PEST MANAGEMENT AND THE ENVIRONMENT
E. F. Legner
Professor of Biological Control
University of California
eflbio@outlook.com
Summary
Improvements in the successful pest management of the
agricultural ecosystem and public health sectors calls for an overhaul of
current procedures. The availability
of specialist personnel to encourage effective management measures backed by
technical research is indispensable.
Without surveillance management tends to descend to environmentally
ineffective or harmful practices, and scheduled routines that do not respond
to periodic environmental changes are counterproductive to sound
management. Inadequacies of current
practices in several examples illustrate the need for research institutions
to augment their participation in management and a return to research funding
by unbiased sources.
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Pest management is a broad concept
that involves considerations of genetics, climate, ecology, natural enemies
and cultural or chemical applications.
Therefore, it is difficult to define this category exactly. A high level of sophistication is required
to manage events in the environment for the efficient production of food and
fiber and the abatement of public health and nuisance pests. A principal objective to the addition of
sound environmental management is the reduction of pesticide usage albeit at
the irritation of large commercial interests (Garcia & Legner 1999,
Pimentel et al. 1991).
Although scientific investigations in colleges and
universities have led to a high level of production and pest abatement,
deployment continues to face obstacles that are largely related to the
absence of competent supervisory personnel.
As expertise resides largely in the research community this group is
encumbered by an academic system that continues to stress research and teaching
and to minimize the deployment aspect.
The most successful programs in environmental management regularly
require five or more years to develop.
Investigator survival in the system demands frequent publication, but
not in the kind of journals that stress implementation. This distracts from
the ultimate goal of deployment, which diminishes the amount of time an
investigator has to be directly involved in an advisory capacity. Several examples of successful projects
that have receded in the absence of this supervision but which could be reactivated
with the proper advisory personnel present, will explain some of the problems
and difficulties involved.
Navel Orangeworm
Management in Almond Orchards
The
almond industry in California has suffered from the invasion of the navel orangeworm, Amyelois transitella (Walker), from Mexico and South America. Two external insect larval parasites, Goniozus legneri Gordh and Goniozus emigratus (Rohwer) and one internal egg-larval parasite, Copidosomopsis plethorica Caltagirone, which
are dominant on the pest in south Texas, Mexico, Uruguay and Argentina, were
successfully established in irrigated and nonirrigated almond orchards in
California (Caltagirone 1966, Legner & Silveira-Guido 1983).
Separate k-value analyses indicated significant regulation of their
navel orangeworm host during the warm summer season. There is a diapause (hibernation) in the
host triggered by several seasonally varying factors, and a diapause in the
parasites triggered by hormonal changes in the host. Possible latitudinal effects on diapause
(hibernation) also are present. The
ability of the imported parasites to diapause with their host enables their
permanent establishment and ability to reduce host population densities to
below economic levels (Legner 1983).
Although
navel orangeworm infestations have decreased with the establishment of the
three parasites (Legner & Gordh 1992),
the almond reject levels are not always below the economic threshold of
4%. Such rejects are sometimes due to
other causes, such as ant damage and fungus infections. In certain years, the peach tree borer, Synanthedon
exitiosa (Say), has been involved as its attacks stimulates oviposition
by navel orangeworm moths and subsequent damage attributed to the latter.
In some orchards, the growers have
sustained a reject level of 2 ˝ percent or less through 2008. Storing rejected almond mummies in
ventilated sheds through winter allows for a build up of natural enemies and
their subsequent early entry into the fields to reduce orangeworm populations
before the latter have an opportunity to increase. Commercial insectaries have harvested Goniozus legneri
from orchards for introductions elsewhere.
Copidosomopsis plethoricus and Goniozus legneri, and
to a lesser extent Goniozus emigratus successfully
overwinter in orchards year after year.
However, only Copidosomopsis
can consistently be recovered at all times of the year. The Goniozus
species are not recovered in significant numbers until early summer. Therefore, pest management in almond
orchards may require periodic
releases of Goniozus legneri to reestablish balances that were
disrupted by insecticidal drift or by the absence of overwintering rejected
almond refuges through aggressive sanitation practices. Although sanitation in this case may
appeal to the grower, it is a costly procedure that also disrupts natural
balances at low pest densities.
