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CLIMATE AS A FACTOR IN THE NATURAL
CONTROL OF ORGANISMS
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underlined categories [ to search for Subject Matter,
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Examples (cherry fruit fly, face fly, red
scale, walnut aphid, spotted alfalfa aphid
Japanese beetle, alfalfa weevil, black scale, Klamath weed, Mexican
bean beetle,
American grizzly bear, aquatic weedds, house fly, eye gnats).
References
[Please refer also to Selected Reviews & Detailed
Research ]
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Introduction
and Discussion Climate
obviously restricts living organisms. Their abundance may be restricted, the
species may not exist in certain climates. However, the manner in which
climate restricts is often more complex than merely the action of physical
forces on the restricted species. Obvious
climatic effects on distribution are seen in polar bears that are confined to
arctic ice, musk oxen to the arctic tundra, and citrus scale insects to areas
where citrus is grown. Possibly, the fruit flies, Anastrepha, Dacus
and Ceratitis are also climatically restricted to latitudes lower than
35E. Indirect climatic
influences on organisms are expressed through physiographic features
(mountains, coasts, etc.), edaphic factors, latitude, photoperiod, altitude,
host plant or animal, phytostructure (vegetation complex and formations),
phenology (synchrony), competition and natural enemies. Competition and
natural enemies are considered here in more detail. There are
many examples of how climate operates through competition and natural enemies
to produce an observed density in a particular area. One example is given by
the density and distribution of the American elm affected by Scolytus multistriatus,
vector of the fungal pathogen causing Dutch elm disease. Infected beetle
vectors have just recently reached the west coast of America, so the density
and distribution of susceptible elms still remains relatively high. The
comparative density of these trees in eastern North America is low, a
fraction of what it was before the vector and pathogen were introduced.
Northern Minnesota and parts of Canada are still relatively untouched because
the vector cannot survive the climate. Very dry areas of the American west
may also similarly prove unsuitable for effective transmission by the vector. The
cherry fruit fly, Rhagoletis indifferens (Curran), occurs in
the higher mountainous regions of northern California. Climatic studies
indicate that this fly can persist in climates typical of California's
Central Valley. Prunus emarginata, its wild host, does not set
fruit reliably south of Lake Tahoe, which is the present southernmost range
of the fly. Thus, the fly, which is found as developing maggots only in
ripening fruit, is faced with an undependable host fruit source: a
consequence of climate acting on the plant and not directly on the insect itself.
Parasitoids of the cherry fruit fly, Opius rosicola Muesebeck
and O. muliebris Mues., extend further south than the host
because of their association with another fruit fly species, Rhagoletis
fausta (O.S.) attacking the same wild plant host, Prunus emarginata. The face
fly, Musca autumnalis DeGeer, and the horn fly, Haematobia
irritans (L.), are both invaded species in North America. The density
of both is reduced respective to available breeding habitat in the eastern
United States because they complement one another in the same habitat, cattle
droppings. Both species appear to be disfavored at latitudes below 34EN. Lat., although horn fly has recently
spread below this latitude. Face fly thus far has not gone beyond, which may
be a photoperiodic restriction. The
California red scale, Aonidiella aurantii (Maskell), used to
occur at high densities throughout citrus areas of California. The
introduction of Aphytis lingnanensis Compere caused the average
density to drop in the inland portions of south California where existing
parasitoids could not cope with the warmer and drier climate. Aphytis lingnanensis
displaced A. chrysomphali in most of the area. The additional
introduction of Aphytis melinus DeBach further reduced scale
density in the drier climatic areas. This species completely displaced the
others in most of their former range. However, Central California retains a
high red scale density because none of the imported parasitoids were as
effective in the colder climate. Recent research by R. F. Luck indicates that
this is a consequence of asynchrony of the red scale crawlers with adult
parasitoids. Prolonged periods of temperatures near freezing in winter
greatly reduce crawler production, with the consequence that parasitoids find
few hosts on which to perpetuate their species. The
walnut aphid, Chromaphis juglandicola (Kalt.) was a serious
pest throughout California walnut growing areas. The parasitoid Trioxys
pallidus (Haliday)--strain #1, was introduced from southern France in
1959, and provided substantial biological control in south California.
