Pesticides, including insecticides and miticides, are still primarily used to “control” or regulate insect and mite pest populations on horticultural plants in order to prevent damage. However, outbreaks of insect and mite pests following pesticide applications are well-known. The common explanations for insect and mite pest outbreaks following pesticide applications are:
Insecticide hormoligosis, which is not the same as resistance, is a phenomenon in which reproduction is stimulated in response to sub-lethal effects of pesticide applications when used at labeled rates or below the label rates. The cause of the stimulation is primarily due to the pesticide. Stimulation of reproduction in pest populations may lead to increased population growth, and thus outbreaks requiring pesticide applications. This could result in an increase in the frequency of pesticide applications, which may enhance the selection pressure placed on pest populations and intensify the potential for resistance.
Insecticides may have an indirect effect on insect pests by reducing or increasing reproduction (e.g., number of eggs laid or offspring produced per female). For example, insecticide hormoligosis has been implicated in increasing the reproduction of several insect species including the green peach aphid (Myzus persicae). Green peach aphid females produce 20 to 30% more offspring when exposed to certain organophosphate insecticides compared to aphids that were not exposed to these insecticides. The increase in reproduction is likely a direct result of the action of the insecticides on the aphids.
Furthermore, spider mites may respond positively to insecticide applications. For example, foliar or drench applications of imidacloprid (Merit) increased the number of eggs laid by twospotted spider mite (Tetranychus urticae) females by 20 to 50%. In addition to increasing reproduction, exposure to sub-lethal rates of pesticides may alter insect sex ratios (female: male) or directly stimulate other life history parameters such as development or longevity. Also, low rates of an insecticide may improve the nutritional quality of host plants thus increasing the reproduction or decreasing the development time of certain pests.
The sub-lethal effects of a pesticide may act as a “stressing agent;” thus stimulating an organism to respond positively to environmental changes. This increases efficiency associated with either development or reproduction, which could be due to enhanced production of specific enzymes or metabolism. In addition, the stimulatory effects of insecticides might be associated with impurities, surfactants, or carries in the formulation and not the actual active ingredient.
Insecticide hormoligosis is less frequently encountered than resistance but can be lead to severe insect or mite pest outbreaks, resulting in damage to horticultural plants. Therefore, it is important to use the recommended label rates and avoid reducing costs by using rates that are below those stated on the label. If anyone has questions or comments regarding this article they may contact the author at firstname.lastname@example.org or 785.532.4750.
Raymond A. Cloyd is a Professor and Extension Specialist in Horticultural Entomology/Integrated Pest Management in the Department of Entomology at Kansas State University.