Goniozus legneri has been reared from codling moth and oriental
fruit moth in peaches in addition to navel orangeworm from almonds. A reservoir of residual almonds that
remain in the trees after harvest is desirable to maintain a synchrony of
these parasites with navel orangeworms in order to achieve the lowest pest
densities. In fact such reservoirs
often exceed 1,000 residual almonds per tree through the winter months, and produce
navel orangeworm densities at harvest that are below 1% on soft-shelled
varieties. Superimposed upon the
system is the diapausing mechanism in both the navel orangeworm and the
parasites (Legner 1983). All of these forces must be considered for
a sound, reliable integrated management.
Almond producers have to make reasonable decisions on whether or not
to remove residual almonds, a very costly procedure, or to use within season
insecticidal sprays. But orchard
managers rarely understand population stability through the interaction of
natural enemies and their prey. Because the
management of this pest with parasitic insects depends heavily on the
perpetuation of parasites in orchards it can only be accomplished by an
understanding of the dynamics involved.
Storing rejected almonds in protective shelters during winter months
increases parasite abundance. This
allows the parasites to reproduce in large numbers for subsequent spread
throughout an orchard in the spring when outdoor temperatures rise. Complete sanitation of an orchard by
removal of all rejected almonds is counter productive to successful
management as this also eliminates natural enemies.
Australian Bushfly
Management in Micronesia
Pestiferous
flies in the Marshall Islands provide a classic example of the adaptation of
invading noxious insects to an area with a salubrious climate. With nearly perfect temperature-humidity
conditions for their development, an abundance of carbohydrate and
protein-rich food in the form of organic wastes and excreta provided by
humans and their animals, and a general absence of effective natural enemies,
several species were able to reach maximum numbers.
There are
principally four types of pestiferous flies in Kwajalein Atoll of the
Marshall Islands, with the African-Australian bush fly, Musca sorbens
Wiedemann, being by far the most pestiferous species. The common housefly, Musca domestica
L., of lesser importance, frequents houses and is attracted to food in
recreation areas. The remaining two types are the Calliphoridae [Chrysomya
megacephala (Fab.), and (Wiedemann)], and the Sarcophagidae [Parasarcophaga
misera (Walker), and Phytosarcophaga gressitti Hall and Bohart). These
latter species are abundant around refuse disposal sites and wherever rotting
meat and decaying fish are available. Most of the fly species differ from the
common housefly and the bush fly in being more sluggish and noisy and by
their general avoidance of humans. Because residents do not distinguish the
different kinds of flies, nonpestiferous types are often blamed as nuisances
when in fact they may be considered to fulfill a useful role in the
biodegradation of refuse and rotting meat.
An initial
assessment of the problem led to the expedient implementation of breeding
source reduction to reduce the housefly, Musca
domestica L., and both the Calliphoridae and Sarcophagidae to
inconspicuous levels. These involved
slight modifications of refuse disposal sites to disfavor fly breeding. These
simple measures resulted in an estimated 1/3rd reduction of total flies
concentrating around beaches and residential areas. Because the housefly
especially enters dwellings, its reduction was desirable for the general
health of the community, and fly annoyances indoors diminished. Thorough surveys of breeding sites and
natural enemy complexes revealed that Musca sorbens reduction would
not be quickly forthcoming, however. A schedule of importation of natural
enemies was begun and other integrated management approaches were
investigated: e.g. baiting and breeding habitat reduction.
