Use recommended label rates

The outbreak of insect and mite pests following applications of insecticides or miticides, has been reported in the literature for many years. In general, the most frequently suggested explanations for insect and mite outbreaks after chemical control applications are:

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1. Elimination of natural enemies that regulate pest populations, which allows for uninhibited growth of insect and mite pest populations.

2. Pesticides may physically modify or alter plant leaf surfaces making them more suitable for colonization by insect and mite pests.

3. Pesticides may influence the nutritional quality or induce changes in plant physiology (referred to as trophobiosis) of the treated plants, which may promote growth and enhance reproduction of insect and mite pests.

4. Pesticides may directly or indirectly physically stimulate insect and mite pests, due to exposure to sub-lethal pesticide concentrations, resulting in an increase in reproduction.

This article will discuss the last explanation.

Insecticide hormoligosis vs. resistance

Insecticide hormoligosis is not the same as resistance. The former is a phenomenon in which reproductive stimulation occurs in response to the sub-lethal effects of insecticides or miticides when used at labeled rates or below the minimum label rates. The actual cause of the stimulation is due to the insecticide or miticide.

Reproductive stimulation of an insect or mite population may lead to increased population growth, and thus outbreaks that require 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 indirect effects on insect pests such as reducing or increasing egg production or fecundity. As such, insecticide hormoligosis has been implicated in increasing the fecundity of several insect species including the green peach aphid (Myzus persicae). Low doses or rates of conventional insecticides have been responsible for increasing the fecundity of the brown soft scale (Coccus hesperdium).

Spider mites have also been shown to respond positively to insecticide applications. For example, foliar or drench applications of imidacloprid increased the fecundity (number of eggs laid) by 20 to 50 percent of twospotted spider mite (Tetranychus urticae). Insecticides may also alter insect sex ratios (female:male) or directly stimulate other life history parameters such as development or longevity as a result of exposure to sub-lethal dosages of pesticides.

Insect growth regulators that act as juvenile hormone mimics such as fenoxycarb (Preclude), kinoprene (Enstar II), and pyriproxyfen (Distance) may reduce reproduction by sterilizing females.

High vs. low rates

High-labeled rates of conventional insecticides may reduce fecundity of thrips although certain concentrations or rates of insecticides may actually increase reproduction. In contrast, low concentrations of insecticides may induce higher reproductive rates of target insect pests.

For example, low-labeled rates of insecticides caused brown soft scale to produce more eggs than normal. Citrus thrips (Scirtothrips citri) females lay more eggs on leaves containing residues of dicofol and malathion. Green peach aphid females produce 20 to 30 percent more offspring when exposed to the organophosphate insecticide azinphosmethyl compared to aphids that have not been exposed to the insecticide. This increase in reproduction may be a direct result of the action of the insecticide on the aphids. The fungicide, manozeb and insecticide carbaryl, and combinations of both result in a higher intrinsic rate of increase (r_m), which is the measure of the growth rate of a population under specified physical conditions, of twospotted spider mite.

Stressing agents

The sub-lethal effects of any insecticide may act as a “stressing agent” thus stimulating an organism to positively respond to environmental changes, which increases efficiency in either development or reproduction. This may be due to an increase in the production of specific enzymes or metabolism. Research has demonstrated that applications of below the minimum label rate of buprofezin (Talus) cause citrus mealybug (Planococcus citri) females to lay more eggs than either novaluron (Pedestal) or the control treatments.

The stimulatory effects of insecticides may be due to impurities, surfactants or carriers in the insecticide formulation and not necessarily to the active ingredient. The sub-lethal effect of insecticides may also influence natural enemies. For example, imidacloprid treatments increase the egg production (≥54 percent) of the predatory mite, Amblyseius (Euseius) victoriensis.

Insecticide hormoligosis is less frequently encountered than resistance, and the actual effect may be primarily due to a decrease in natural enemy abundance, or water and/or nutritional imbalances in plants. In addition, there may be selection for a genetic strain or change in the insect or mite pest population. However, it is important to use the manufacturer’s recommended rates provided on the product label, and avoid trying to cut-costs by using rates that are below those stated on the label; unless there is supporting data indicating that control may be obtained using less than the minimum label rate.

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- Raymond Cloyd