 Raymond Cloyd
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Insect growth regulators (IGRs) are insecticides or compounds that, generally, disrupt the molting process or modify growth and development of insect pests and some mite pests. They don’t directly kill insect pests like contact insecticides, but interfere with the normal mechanisms of development, which results in insect pests dying before they reach adulthood. IGRs typically kill insect pests in three to 10 days; however, the specific IGR, the target insect pest, and life stage may all affect the time required to kill insect pests. IGRs tend to reduce insect pest populations over time — not immediately.
Maximizing effectiveness
As with any insecticide, thorough coverage of all plant parts is important to obtain sufficient control of insect pests; however, the most important aspect is timing of application when the susceptible immature life stage(s) or instar(s) are present. For example, many IGRs are active on the early life stages, such as the first and second instars, more so than the third and fourth instars. This is the main reason why IGRs generally take longer to provide regulation/suppression when dealing with overlapping generations. In addition, they tend to be more active on insect pests that undergo complete metamorphosis (egg, larva, pupa and adult), such as fungus gnats, shore flies, caterpillars and whiteflies, than those that have incomplete metamorphosis (egg, nymph and adult), including mealybugs, thrips and scales.
Preventing resistance
The use of similar modes of action, regardless of the type of insecticide, even an IGR, places undo selection pressure on insect pest populations, resulting in an increase in the frequency of resistant individuals.
Understanding differing classifications
IGRs are classified into three general categories based on their mode of action: juvenile hormone mimics (or analogs), chitin synthesis inhibitors and ecdysone antagonists. Analogs arrest development and cause insect pests to remain in young or immature stages, preventing completion of the life cycle. Chitin synthesis inhibitors interfere with enzymes during the molting process that stimulate the production and formation of chitin, which is an essential component of an insect’s exoskeleton. As such, insect pests fail to reach adulthood because they die in the young stage, or they may mature into sterile adult females. Ecdysone antagonists disrupt the molting process of insect pests by inhibiting metabolism of the molting hormone — ecdysone.
Indirect effects on adult stages
Although IGRs are, generally, primarily active on the young/immature stages of insect pests, studies have demonstrated that IGRs may indirectly affect the adult stages of certain insect pests, such as whiteflies, by reducing female reproduction or egg viability. This is important in regards to pest management, as this means that another development stage may be negatively affected. For example, suppressing reproduction by treating adult female whiteflies is an important long-term effect, which may decrease the number of individuals present in future generations.
Working with biological controls
In general, IGRs are directly less harmful to biological control agents than conventional broad-spectrum pest-control materials (insecticides and miticides). However, studies have shown that IGRs may be directly or indirectly harmful to certain natural enemies. The overall effect of IGRs on biological control agents depends on a number of factors, including type of natural enemy (parasitoid or predator), life stage (egg, larva, pupa and adult) sensitivity, rate of application, timing of application, and type and mode of action of IGR used. Another factor to consider is that the harmful effects of IGRs may not be associated with the active ingredient, but due to inert ingredients such as surfactants, adjuvants, carriers or solvents in the formulation.
Cloyd is a frequent contributor to Greenhouse Management and a professor and extension specialist in ornamental entomology/integrated pest management for the department of entomology at Kansas State University. You can write Raymond at rcloyd@ksu.edu.
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