However, this strain was unable to establish in northern California due to
summer heat. Trioxys pallidus--strain #2, was introduced from
Iran in 1968, and was successfully established in the north, with a
subsequent great drop in aphid density. Climate influenced the density of
this aphid through the actions of its parasitoids. Another aphid species that
was incumbered by C. juglandicola through competitive
exclusion, now appears in larger but not problematic numbers. Thus, climate,
acting on the parasitoids of one aphid influenced the abundance of the other
aphid. The
spotted alfalfa aphid, Therioaphis trifolii (Monell), has three
parasitoids in California: Trioxys
complanatus Quilis Perez, Praon exsoletum (Nees), and Aphelinus
asychis Walker. All three parasitoids plus native predators give good
biological control of the aphid in California. These parasitoids are not
co-extensive, and each one only partly covers the range of the host.
Bioclimatic studies suggest that climate is responsible for the limitations
on the distribution of these natural enemies, and competition can only occur
in the overlapping areas. Climate influences the outcome of competition,
involving differential abilities of species to oviposit first and to
diapause. All three parasitoids coexist because seasonal changes favor
different species. The
Japanese beetle, Popillia japonica Newman, has a tachinid
parasitoid, Hyperecteina aldrichi Mesn., which is the principal
agent holding the beetle density down in northern Japan. In the eastern
United States, although the tachinid is established permanently, it was not
able to maintain the beetle at a low density. The climate in America
precluded synchronization of the life cycles of the host and parasitoid. The
tachinid emerges earlier in the spring than the beetle and dies before it can
find adult beetles to parasitize. This is thought to be due to the heavier
show cover and cold in Japan which delays the emergence of both species until
the sudden onset of spring, when both parasitoid and host emerge from
the soil together. In America, the soil warms up earlier and more gradually
due to the lack of such heavy snow. This results in the early and fatal
emergence of many of the tachinid parasitoids. The
density of the alfalfa weevil, Hypera postica (Gyll.), is kept
low by an ichneumonid parasitoid, Bathyplectes curculionis
(Thom.), in the San Francisco Bay area of California. This parasitoid is less
effective in the coastal range mountains and least effective in the Central
Valley. This results in the beetle being most abundant in the Central valley
and least abundant on the coast. Reasons for this lie more with the effects
of climate on the biology of the host rather than on the parasitoid. In the
cool, mild climate of the coast, the weevil population is composed of all
stages of development for most of the year. This results in the presence of
weevil larvae, the stage attacked, for a prolonged period of time. Under the
more extreme temperature conditions of the Central Valley, members of the
host population are closely synchronized developmentally, so that larvae are
present for only a relatively short period of time in the spring. This is not
favorable to the optimum performance of the parasitoid. The same lake of
close synchrony between alfalfa weevil and the introduced parasitoid, Tetrastichus
incertus (Ratz.) is thought to explain the lack of more effective
biological control of this weevil in the eastern United States. Metaphycus
helvolus (Compere) is very effective in coastal southern California,
where it keeps the black scale, Saissetia oleae (Bern.) down to
non-economic densities. This parasitoid cannot function optimally in the
Central Valley due to the longer winter months when the host stage attacked
is not available. Oleander duff (dead leaves beneath the bushes) protects
hosts from cold and simulates the necessary microhabitat in the Central
Valley, allowing scale stages to overlap as the is case on the coast. The
planting of oleander in the Central Valley, then, can contribute to an
increased level of biological control by offsetting climatic stresses. The Klamath weed, Hypericum perforatum
L., was successfully reduced to nonsignificant densities in the northwestern
United States, Chile and portions of other continents by Chrysolina
weevils. Only partial success was achieved in Australia, however, where the
climate proved to be more severe on beetle performance. Summer rainfall
favored host plant increases there while the beetle is dormant. Hence,
climate is responsible indirectly for a grater density of the Klamath
weed in Australia. The
Mexican bean beetle, Epilachna varivestis Muls., is not as
severe a pest in Mexico as it is in the United States. Less severe winters in
Mexico allow the tachinid parasitoid, Paradexodes epilachnae
Aldrich to be more effective. The parasitoid cannot diapause in the United
States. The
American grizzly bear, Ursus horribilis, presently ranges north
of California to Alaska, attaining its highest population density in Canada
and Alaska. This bear was driven out of California and most of the Northwest
by humans, who settled in the milder climatic areas. Thus, the milder
climates by favoring the human competitor, reduced the grizzly bear density. Aquatic
weeds, Potamogeton pectinatus, Eriochara spp., Myriophyllum,
and Hydrilla, in some portions of the irrigation system of
southeastern California and western Arizona, are maintained at low density by
two herbivorous cichlid fish, Sarotherodon mossambica and Tilapia
zillii. (Hauser et al. 1976 ).
