Repellency of pesticides: impact on natural enemies

Pesticides can have direct and/or indirect effects on natural enemies; such as, parasitoids and predators. Direct effects are associated with acute mortality or survival (longevity), over a specified period of time, of the life stages of natural enemies including the egg, larva, nymph or adult. Indirect effects involve inhibition of feeding behavior (for predators) and parasitism (for parasitoids); a decrease in female reproduction; and a reduction in foraging behavior (Fig. 1).

These indirect effects may be associated with repellency. The effects of repellency on natural enemies are considered indirect or sublethal due to potential influences on foraging behavior and orientation (physical direction or position of something). Any indirect adverse effects of pesticides due to repellency can interfere with foraging behavior and parasitism (Fig. 2). In fact, a number of insecticides are known to have repellent activity with potential effects on the foraging or searching behavior of natural enemies. Repellency is primarily affiliated with reduced contact by natural enemies with a host (prey) treated with an insecticide. Therefore, repellency may decrease parasitoid and predator efficiency by reducing the probability of encountering prey (Fig. 3), which can lead to a change in spatial distribution of predators and parasitoids; and more importantly, a reduction in regulating insect or mite pest populations. The factors that can influence repellency include: pesticide formulation, residual activity (persistence), and pesticide susceptibility to sunlight (ultra-violet light) degradation.

It is important to understand that repellency can prevent natural enemies from effectively regulating pest populations, thus leading to increases in pest populations over time. A strong repellent effect can prevent individuals from contacting pesticide residues, which could result in a reduction in the number of eggs laid and offspring produced, consequently decreasing the number of individuals in future generations. This will decrease the ability of natural enemies to effectively regulate pest populations.

Bifenthrin and permethrin (pyrethroids), and chlorpyrifos and malathion (organophosphates) are insecticides with repellent activity. In fact, many pyrethroid-based insecticides are known repellents acting on the sensory nervous system, which can affect foraging, feeding, and reproduction. Natural enemy responses to the repellent effects of pyrethroids may be an innate behavioral adaptation to reduce the risk of exposure to pesticide residues. These insecticides may repel (or even irritate) predators by acting directly on the central or peripheral nervous system. Botanical insecticides may also have repellent effects on natural enemies, which can reduce the time spent by adults searching for hosts (prey) on leaves and diminishes the number of prey attacked by predators or eggs laid into prey by female parasitoids.

Certain fungicides have repellent effects on natural enemies such as the predatory mite, Phytoseiulus persimilis, which could reduce the ability of the predatory mites to effectively regulate twospotted spider mite (Tetranychus urticae) populations. It should be noted that any repellent effects may not be attributed to the active ingredient of a given pesticide but instead to the inert ingredients (e.g. adjuvants) in the formulation. If a pesticide is needed to help an existing biological control program, be sure to read the label thoroughly. If necessary contact the manufacturer to obtain information associated with any repellent effects of a given pesticide (insecticide, miticide, or fungicide).

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu or 785-532-4750