Bush Fly Origin
and Habits. -- This species is known as the bazaar fly in North Africa, a
housefly in India, and the bush fly in Australia (Yu 1971). It was first
described from Sierra Leone in West Africa in 1830 where it is a notorious
nuisance to humans and animals. The flies are attracted to wounds, sores, and
skin lesions, searching for any possible food sources such as blood and other
exudations. Although not a biting species, its habits of transmitting eye
diseases, enteric infections, pathogenic bacteria and helminth eggs make it a
most important and dangerous public health insect (Bell 1969, Greenberg 1971,
Hafez and Attia 1958, McGuire and Durant 1957)
The bush fly has
spread through a major portion of the Old World, Africa and parts of Asia
(Van Emden 1965). In Oceania its distribution is in AustraIia (Paterson and
Norris 1970); New Guinea (Paterson and Norris 1970); Samoa and Guam (Harris
and Down 1946); and the Marshall Islands (Bohart and Gressitt 1951). In
Hawaii Joyce first reported it in 1950. Later Hardy (1952) listed it in the Catalog
of Hawaiian Diptera, and Wilton (1963) reported its predilection
for dog excrement. The importance of
bush fly increased in the 1960's when it was incriminated as a potential
vector of Beta-haemolytic streptococci in an epidemic of acute
glomerulonephritis (Bell 1969).
On the islands of
Kwajalein Atoll a substantial portion of the main density of Musca sorbens
emanated from dog, pig and human feces.
Inspections of pig droppings in the bush of 10 widely separated islets
revealed high numbers of larvae (over 100 per dropping), making this dung, as
in Guam (Bohart and Gressitt 1951), a primary breeding source in the Atoll.
Pigs that are corralled on soil or concrete slabs concentrate and trample
their droppings making them less suitable breeding sites. In such situations
flies were only able to complete their development along the periphery of
corrals. Coconut husks placed under
pigs in corrals results in the production of greater numbers of flies by
reducing the effectiveness of trampling.
Kitchen and other organic wastes were not found to breed M. sorbens,
although a very low percentage of the adult population could originate there
judging from reports elsewhere. Nevertheless, this medium is certainly not
responsible for producing a significant percentage of the adult densities
observed in the Atoll.
Management
Efforts Worldwide. -- Successful partial reduction of bush fly had been
achieved only in Hawaii through a combination of the elimination of breeding
sites, principally dog droppings, and the activities of parasitic and
predatory insects introduced earlier to combat other fly species, e.g., Musca
domestica (Legner 1978). The density of-bush fly
varies in different climatic zones in Hawaii, but the importance of this fly
is minimal compared to Kwajalein. At times hymenopterous parasites have been
found to parasitize over 95% of flies sampled in the Waikiki area (H-S. Yu,
unpublished data). Other parts of
Oceania were either not suitable for the maximum effectiveness of known parasitic
species (e.g. Australia) or the principal breeding habitats were not
attractive to the natural enemies. Therefore, in Australia a concerted effort
has been made to secure scavenger and predatory insects from southern Africa
that are effective in the principal unmanageable fly producing source, range
cattle and sheep dung (Bornemissza 1970).
Kwajalein Atoll.
-- Integrated fly management had reached a level of partial success by 1974.
Initial surveys for natural enemies of M. sorbens revealed the
presence of four scavenger and predatory insects, the histerid Carcinops
troglodytes Erichson, the nitidulid Carpophilus pilosellus
Motschulsky, the tenebrionid Alphitobius diaperinus (Panzer), and the
dermapteran Labidura riparia (Pallas). Dog numbers were significantly
reduced and all privies were reconstructed or improved on one island,
Ebeye. Dogs were reduced or tethered
on Kwajalein Island and refuse fish, etc., disposed of thoroughly on
l1leginni and other islands with American residents. Importations of natural enemies were made
throughout the Atoll, and the average density of M. sorbens on Ebeye was subsequently reduced from an estimated
8.5 flies attracted to the face per minute, to less than 0.5 flies per
minute, which was readily appreciated by the inhabitants. The single most important cause appeared
to be the partial elimination of breeding sources, with natural enemies
playing a secondary role.
For the further
reduction of bush fly numbers the integration of a nondestructive
insecticidal reduction measure was desirable. Sugar bait mixtures that have been used for houseflies in years
previous to 1972 were wholly ineffective for killing adult M. sorbens
due to their almost complete lack of attractiveness. However, a variety of decomposing
foodstuffs including rotting eggs and rotting fish sauces were very highly
attractive. Experiments using a 6-day old mixture of one-part fresh whole
eggs to one part water (Legner et al. 1974) attracted
over 50,000 bush flies that were then killed by a 0.5 ppm Dichlorvos (R)
additive. The poisoned mixture was
poured in quantities of 100 mI. each in flat plastic trays with damp sand at
20 sites in the shade and spaced every 10 meters along a public beach on
Kwajalein. Baits placed above the
height of 1m or against walls in open pavilions were only weakly attractive.
