PSI: The case of the non-growing pest

Insect growth regulators (IGRs), are compounds that mimic the action of specific hormones, which disrupt the molting process or modify growth and development of insect (or mite) pests. Insect growth regulators do not directly kill insect pests, but may interfere with the normal mechanisms of development, thus leading to insect pests dying before they reach the adult stage. IGRs, in general, kill insect pests within three to 14 days. Although the time required to kill an insect pest is dependent on the particular IGR, the target insect pest and the subsequent age of the susceptible life stage. Insect growth regulators reduce pest populations over time — not immediately. Some IGRs may actually cause insect pests to stop feeding before they die. Furthermore, certain IGRs may affect egg, larva, nymph and/or pupa development. IGRs may inhibit metamorphosis and negatively affect female and male reproduction and egg viability. IGRs are primarily used to kill the young or immature stages of plant-feeding insects including caterpillars, fungus gnats, leafminers, mealybugs, scales, shoreflies, thrips and whiteflies. This is why application timing is critical in order to maximize the effectiveness of IGRs. The short residual activity of IGRs, and the fact that their longevity may be impacted by exposure to ultra-violet (UV) light, means that repeat applications are usually required.
 

Categories of IGRs

Insect growth regulators may be placed into three distinct categories based on mode of action. These include 1) juvenile hormone mimics or analogs, 2) ecdysone antagonists (substances acting against and blocking an action) or agonists (substances acting like another substance that stimulate an action), and 3) chitin synthesis inhibitors (Table 1). Juvenile hormone mimics or analogs arrest development causing insect pests to remain in an immature stage; thus preventing them from completing their life cycle. Examples of juvenile hormone mimics include fenoxycarb, kinoprene, and pyriproxyfen. Ecdysone antagonists or agonists interfere with or disrupt the molting process of insect pests by inhibiting metabolism of the molting hormone—ecdysone. An example of an ecdysone antagonist is azadirachtin, which is the active ingredient in a number of commercially available products. Chitin synthesis inhibitors interfere with enzymes that stimulate synthesis and formation of chitin, an essential component of an insect’s exoskeleton (skin). As a result, insect pests fail to reach adulthood because they die in the immature stage, or they mature into sterile adults (male and/or female).
 

Chitin synthesis inhibitors

There are distinct differences among some of the chitin synthesis inhibitors. For example, diflubenzuron directly inhibits chitin production. Chitin is a protein-like compound responsible for the strength and resilience of the exocuticle (middle layer of the cuticle). Buprofezin, however, prevents certain hormones from decreasing during initial molting resulting in improper digestion of the old cuticle. This prevents proper development of the new cuticle. Cyromazine causes the cuticle to be abnormally rigid (hard) or stiff resulting in decreased elasticity, which reduces the ability of insect pests to move and feed. There is also a mite growth regulator, etoxazole that inhibits chitin synthesis. Etoxazole (TetraSan) is a compound that hampers the molting process during mite development. This mite growth regulator is active on the eggs, larvae and nymphs with no direct effects on adult mites.
 

Indirect effects on adult stages

Although IGRs may only be directly active on the young stages of insect pests, certain IGRs may indirectly affect the adult stages by reducing reproduction or egg viability. Egg viability is defined as those eggs that hatch into live young. IGRs may also suppress reproduction by preventing the formation of ovaries in the adult female. Consult the label to determine those IGRs that have indirect effects on the adult stage of insect pests.

The fact that some IGRs indirectly impact the adult stages of certain insect pests is important in regards to pest management because this means that another developmental life stage may be negatively affected. Suppressing reproduction of adult females may be helpful for long-term pest management because this may reduce the number of generations that occur during the cropping cycle, which may subsequently decrease the number of insecticide applications needed.
 

Effects

Insect growth regulators, in general, are less directly harmful to natural enemies such as parasitoids and predators than conventional broad-spectrum pesticides (insecticides and miticides). However, studies have shown that IGRs may be directly or indirectly harmful to natural enemies. The overall effect of IGRs depends on a number of factors including 1) type of natural enemy (parasitoid or predator), 2) life stage (egg, larva, pupa and adult) sensitivity, 3) application rate, 4) timing of application, and 5) type and mode of action of IGR used. It is important to note that any harmful effects of IGRs may not be associated with the active ingredient but due to the inert ingredients such as adjuvants, carriers or solvents in the formulation.

Raymond Cloyd 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.