Neither fish species can overwinter reliably north of the 33E N. Lat. parallel, and consequently
aquatic weeds are potentially denser up north. The encephalitis vector Culex
tarsalis breeds extensively in surface mats formed by living and dead
parts of these aquatic weeds. The average density of this mosquito has been
drastically reduced by both habitat removal actions and direct predation of
these fish. Thus, climate favoring the existence of the fish in the south has
set up conditions whereby mosquitoes are effectively reduced, and this
lowered the incidence of encephalitis. The
common house fly, Musca domestica L., is effectively reduced by
the actions of Spalangia endius Walker below 37E N. Lat. Above this parallel, house
flies are not as easily controlled or maintained at low densities without
annual parasitoid releases. The greater impact of S. endius
below 37E N. may account
for the lower densities in the south. Chloropid eye gnats of the genus Hippelates
have effective natural enemies in portions of the Neotropical area, of which Trybliographa
spp and Trichopria spp. seem most significant. This genus may have
recently invaded California and the southern United States in the absence of
adapted natural enemies. Climate favors the pests but restricts the more
tropical natural enemies. (see Research). Climate may be changed to favor
the activities of natural enemies and result in a lowering of the pest
density. For example, windbreaks planted around citrus groves in southern
California raise the average relative humidity in the orchards, favoring
parasitoid performance. Also, warm water overwintering areas are provided for
herbivorous cichlid fish (Sarotherodon and Tilapia ) either
deliberately or accidentally, resulting in greater aquatic weed, mosquito and
chironomid midge control the following year (see Research). The greenhouse whitefly parasitoid, Encarsia
formosa, is favored by cooler temperatures. Therefore, care is
necessary to maintain temperatures in the favorable range for minimum
whitefly densities. Many cultural practices in agriculture in reality change
microclimates to favor natural enemies in order to produce synchrony with the
hosts. It is
obvious that climate can exert a major impact on the results of biological
control importations. For example, a colonized natural enemy may fail to
establish, it may not spread throughout the range of the host; and even if
the natural enemy becomes coextensive with its host, climate may prevent
effective control from occurring. Knowledge
of why a particular importation and colonization is limited or enhanced will
give valuable insight into requirements for improvement in biological
control. New natural enemies or new strains of old species may be sought with
the appropriate characteristics to provide the needed control capability.
