CLIMATE AS A FACTOR IN THE NATURAL
CONTROL OF ORGANISMS
----Please CLICK on desired underlined categories [ to search for Subject Matter, depress Ctrl/F ]:
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).
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.
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?
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