water include mortality during
irrigation and impacts of RH. Ferro & Southwick (1984) and Ferro et al.
(1979) reviewed the importance of RH on small arthropods. Crop architecture
and watering regimes cause large deviations from ambient temperature and
humidity levels (Ferro & Southwick 1984) within foliage boundary layer
microhabitats. Irrigation of soybean caused a substantial decrease in canopy
temperature and a 16% increase in RH at 15 cm above the ground (Downey &
Caviness 1973). Prolonged periods of such irrigation effects can have
important consequences for natural enemies because developmental time and
therefore population growth and synchrony are related to temperature and RH.
This may be illustrated in the case of the tachinid Eucelatoria armigera
(Coquillett), which completes development at different rates depending on
temperature and host species (Jackson et al. 1969). Holmes et al. (1963) showed that parasitism levels of the wheat stem sawfly
by Bracon cephi (Gahan) were enhanced by
soil moisture and temperature levels that slow plant ripening. Force &
Messenger (1964) showed that a few degrees dramatically affect changes of the
innate capacity for increase (r) in parasitoids under laboratory conditions. Cotesia medicaginis reaches its maximum longevity at 55% RH;
longevity decreased markedly at levels above and below this value (Allen
& Smith 1958). However, it was not deemed an important factor in
determining parasitism levels of Colias
spp. larvae in alfalfa. But it is known that armored scale parasitoids in
arid citrus groves require irrigated conditions for maximum biological
control (DeBach 1958b). The vertical profile and general microclimate depend
not only on water inputs but on mulching, row direction, windbreaks and crop
spacing (Hatfield 1982). The severity of effects caused by drought conditions
depends on many factors, including availability of free water and nectar in
the habitat. Bartlett (1964) reported that caged Microterys flavus
(Howard) was able to function well at extremely low RH if provided with honey
and water. Semiochemicals.--The knowledge that parasitic insect behavior is influenced
by chemicals produced by their hosts stimulated considerable interest in the
use of semiochemicals for manipulating predators and parasitoids in the
field, especially for aggregating and/or retaining released parasitoids in
target areas (Gross 1981). The various opportunities for and limitations of
manipulating natural enemies with semiochemicals were reviewed by Vinson
(1977), Nordlund et al. (1981a,b, 1988), Powell (1986) and Hagen (1986).
Lewis et al. (1976) suggest that host or prey selection is the most important
step in the searching behavior of entomophagous insects that can be
manipulated to improve biological control. Semiochemicals should be used to
increase effective establishment of imported species, improving performance
and uniform distribution of released species throughout a target area and
optimizing abundance and performance of naturally occurring natural enemies
(Greenblatt & Lewis 1983). It is possible to devise three main habitat
management technique with semiochemicals: (1) strategies directed at
improving habitat characteristics such as the use of semiochemicals to make
crops more attractive or to define a more complex mosaic of local search
areas (Altieri et al. 1981). Gardner & van Lenteren (1986) nevertheless
give an exception. (2) Enhancing host plant characteristics; breeding
programs directed at improving chemical attractiveness of crops or crops with
extrafloral nectaries. (3) Mimicking high pest densities through applications
of diatomaceous earth or artificial eggs impregnated with kairomones (Gross
1981). Drift of
Pesticides.--Low
level inputs of insecticides to nontarget areas result from aerial
applications. Half the material applied to a field under ideal conditions can
drift a considerable distance downwind (Ware et al. 1970). Although a great
deal is known about the effects of direct spraying of various insecticides on
natural enemies, there is not much experimental work to determine the effects
of low level inputs. Biological control can be disrupted given sufficient
frequency, intensity and toxicity of sprays (Ridgway et al. 1976, Riehl et
al. 1980). The ratio of natural enemies to herbivores was increased by low,
drift-level concentrations of carbaryl, and arthropod abundance dropped
significantly more in an old field than it did in a corn monoculture. It was
suggested that low concentrations of insecticides have different effects on
herbivores and natural enemies depending on whether the nontarget habitat is
a crop field or a field of natural vegetation which serves as a source of
colonizers. However, such impacts cannot be predicted from knowledge of
effects at high concentrations (Risch et al. 1986). Drift of chemicals may be
minimized by making applications when winds are less than 2 m/sec, using
adjuvants, formulating inert emulsions and using large droplet sizes
(Gebhardt 1981). Windbreaks surrounding field and regional wide spray
synchrony are forms of cooperative efforts for drift reduction of the effects
of low level pesticide applications. The application of herbicides to crop fields can have
nontarget effects similar to low-level insecticide application. Baker et al.
