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CLIMATE AS A FACTOR IN THE NATURAL
CONTROL OF ORGANISMS
(Contacts)
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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. Humans, who settled in the milder climatic areas, drove
this bear out of California and most of the Northwest. 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). Cooler temperatures favor the greenhouse
whitefly parasitoid, Encarsia formosa. 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. |