After 48 hours, flies were reduced to inconspicuous levels all over Kwajalein
Island. This condition endured for at
least three days after which newly emerging and immigrating flies managed to
slowly increase to annoying levels as the baits ceased to be attractive. But
the former density of flies was never reached even one week after the
baiting; and these populations were subsequently reduced to even lower levels
by applying additional fresh poisoned baits.
Baiting was
extended to other islands in the Atoll with the result of sustained
reductions of bush flies to below general annoyance levels (less than 0.01
attracted per minute on Kwajalein, Roi-Namur, Illeginni and Meck Islands.) A new attractant that augmented the
rotting egg mixture consisted of beach sand soaked for one week in the
decomposing body fluids of buried sharks. This new attractant was far
superior to rotting eggs both in rate and time of attraction, the latter
sometimes exceeding 5 days. The baiting method could be used effectively if
applied initially twice a week, and only biweekly applications were necessary
in the following months.
After January
2000 in the absence of specialist supervision the baiting procedure in the
Atoll has not continued with the sophistication initially determined
necessary. In the absence of
supervision the flies were not adequately reduced. Periodic personnel changes precluded the passing on of accurate
information critical to managing the fly densities. Of vital importance is habitat reduction, the proper
preparation of baits and the latter’s placement in shaded wind calm areas of
the islands. Because such sites are
generally out of sight of the public, baiting has rather shifted to populated
areas where only very conspicuous but nonpestiferous species of flies are
attracted to the baits in large numbers.
Sometimes even ammonia baits were substituted that attract harmless
blow fly species but not the targeted bush fly.
Aquatic Weed Management
by Fish in Irrigation Systems
Imported fish species have been used for clearing aquatic vegetation
from waterways, which has also reduced mosquito & chironomid midge
abundance. In the irrigation systems,
storm drainage channels and recreational lakes of southern California, the
California Department of Fish and Game authorized the introduction of three
species of African cichlids, Tilapia zillii
(Gervais), Oreochromis (Sarotherodon) mossambica
(Peters), and Oreochromis (Sarotherodon) hornorum
(Trewazas). These became established over some 2,000 ha. of waterways (Legner
& Sjogren 1984). Their establishment reduced the biomass of emergent
aquatic vegetation that was slowing down the distribution of irrigation water
but that also provided a habitat for such encephalitis vectors as the
mosquito Culex tarsalis Coquillet. Previous aquatic weed reduction
practices had required an expensive physical removal of vegetation and/or the
frequent application of herbicides.
One species, Tilapia zillii can reduce mosquito populations by
a combination of direct predation and the consumption of aquatic plants by
these omnivorous fishes (Legner & Fisher 1980; Legner & Murray 1981,
Legner & Pelsue 1983). As Legner & Sjogren (1984) indicated, this is
a unique example of persistent biological suppression and probably only
applicable for relatively stable irrigation systems where a permanent water
supply is assured, and where water temperatures are warm enough in winter to
sustain the fish (Legner et al. 1980). A three-fold advantage
in the use of these fish is (1) clearing of vegetation to keep waterways
open, (2) mosquito abatement and (3) a fish large enough to be used for human
consumption. However, optimum management of these cichlids for aquatic weed reduction
often is not understood by irrigation district personnel (Hauser et al.
1976, 1977; Legner 1978), with the result that competitive displacement by
inferior cichlids minimize or eliminate T. zillii,
the most efficient weed eating species (Legner 2000).