(1985) showed that Orius
spp. and Nabis spp.
densities were decreased by monosodium methanearsenate, but not the abundance
of spiders, Geocoris spp.,
Hymenoptera and coccinellids. Herbicides may also modify weed species
composition in fields and thereby affect natural enemies. Other Pollutants (Dust).--Dust and
pollutants of different kinds may influence the efficiency of predators and
parasitoids. Environmental management includes consideration of the placement
of the sources and control of pollutant influx with respect to agricultural
fields. It has long been known that some pest outbreaks are caused or
enhanced by dust on crop foliage. Bartlett (1951) found that many inert dusts
rapidly killed Aphytis chrysomphali (Mercet) and Metaphycus luteolus (Timberlake). DeBach (1985a) demonstrated an
increase in California red scale populations on citrus trees in response to
road dust. Mechanisms may be mechanical interference or desiccation (Edmunds
1973). It is possible also that leaf temperature, which can be raised 2-4°C
by dust cover (Eller 1977) is a factor. Planned placement of roads and timing
of cultivation can reduce the level of dust on crops. Strawberry growers in
California profit from daily or twice daily watering of roadways through the
reduction in losses from mites, as predaceous mites are apparently inhibited
by dust. Gaseous air pollutants
are more difficult to detect and to control. Sulphur dioxide is a common
effluent that has known negative effects on a variety of organisms (Petters
& Mettus 1982), including honeybees (Ginevan et al. 1980). But acute
exposure of female Bracon hebetor (Say) to sulphur
dioxide in air causes no reduction in fertility and fecundity. Petters &
Mettus (1982) suggested that damage to parasitic wasps may develop in the
earlier stages or behavioral avoidance of contaminated areas may explain
reports of lower parasitoid and higher herbivore levels near sources of
sulphur dioxide pollution. Melanic morphs of the generalist coccinellid predator
Adalia bipunctata (L.) occur disproportionately often in the
vicinity of coal processing plants in Great Britain. Although earlier
investigators suggested a mechanism involving selective toxicity of air
pollutants, Muggleton et al. (1975) attributed the differences to sunshine
levels. Whether or not the coloration of such predators affects their
efficiency as biological control agents is unknown. Other sources of
contamination include auto traffic, drainage from selenium rich soils
(Gerling 1984), and ozone (Trumble et al. 1987). Literature stresses effects
on herbivores, and little is known about effects on natural enemies. Lead as
a contaminant from auto exhaust has been shown to concentrate in higher
trophic levels (Price et al. 1974). Some pollutants are inadvertently added
to the crop with soil amendments, such as sludge, manure and chemical
fertilizer (Wong 1985). Culliney et al. (1986) found a general response of
low arthropod diversity when sludge containing heavy metals and toxic
chemicals was applied to cole crops. Mechanisms
Involved in Enhancing Natural Enemies Insights into the biological mechanisms for environmental
management that enhances biological control can be obtained from an
examination of host selection processes of entomophages, which includes host
or prey habitat location, host or prey location and host or prey acceptance
(Vinson 1981). Designing crop habitats for effective biological control
requires an understanding of such mechanisms. During migration and habitat
location the effective environment may be the local area, a regional
landscape or a series of distant habitat patches with long distances between
them. The interplay of colonizer source location, wind patterns, vegetation
texture and host or prey density becomes important on a large scale. Maximum
levels of natural control require at the onset both sufficient numbers of
natural enemies and temporal synchrony of these invasions. Regional
environmental management for enhancing the success of habitat location by
natural enemies should focus on the arrangement of colonizer sources in
relation to target sites of potential pest problems as well as on timing of
natural enemy colonization. Rabb (1978) addressed these needs when he
criticized the propensity of single commodity, closed system approaches to
pest management in research and decision making as deficient for problems
which demand attention to large unit ecosystem heterogeneity. Natural enemies vary in their dispersal range, and migration
often occurs in high currents along paths of turbulent convection. Even weak
flying insects can disperse over long distances and across wide areas by
exploiting the ephemeral but very structured nature of air movement
(Wellington 1983). For example, robust hosts and minute parasitoids can
exhibit coupled displacement in long distance migration, as shown by the
Australian plague locust Chortoicetes
terminifera Walker and its
egg parasitoid Scelio fulgidus Crawford which
disperse independently on wind currents to the same location (Farrow 1981).