Many past failures may be re-evaluated, and renewed attempts made at foreign
exploration and importation. Climate
may be "changed" to favor the activities of natural enemies and
lower pest densities. Examples are the planting of oleander in California's
Central Valley, installing windbreaks around citrus in southern California
which is thought to raise the average humidity in the orchards, and providing
warm water overintering refuges for subtropical cichlids in California's
irrigation system. Exercise 8.1--Describe the way California red scale, Aonidiella
auranti, density is held down at different levels by its natural
enemies in different climatic zones of southern California. Exercise 8.2--Use the walnut aphid, Chromaphis juglandicola,
example to illustrate how climate acts to determine the level of a host
population by acting on the activity of specific parasitoids. Exercise 8.3--Discuss the interaction of climate and the Chrysolina
weevils on Klamath weed. Exercise 8.4--Describe the apparent climatic influences in the ecology
of the following: cherry fruit fly, face fly, spotted alfalfa aphid, Japanese
beetle, alfalfa weevil, black scale, Mexican bean beetle, encephalitis
incidence, house flies, a large mammal. Exercise 8.5--Can you name a single action that
would result in an increased encephalitis threat in southeastern California
where herbivorous cichlid fish are established? REFERENCES: [ Additional references may be found at
MELVYL Library ] Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook
of Biological Control: Principles and Applications. Academic Press, San
Diego, CA. 1046 p. DeBach, P. 1965. Weather and the success of parasites in
population regulation. Canad. Ent. 97: 848-63. DeBach, P., T. W. Fisher & J. Landi. 1955. Some effects
of meteorological factors on all stages of Aphytis lingnanensis,
a parasite of the California red scale. Ecology 36: 743-53. Flanders, S. E. 1940. Environmental resistance to the
establishment of parasitic Hymenoptera. Ann. Ent. Soc. Amer. 33: 245-53. 158. Hauser, W. J., E. F. Legner, R. A. Medved & S. Platt. 1976.
Tilapia-- a management tool for
biological control of aquatic weeds and insects. Bull.
Amer. Fisheries Soc. 1(2): 15-16. Legner, E. F. 1983. Imported cichlid behaviour in
California. Proc. Intern. Symp. on Tilapia in aquaculture, Nazareth,
Israel, May, 1983, p. 8-13. Tel-Aviv Univ. Publ. 59-63. Legner, E. F. 1986. The requirement for reassessment of
interactions among dung beetles, symbovine flies and natural enemies. Ent.
Soc. Amer. Misc. Publ. 61: 120-131. Legner, E. F. & D. J. Greathead. 1969. Parasitism of
pupae in East African populations of Musca domestica and Stomoxys
calcitrans. Ann. Ent. Soc. Amer. 62: 128-33. Legner, E. F. & R. A. Medved. 1973. Influence of Tilapia
mossambica (Peters), T. zillii (Gervais) (Cichlidae) and
Mollienesia latipinna LeSueur (Poeciliidae) on pond populations
of Culex mosquitoes and chironomid midges. J. Amer. Mosq. Contr.
Assoc. 33: 354-64. Legner, E. F. & G. S. Olton. 1971. Distribution and
relative abundance of dipterous pupae and their parasitoids in accumulations
of domestic animal manure in the southwestern United States. Hilgardia
40(14): 505-35. Legner, E. F., R. A. Medved & F. Pelsue. 1980. Changes
in chironomid breeding patterns in a paved river channel following the
adaptation of cichlids of the Tilapia mossambica-hornorum
complex. Ann. Ent. Soc. Amer. 73: 293-99. Legner, E. F., G. S. Olton, R. E. Eastwood & E. J.
Dietrick. 1975. Seasonal density, distribution and interactions of predatory
and scavenger arthropods in accumulating poultry wastes in coastal and
interior southern California. Entomophaga 29: 269-83. Legner, E. F., W. J. Hauser, T. W. Fisher & R. A.
Medved. 1975. Biological aquatic weed control by fish in the lower Sonoran
Desert of California. Calif. Agric. 29(11): 8-10. Messenger, P. S. 1970. Bioclimatic inputs to biological
control and pest management programs. Proc. Conf. N.Carolina St. Univ.,
Raleigh, March 25-27, "Concepts of Pest Management." p. 84-102. Messenger, P. S. 1971. Climatic limitations to biological
controls. Proc. Tall Timbers Conf. on Ecol. Animal Control by Habitat
Management 3: Feb. 25-27, Tallahassee, Fla. p. 97-114. Messenger, P. S. & R. van den Bosch. 1971. The
adaptability of introduced biological control agents. In: C. B.
Huffaker (ed.), "Biological Control." Plenum Press, N.Y. p. 68-92. Nicholson, A. J. 1933. The balance of animal populations. J.
Anim. Ecol. Suppl. to Vol. 2(1): 132-78. |