The Impact of Pesticides on Natural Enemies

Most biological control agents, including predators and parasitoids, at work in our agricultural and urban environments are naturally occurring ones, which provide excellent regulation of many pests with little or no assistance from humans. The existence of naturally occurring biological control agents is one reason that many plant-feeding insects do not ordinarily become economic pests. The importance of such agents often becomes quite apparent when pesticides applied to control one pest cause an outbreak of other pests because of the chemical destruction of important natural enemies. There is great potential for increasing the benefits derived from naturally occurring biological controls, through the elimination or reduction in the use of pesticides toxic to natural enemies.

Problems associated with reliance on chemical control include the development of pesticide resistance in important pest species. This encourages an increase in dosage and number of pesticide applications which magnifies the adverse effects on natural enemies. Pests may also resurge because of the destruction of predators and parasitoids, breeding without restraint from natural enemies. A secondary pest outbreak may occur when natural enemies of a pest, which were not the target of the application, are destroyed. The second species, released from the pressure imposed by its enemies now may increase to damaging numbers and require further insecticidal treatment. If pesticides eliminate natural enemies, populations of pests may increase dramatically. There is the potential for these pests to emigrate to surrounding habitats, and they may end up damaging crops at considerable distances from the site where the application took place. The impact of pesticides may extend over long periods of time and large areas or may last until the delicate numerical balance is reestablished. If pesticides are used often, the normal balance may never be achieved.

Pesticides often inflict severe mortality on both pest and natural enemy because of their basic physiological similarities they are both arthropods. There can also be indirect impacts through destruction of alternative prey. Pesticides influence the biology of insects in even more subtle ways and may affect their development, egg production, and the ability to find prey. However, much less is known about the effects of chemical pesticides on natural enemies than on pests because the pest is usually the primary object of pesticide research. Natural enemies come in contact with pesticides through a variety of means, including direct exposure to the chemical, contact with the pesticide residue, or through the food chain. Parasitoids or predators may encounter fallout of the toxicant while on plants or soil surfaces or while flying. They may also ingest the toxic material while feeding on plant material to obtain nutrients or water, through predation, through host feeding by adult parasitoids, or as immature parasitoids consuming the host. The greater susceptibility of natural enemies to low concentrations of pesticides often means that during reentry into treated habitats, predators and parasitoids are subjected to toxic residues longer than the pests. Pesticides also have sublethal effects on natural enemies. Herbicides, fungicides and all major classes of insecticides may reduce feeding, egg laying, or egg hatch. The developmental rate may be lengthened or the sex ratio changed resulting in fewer females produced, and there may be behavioral effects on prey searching, prey acceptance, and mobility. Although the focus of pesticide studies has been on the mortality of isolated species, the shift to IPM requires that the diverse and often subtle pesticide side-effects among the plant, pest, and natural enemy be more thoroughly investigated.

Selectivity is the use of pesticides to kill pests while not affecting their natural enemies. Physiological selectivity is an inherent property of a pesticide at a particular dose, regardless how it is used. It involves movement of pesticide on or within the arthropod's body, its activation, or degradation, and excretion. Physiological selectivity may also be attained through development of resistant strains of natural enemies or may be achieved or improved by dose manipulation. Ecological selectivity depends on pesticide manipulation through timing or placement and is achieved by applying pesticides in ways that minimize the exposure of natural enemies while killing their hosts or prey. This includes using different pesticide formulations and altering the timing of application, the method of application, and the spatial distribution of the treatment. It exploits the differences in the biology of natural enemies and their host or prey and includes aspects of the life history, movement, and spatial distribution. Applications may be made to only a portion of the plant. Since pests and natural enemies differ in patterns of movement, spot treatments or stratified applications may be made within a field or crop. This may include treatment of field perimeters, strips or selected plots, or the use of trap crops that are treated separately. Application can also be made to avoid drift in refuges or sheltered areas adjacent to crops where natural enemies might be resting or feeding. Timing pesticide applications to take advantage of differences in pest and natural enemy biology has probably been the single most common and effective means used to achieve ecological selectivity. Implementation of such tactics requires knowledge of the biology and development of both the pest and natural enemies.

Manipulation of the formulation can limit or direct the distribution of pesticide residues and can influence its uptake and penetration in the natural enemy. The side-effects on natural enemies are highest with emulsifiable concentrates and wettable powders. Baits and seed treatments can drastically decrease pesticide residues in the environment in addition to concentrating chemicals where they are effective and selective. Granular materials may also control pests selectively while conserving natural enemies through spatial separation of the pesticide and natural enemy.

Pesticide resistance has been observed in some beneficial insects and mites and has been more commonly reported for predators than parasitoids. From an IPM perspective, resistance levels in natural enemies often must be substantial (10 to 100 fold) before significant selectivity benefits can be obtained in the field because natural enemies are often intrinsically more susceptible than are pests. Lower levels of resistance may be of some value in achieving pesticide selectivity when combined with ecological selectivity, and in allowing the reentry of natural enemies from refuges or sheltered areas into treated habitats as pesticide residues are dissipating. The potential for genetic improvement of resistance by altering genetic characteristics of a species through hybridization, artificial selection or genetic engineering has been a major stimulus in pesticide resistance research on beneficial organisms.

The successful combination of pesticide use and biological control in an IPM program is dependent more on knowledge of the system the ecology and the behavior of pests and natural enemies than on the availablility of tools and techniques. The best approach to preserving effective biological control of natural enemies is a combination of tactics including an understanding of the biology and behavior of arthropods, detailed monitoring of life history and population dynamics of pests and natural enemies, employment of selective pesticides, use of the least disruptive formulation of the pesticide, application only when absolutely necessary, basing chemical control on established economic injury levels, and application at the least injurious time. By conserving and protecting natural enemies we permit them to operate to their full potential as naturally occurring sources of biological control in the urban and agricultural environment.

- Larry Charlet, USDA-ARS, Northern Crop Science Lab, Fargo, North Dakota

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