Cumulative numbers over a growing season may be irrelevant if immigration
rates of natural enemies are very slow in relation to rising levels of the
pest (Doutt & Nakata 1973, Letourneau & Altieri 1983, Williams 1984).
Information on source constitution, phenology and flight patterns are
necessary to design and manage regional scale agroecosystems for optimal
biological control. Flight capacity studies and mathematical models to
describe movement patterns based on continuous diffusion or discrete random
walk equations have focused on predicting dispersal and migration of
herbivores (Okubo 1980, Stinner et al. 1983, 1986). Biological information
coupled with predictive models of natural enemy movement may aid in
predicting synchrony (Duelli 1980), but many times synchronies are difficult
to achieve because local species are adapted to exploit natural conditions of
prey or host phenologies. For example, coccinellid beetles in California
estivate during times of prey availability; irrigated crops provide a
continuous food supply that was not available in an area before agricultural
expansion had occurred (Hagen 1962). While locating hosts or prey, factors such as the physical
texture of plant surfaces, structural attributes of plants, microclimatic
conditions and patch heterogeneity interplay. Flaherty (1969) showed enhanced
control of herbivorous mites on grape vines with Johnson grass cover. The
grass acted as a source of predaceous mites. In this study involving prey
location, and in the habitat location phase study of Doutt & Nakata
(1973), the cumulative total number of natural enemies was not as important
as the temporal synchrony with growing herbivore populations. During host or
prey acceptance and predation or parasitism, environmental factors operate
indirectly through their effects on host or prey behavior, host or prey
quality and alter levels of vulnerability of natural enemies to mortality
factors. Examples of the mechanisms of host or prey selection on all levels
of natural enemy behavior were given by many authors. Activities other than those directly associated with predation
or parasitism are migration to overwintering sites, mating, and the
acquisition and use of resources other than the primary prey or hosts. The
interdependence and variability of resource needs and factors such as
proximity and availability of resources in time become vital aspects of the
environment. These are factors of habitat suitability for natural enemies. A
reduction of the relative energy expenditure needed, in a particular
environment, to fulfill the resource needs of a particular
parasitoid/predator will increase its efficiency as a biological control
agent. Conservation of natural enemies through habitat management techniques
adapted to the prevailing agronomic schemes can be of great benefit. Small
changes in agricultural practices may increase natural enemy populations or
enhance efficiency. But predators and parasitoids are extremely diverse and
each family represents a particular range of responses to environmental
modification. There are numerous examples of habitat management techniques
that have been shown to increase the effectiveness an abundance of specific
predator groups. Theoretical
Aspects of Management Natural Enemy-Free
Space.--Probably the
most general level of theory to guide habitat management for biological
control is that of ecological and/or evolutionary escape from
predators/parasitoids. Price (1981) acknowledged in his theory of natural
enemy-free space, that pest irruption is a likely consequence of agricultural
practices that foster the spatial and temporal isolation of herbivores from
their natural enemies. Pest introduction to a novel environment is a classic
example (Price 1981, Altieri & Letourneau 1982, 1984; Risch 1987).
