Using And Making A Biological Key Answer Key

[Boleslao Drinker]

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Biological control is the use of living organisms to suppress pest populations, making them less damaging than they would otherwise be. Biological control can be used against all types of pests, including vertebrates, plant pathogens, and weeds as well as insects, but the methods and agents used are different each type of pest. This publication will focus on the biological control of insects and related organisms.

Pests are those species that attack some resource we human beings want to protect, and do it successfully enough to become either economically important or just a major annoyance. They are only a tiny fraction of the insect species around us. Even many of the species we would recognize as important pests only occasionally do significant damage to us or our resources.

Natural enemies play an important role in limiting the densities of potential pests. This has been demonstrated repeatedly when pesticides have devastated the natural enemies of potential pests. Insects which were previously of little economic importance often become damaging pests when released from the control of their natural enemies. Conversely, when a non-toxic method is found to control a key pest, the reduced use of pesticides and increased survival of natural enemies frequently reduces the numbers and damage of formerly important secondary pest species.

Predators: Many different kinds of predators feed on insects. Insects are an important part of the diet of many vertebrates, including birds, amphibians, reptiles, fish, and mammals. These insectivorous vertebrates usually feed on many insect species, and rarely focus on pests unless they are very abundant. Insect and other arthropod predators are more often used in biological control because they feed on a smaller range of prey species, and because arthropod predators, with their shorter life cycles, may fluctuate in population density in response to changes in the density of their prey. Important insect predators include lady beetles, ground beetles, rove beetles, flower bugs and other predatory true bugs, lacewings, and hover flies. Spiders and some families of mites are also predators of insects, pest species of mites, and other arthropods.

Parasitoids: Parasitoids are insects with an immature stage that develops on or in a single insect host, and ultimately kills the host. The adults are typically free-living, and may be predators. They may also feed on other resources, such as honeydew, plant nectar or pollen. Because parasitoids must be adapted to the life cycle, physiology and defenses of their hosts, they are limited in their host range, and many are highly specialized. Thus, accurate identification of the host and parasitoid species is critically important in using parasitoids for biological control.

Pathogens: Insects, like other animals and plants, are infected by bacteria, fungi, protozoans and viruses that cause disease. These diseases may reduce the rate of feeding and growth of insect pests, slow or prevent their reproduction, or kill them. In addition, insects are also attacked by some species of nematodes that, with their bacterial symbionts, cause disease or death. Under certain environmental conditions, diseases can multiply and spread naturally through an insect population, particularly when the density of the insects is high.

An example of an established population of an insect pathogen which has been successfully controlling its host is the fungus Entomophaga maimaiga, a pathogen of the gypsy moth. This fungus is believed to have been introduced about 1911, but was not discovered in forests until 1989, when it was widespread and abundant in New England. It has continued to control gypsy moth populations here for several years. It overwinters in leaf litter as resting spores, which germinate when gypsy moth larvae are present. First-instar caterpillars are dispersed by wind, and those that fall to the forest floor are probably infected while crawling to a tree. While these larvae are feeding in the tree canopy, if there is adequate rainfall, the fungus in their bodies produces spores that spread to other caterpillars. If conditions are suitable, this infection cycle will occur again during the larval stage. Large caterpillars rest during the day in forest litter, where they are also susceptible to infection by germinating resting spores. In late June, as infected caterpillars die in large numbers, new resting spores are produced to survive the next winter. This biological control agent is dependent on rain at appropriate times during the season to be successful.

There are three primary methods of using biological control in the field: 1) conservation of existing natural enemies, 2) introducing new natural enemies and establishing a permanent population (called "classical biological control"), and 3) mass rearing and periodic release, either on a seasonal basis or inundatively.

Reducing pesticide use: Most natural enemies are highly susceptible to pesticides, and pesticide use is a major limitation to their effectiveness in the field. The original idea that inspired integrated pest management (IPM) was to combine biological and chemical control by reducing pesticide use to the minimum required for economic production, and applying the required pesticides in a manner that is least disruptive to biological control agents. The need for pesticides can be reduced by use of resistant varieties, cultural methods that reduce pest abundance or damage, methods of manipulating pest mating or host-finding behavior, and, in some cases, physical methods of control. Many IPM programs, however, have not been able to move beyond the first stage of developing sampling methods and economic thresholds for pesticide application.

The effect of a pesticide on natural enemy populations depends on the physiological effect of the chemical and on how the pesticide is used -- how and when it is applied, for example. While insecticides and acaricides are most likely to be toxic to insect and mite natural enemies, herbicides and fungicides are sometimes toxic as well. A database has been compiled on the effects of pesticides on beneficial insects, spiders and mites (summarized in Croft 1990 and Benbrook 1996). This database compares the toxicity of different pesticides and the "selectivity ratio" -- the dose required to kill 50% of the target pest divided by the dose that kills 50% of the affected natural enemy species. Among the insecticides, synthetic pyrethroids are among the most toxic to beneficials, while Bacillus thuringiensis and insect growth regulators were among the least toxic. In general, systemic insecticides, which require consuming plant material for exposure, and insecticides that must be ingested for toxicity affect natural enemies much less than pests.

Pesticides may also have more subtle effects on the physiology of natural enemies than direct toxicity. Several fungicides, such as benomyl, thiophanate-methyl, and carbendazim, inhibit oviposition by predacious phytoseiid mites. Certain herbicides (diquat and paraquat) make the treated soil in vineyards repellent to predacious mites.

The impact of pesticides on natural enemies can be reduced by careful timing and placement of applications to minimize contact between the beneficial organism and the pesticide. Less persistent pesticides reduce contact, especially if used with knowledge of the biology of the natural enemy to avoid susceptible life stages. Spot applications in the areas of high pest density or treatment of alternating strips within a field may leave natural enemies in adjacent areas unaffected. The effectivenss of limiting the areas treated may depend on the mobility of the natural enemy and the pest.

Natural enemies are generally not active during the winter in the Northeast, and thus, unless they are re-released each year, must have a suitable environment for overwintering. Some parasitoids and pathogens overwinter in the bodies of their hosts (which may then have overwintering requirements of their own), but others may pass the winter in crop residues, other vegetation, or in soil. A classic example is the overwintering of predacious mites in fruit orchards. Ground cover in these orchards provides shelter over the winter, refuge from pesticides used on the fruit trees, and a source of pollen and alternate prey.

The adults of many predators and parasitoids may require or benefit from pollen, nectar or honeydew (produced by aphids) during the summer. Many crop plants flower uniformly for only a short time, so flowering plants along the edges of the field or within the field may be needed as supplemental sources of pollen and nectar. However, diversification of plants within the field can also interfere with the efficiency of host-finding, particularly for specialist parasitoids. Populations of generalist predators may be stabilized by the availability of pollen and alternative prey, but the effectiveness of the predators still depends on whether they respond quickly enough, either by aggregation or multiplication, to outbreaks of the target pest. Thus, diversification of plants or other methods of supplementing the nutrition of natural enemies must be done with knowledge of the behavior and biology of the natural enemy and pest.

For example, the native lady beetle Coleomegilla maculata is a potentially important predator of the eggs and early instar larvae of Colorado potato beetle. The population feeding on the potato beetle depends on the availability of aphid prey in surrounding fields, including crops of alfalfa, brassicas, cucurbits, and corn, and on the availability of pollen from corn and several weeds, such as dandelion and yellow rocket. Although this predator does not currently control Colorado potato beetle on its own, more knowledge about managing C. maculata populations in the agricultural landscape could make it more effective.

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