Chapter 11

Integrated Pest Management on Organic Farms

Insect Growth Regulators

Insect growth regulators (IGRs) have proven extremely effective as components in IPM programs for control of insects which have become resistant to standard insecticides. Typically, IGRs are less harmful to the environment and more compatible with pest management systems that include biological controls. They generally don’t affect nontarget species—such as humans, birds, fish, or other vertebrates. Insect growth regulators are compounds that mimic the action of hormones to disrupt the molting process and modify growth of insect or mite pests. They do not kill insects directly, but interfere with normal development so insects die before they mature. Some cause insects to stop feeding. Others affect egg, larval, nymphal, or pupal development. They inhibit metamorphosis and may negatively affect reproduction and egg viability. Insect growth regulators are primarily used to kill immature stages of plant-feeding insects, including caterpillars, fungus gnats, leafminers, mealybugs, scales, shoreflies, thrips, and whiteflies.

How Insect Growth Regulators Work

Insect growth regulators (IGRs) use a different and more selective mode of action; they disrupt the growth process of insects, preventing them from the reaching reproductive stage. IGRs have a complex mode of action that precludes insects from rapidly developing resistance. IGRs can work in one of several ways: (1) they can mimic juvenile hormones, so that insects never enter the reproductive stage of development; (2) they can interfere with the production of chitin, which makes up the shell of most insects; or (3) they can interfere with the molting process. For most IGRs, there are minimal re-entry restrictions. IGRs typically take several days to have an effect on pest populations.

Juvenile Hormone Analogs and Mimics

When applied to an insect, these abnormal sources of juvenilizing agent can have striking consequences. For example, if the normal course of events calls for a molt to the pupal stage, an abnormally high level of juvenilizing agent will produce another larval stage or produce larval-pupal intermediates.

Chitin Synthesis Inhibitors

These prevent the formation of chitin, a carbohydrate that is an important structural component of the insect's exoskeleton. When treated with one of these compounds, the insect grows normally until the time to molt.

Anti-juvenile Hormone Agents

Anti-juvenile hormone agents cancel the effect of juvenile hormone by blocking juvenile hormone production. For example, an early instar treated with an anti-juvenile hormone agent molts prematurely into a nonfunctional adult. A disadvantage of these chemicals is that they are so selective that they may not be economic for a manufacturer to develop.

Application of Insect Growth Regulators

Application timing is important with insect growth regulators. Products have a brief period of residual activity, and exposure to ultraviolet light can affect longevity, so repeat applications usually are warranted. Insect growth regulators are most active on the early life stages of insects that undergo complete metamorphosis (egg, larva, pupa, and adult). They should be applied as soon as susceptible life stage(s) are noticed, thoroughly covering all plant parts. Population regulation may take longer when generations overlap. Death occurs in 3 to 14 days depending on the product, target pest, and life stage. Insect growth regulators decrease populations over time.

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