Temporary release of pests also occurs under conditions of insecticide caused
pest resurgence and secondary pest outbreaks. Evolutionary changes in native
crop pests (Host shifts) is still another process that may result in a
reduction of predation/parasitism. Island Biogeographic
Theory.--Cultivated
areas are insular in nature, which has motivated several analogies regarding
crops as islands available for colonization by arthropods (Strong 1979, Price
& Waldbauer 1982, Simberloff 1985). The development of arthropod communities
in crops was analyzed using MacArthur & Wilson's (1967) theory of island
biogeography, which allows the prediction of colonization rates and
mortality/emigration rates, on a comparative basis, with respect to crop
area, distance from the sources of colonizers, and crop longevity (assuming
that the system has aspects of equilibrium). The species composition,
structure and abundance of arthropods colonizing a crop field are the result
of highly dynamic processes and the assumption of equilibrium is often
inappropriate, however. But some predictions from the theory seem possible. One example is that species richness is positively correlated
to size on oceanic islands. Similarly in mainland communities, the number of
herbivores associated with a plant is a positive function of the local area
planned to or covered by that species (Strong 1979). Larger host islands
probably collect more individuals by random probability of encounter. Also,
patch detection by dispersers may increase with size. The effect of an
increase in the number of herbivores with an increase in size is important
for consideration in pest management strategies. But any increase in species
diversity must be defined by the proportion in each trophic level, and if
possible by the component species' biologies before it can be analyzed for
pest management potential. MacArthur & Wilson's (1967) model treated all
members of s species source pool as equivalent colonizers. The application of
this theory to dynamic and temporary crop islands requires the consideration
not only of the number of species and pattern of occurrence, but the order of
colonizer establishment by trophic level (Altieri & Letourneau 1984,
Robinson & Dickerson 1987). Extinction rates depend upon resource availability in a system.
Because the plants are supplied to the system or reset at certain intervals
(Levins & Wilson 1980), the resource base may be more predictable for
herbivores at least early in the season. The immigration rates of natural
enemies to large expanses of monoculture may be similarly increased, though
spread from the edges may be slow and thus favor the development of herbivore
populations. The equilibrium theory of biogeography does not allow for
comparisons of single, large crop fields versus a network of several small
fields of the same total area, yet the contrasting designs are likely to
differ in terms of suitability for biological control (Price 1976). Even though most theory based on island community development
poses questions and organizes thought on crop design, the barriers to its
application are (1) frequent disturbance of most crop fields reduces the
rigor and applicability of equilibrium models; (2) the few current empirical
data available on diversity, size and distance relationships do not constitute
a sufficient basis for environmental design recommendations (Simberloff
1985); (3) the theory does not distinguish pests and beneficial organisms
(Stenseth 1981); (4) economic impact of changing island size must be viewed
as exceedingly risky until demands for more certainty in the theory are met
(Simberloff 1985). However, Liss et al. (1986) presented a modification of
the MacArthur & Wilson (1967) model that incorporates colonizer source
composition and changes in island habitats over time. Consumer Dynamics.--Studies of consumer dynamics become important after the
natural enemies are within the habitat of their prey or hosts, in order to
predict the outcome of their interactions. Trophic interaction studies in
manipulated and natural systems have focused on two trophic levels, such as
plant-herbivore, host-parasitoid and predator-prey. Theory and data both
demonstrate the regulation of populations at the lower trophic level (plant,
prey or host) by natural enemies (Clark & Dallwitz 1975, Mattson & Addy
1975, Murdoch & Oaten 1975, Podoler & Rogers 1975, Morrow 1977,
Gilbert 1978, Hassell 1978, May & Anderson 1978, Clark & Holling
1979, Murdoch 1979, McClure 1980, Kareiva 1982). On the other hand, natural
enemies have been ineffective in other cases studied (Southwood & Comins
1976, Strong et al. 1984, Walker et al. 1984). The effectiveness of natural
enemies as regulators of herbivore populations depends not only on behavioral
and developmental responses of individual predators and on responses of the
entire population to changes in prey or host densities (Murdoch 1971, Murdoch
& Oaten 1975, Fox & Murdoch 1978), but also on variation in plant
parameters such as density, secondary compounds and associated plants
species. The ability of natural enemies to regulate the herbivores depends on
the herbivore population's intrinsic growth rate (r), which in turn reflects
the quality of the plant diet. Small changes in r caused by slight
differences in plant quality, such as variety, secondary chemistry, nutrients,
may determine whether or not parasitoids or predators can control the
herbivore populations (Lawton & McNeill 1980, Price et al. 1980). The
effectiveness of regulation also reflects subtle differences in the timing of
population events in both predator and prey populations (Hassell 1978, May