The three imported fish species varied in
their influence in different parts of the irrigation system. Each fish species possessed certain
attributes for combating the respective target pests (Legner & Medved 1973a,
b). Tilapia zillii was best able to perform both
as a habitat reducer and an insect predator. It also had a slightly greater
tolerance to low water temperatures, which guaranteed the survival of large
populations through the winter months; while at the same time it did not pose
a threat to salmon and other game fisheries in the colder waters of central
California. It was the superior game species and most desirable as human
food. Nevertheless, the agencies supporting the
research (mosquito abatement and county irrigation districts) acquired and
distributed all three species simultaneously throughout hundreds of
kilometers of the irrigation system, storm drainage channels and recreational
lakes. The outcome was the permanent and semi permanent establishment of the
two less desirable species, S. mossambica and S.
hornorum over a broader portion of the distribution range. This
was achieved by the competitively advantaged Sarotherodon species that
mouth-brood their fry, while T. zillii did not have this
attribute strongly developed. It serves as an example of competitive
exclusion such as conjectured by Ehler (1982). In the clear waters of some
lakes in coastal and southwestern California, the intense predatory behavior
of S. mossambica males on the fry of T.
zillii could be easily observed, even though adults of the
latter species gave a strong effort to fend off these attacks.
This outcome was not too serious for
chironomid reduction in storm drainage channels because the Sarotherodon
species are quite capable of permanently suppressing chironomid densities to
below annoyance levels (Legner et al. 1980). However, for the management of
aquatic weeds, namely Potamogeton pectinatus L., Myriophyllum
spicatum var. exalbescens (Fernald) Jepson, Hydrilla
verticillata Royle and Typha species, they showed
little capability (Legner & Medved 1973b). Thus, competition excluded T.
zillii from expressing its maximum potential in the irrigation
channels of the lower Sonoran Desert and in recreational lakes of
southwestern California. Furthermore, as the Sarotherodon
species were of a more tropical nature, their populations were reduced in the
colder waters of the irrigation canals and recreational lakes. Although T.
zillii populations could have been restocked, attention was
later focused on a potentially more environmentally destructive species, the
White Amur, Ctenopharyngodon idella
(Valenciennes), and other carps. The competitively advantaged Sarotherodon
species are permanently established over a broad geographic area, which
encumbers the reestablishment of T. zillii in storm drainage channels
of southwestern California.
Managment of Filth Fly
Abundance in Dairies and Poultry Houses
The most
important of muscoid fly species are broadly defined as those most closely
associated with human activities. Breeding habitats very from the organic
wastes of urban and rural settlements to those provided by various
agricultural practices, particularly ones related to the management and care
of domestic animals. Their degree of relationship to humans varies
considerably with the ecology and behavior of the fly species involved. Some
are more often found inside dwellings.
Research to
reduce fly abundance has centered on the highly destructive parasitic and
predatory species, such as the encyrtid Tachinaephagus zealandicus
Ashmead, five species of the pteromalid genus Muscidifurax, and Spalangia species that
destroy dipterous larvae and pupae in various breeding sources. The natural enemies are capable of
successful fly suppression if the correct species and strains are applied in
the right locality (Axtell & Rutz 1986, Legner et al. 1981 , Mandeville et al. 1988, Pawson
& Petersen 1988). Other approaches have included the use of pathogens and
predatory mites, and inundative releases of parasites and predators (Ripa
1986, 1990). Although partially successful, none of these strategies have
become the sole method for fly abatement, and the choice of a ineffective
parasite strain may have detrimental results (Legner 1978). Instead, the focus is on integrated
management including habitat reduction, adult baiting and aerosol treatments
with short residual insecticides. Also, it is generally agreed that existing
predatory complexes exert great influences on fly densities (Geden &
Axtell 1988) and that many natural enemies of these flies have a potential to
significantly reduce their abundance if managed properly (Legner 2000,
Mullens 1986, Mullens et al. 1986). Because climatic and locality differences dictate which
abatement strategies are effective, simple instructions to the public are
impossible and the involvement of skilled personnel is required. Of primary importance for successful
management is the provision of relatively stable breeding habitats and their
natural enemy complexes. Periodic
cleaning operations should stress the partial removal of breeding sites and
the deposition of such waste into large stacks that favors the generation of
destructive heat while minimizing the area and attractiveness for fly
oviposition. Nevertheless, this
management procedure is difficult for abatement personnel to grasp in the
absence of competent supervision.
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