& Anderson 1978). Theory and data on interactions involving three trophic
levels in a complex habitat are ultimately more suitable as a basis for
environmental management strategies (Price 1986, Duffey & Bloem 1986,
Barbosa & Letourneau 1988). Therefore, the goal of such preemptive
measures of pest control is to avoid the provision of enemy-free space in
agricultural environments and instead to present pests simultaneously with
deleterious effects caused by their natural enemies and with selectively
defensive or suboptimal properties of their food plants. Studying systems as
communities of at least three trophic levels can contribute an understanding
of complex interactions that is different from that likely to be gained
purely as a byproduct of results from two level studies (Orr & Boethel
1986). Vegetation Diversity
& Patch Size.--Two hypotheses were proposed by Root (1973) to explain the
tendency for low herbivore abundance in diverse vegetation. The Resource
Concentration hypothesis, which predicts that many herbivores, especially
those with a narrow host range, are more likely to find, survive and
reproduce on hosts that are in pure or nearly pure stands. The Enemies
hypothesis incorporates the third trophic level that Root (1973) predicted
that vegetation would provide more resources for natural enemies (e.g.,
alternate hosts, refugia, nectar and pollen) and thus herbivore irruption
would be rapidly checked by a higher diversity and abundance of natural
enemies. Sheehan (1986) extended the resource concentration concept to
predict that specialist natural enemies will respond to mixed vegetation
differently, and probably less favorably, than will generalist predators and
parasitoids, because of the importance of alternate prey for generalists. The
designation of host/prey specialization categories, however, tends to rely
only on one aspect of the resource spectrum of parasitoids and predators
(Letourneau 1987). A range of species characteristics, such as relative vagility,
resource needs, and habitat location cues may determine the response of
parasitoids and predators to vegetational diversity. Maintaining heterogeneity within an agroecosystem may also
affect the success of establishment of imported biological control agents.
The debate over the degree to which ultimate levels of regulation are
attained by single versus multi species releases in classical biological
control continues, but analyses of environmental factors as raw materials or
as constraints are rarely considered (Beirne 1985). Factors such as species
richness, climatic gradients and disturbance levels are important in
assessing the susceptibility of large scale communities to biological
invasion (Fox & Fox 1986). Optimal Foraging.--During the host/prey selection process, natural enemies
exhibit a chain of responses to stimuli. The objectives of biological control
are to exploit natural processes that allow maximum prey encounter and
foraging rates by natural enemies, and therefore, this body of theory is useful
for predicting enhancement mechanisms and for evaluating the consequences of
under and overexploitation. The aggregation of foraging parasitoids in patches of higher
host density has been a critical feature thought to be responsible for
successful biological control (Beddington et al. 1978, may & Hassell
1981). Models of optimal patch use predict predation/parasitism levels
between patches, based on host/prey densities (see Cook & Hubbard 1977,
Waage 1979, Iwasa et al. 1984), but the power of these models varies. Murdoch et al. (1985) examined the importance of this searching behavior
using the successful olive scale/Aphytis
paramaculicornia DeBach
& Rosen - Coccophagoides
utilis Doutt system. These
parasitoids do not aggregate in areas of high host density. Waage (1983) did
find that Diadegma spp.
attacking Plutella xylostella (L.) aggregated in
patches with greater host density, yet the proportion of hosts parasitized at
high host densities was not greater. Roland (1986) found similar results with
Cyzenis albicans; whether or not the eggs are clumped, the level
of parasitism is similar. Predictive models can be used to clarify the
mechanisms involved in natural enemy behavior and their importance. It might
be possible to take advantage of the simple rules that foragers use for
decisions on how long to remain in a patch, which hosts or prey to seek and
accept, when and where they will oviposit and especially for hymenopterous
parasitoids, what the sex ratio will be. If these decisions are made in response
to environmental cues, then they are potential field tools (Kareiva &
Odell 1987). Dicke et al. (1985)
found that searching eucoilid parasitoids remained longer in a patch with
moderately higher kairomone concentrations regardless of the actual density
of Drosophila melanogaster Meigen. Charnov
& Skinner (1985) recommended careful reflection of both the proximate
causes of such responses and the evolutionary causes as complementary
approaches that enhance theory and application. It is also
necessary to consider the ultimate population effects on natural enemies
given habitat manipulations that exploit behavioral cues and maximize prey
reduction. A recent example giving particular attention to predator fitness
shows that although juvenile mantids exhibit a strong Type II functional
response, such behavior rapidly increases beyond the maximum gain in
characteristics related to fitness (Hurd & Rathet 1986). In any case,
natural enemy response to environmental manipulation should benefit through
life table studies over many generations (Hassell 1986) and optimal foraging
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