The IGR playbook

Modern pest management approaches are built on a simple ecological principle: You need to either increase the death rate or reduce the birth rate to suppress a pest population. Most insecticides and miticides are effective at increasing the death rate of nymphs or adults, but not all of them excel at reducing the birth rate among adults or from eggs. Insect and mite growth regulators (collectively called IGRs) are different. They can increase the death rate and some can reduce the birth rate. When used effectively, IGRs can break the pest life cycle and achieve long-term control of a pest population.

Before we talk about how IGRs kill a pest, we need to understand the molting and reproductive processes. All insects and mites must molt or shed their old exoskeleton (cuticle or “skin”) as they grow from a nymph to an adult. The molting (metamorphosis) and reproductive processes are an intricate dance of multiple hormones. For this discussion, we will focus on two very important hormones — 20-hydroxyecdysone (20E) and juvenile hormone (JH). The proper balance between 20E and JH is the key to properly triggering molting and egg production, forming exoskeleton (where chitin is an important component), generating yolk proteins and other growth and reproductive processes in insects. 20E and its relatives (not JH) are important for mite development and reproduction. Immature insects and mites cannot develop to the next stage and eggs cannot be laid or will not hatch if this balance of normal hormonal functions is disrupted.

IGRs work by disrupting the normal functions of either 20E or JH and, therefore, disrupting the balance between these hormones. IGRs can generally be grouped based on their target hormone or process (Table 1).

Juvenile hormone analogs: This group includes insecticides in IRAC (Insecticide Resistance Action Committee) Group 7. These insecticides mimic JH and trick insects into stopping development and completion of life cycle. The affected insects eventually die.

Chitin synthesis inhibitors: This group includes insecticides and miticides in IRAC Group 10, 15, 16, and 17. Each of these IRAC Groups work slightly differently and target different groups of pests. In general, chitin synthesis inhibitors interfere with the normal function of 20E or other related hormones and prevent the normal production or deposition of chitin in the cuticle. The affected insects and mites either cannot develop cuticles or produce disformed, rigid, or brittle cuticles.

Ecdysone receptor agonists: Members of this group are in IRAC Group 18. They work by mimicking 20E and related hormones, which induce pre-mature molting and stop feeding. Molting at the wrong time and feeding cessation kill the affected insects.

Lipid biosynthesis inhibitors: Insecticides in Group 23 are considered IGRs according to the current IRAC classification. Insecticides in this group inhibit the production of acetyl CoA carboxylase, which is essential to the normal production of lipids. Since various types of lipids are critical components in tissues and cuticles, the inhibition of lipid biosynthesis essentially stops the development of insects and mites.

Azadirachtin: Azadirachtin has been used as an insecticide for decades, yet we still do not know how it works. In addition to having antifeedant and direct toxicity activities, azadirachtin can affect insect development and reproduction through disrupting 20E or JH. Recent research suggested that azadirachtin may act as an ecdysone antagonist.

Several IGRs have the added benefits of affecting pest reproduction. An adult developed from a nymph that survives treatment may be sterile. An adult, even if it is not killed by an IGR, may lay fewer eggs because the IGR disrupts the balance of hormones needed for egg maturation and laying. The produced eggs may not contain sufficient yolk, chitin, or other ingredients to ensure successful embryonic development. The effect of IGRs on insect reproduction can be dramatic. For example, lettuce aphid nymphs exposed to azadirachtin (active ingredient of Ornazin) produced 87% fewer offspring when they became adults. The hatching rates of eggs produced by cottony cushion scale and citrus mealybug adults treated with buprofezin (active ingredient of Talus) were less than 20% and 30%, respectively. Pyriproxyfen and etoxazole also have similar effects on insect and mite reproduction.

Tips on using IGRs.

Rotation is a must. Insects and mites can develop resistance to IGRs. This point is clearly illustrated with Q-biotype whitefly’s resistance to neonicotinoids, buprofezin, and pyriproxyfen. A pesticide treatment program should rotate among different modes of action or IRAC group numbers. You will also need other modes of action to kill adults, which are not affected by some IGRs, if you have a pest population with mixed life stages. To find the IRAC numbers of pesticides registered for greenhouse and nursery uses and examples of rotation programs, visit sepro.com/bugposter.

Some IGRs can affect adults, but they work best against eggs and nymphs. Scout regularly to identify the predominant life stage of the pest population. This information will help you find the most effective application timing and placement of an IGR in your rotation program.

Know the target spectrum of an IGR. No IGR is effective against all pests. For example, cyromazine is for flies (fungus gnats and leafmining flies) and methoxyfenozide is for caterpillars. Talus (buprofezin) is my go-to for scale insects and mealybugs, and pyriproxyfen and azadirachtin (Ornazin) drench are great for fungus gnat larvae. Read the labels and choose the right IGR for the target pests.

Most IGRs work by contact. Therefore, you need to achieve good coverage by using high volume spray or thorough drench, adding a surfactant (spreader-sticker) to the spray solution, and using spray equipment that get the spray solution to where the pests are. Some IGRs are translaminar (such as etoxazole and spiromesifen) or systemic (such as spirotetramat and Ornazin), but the above cautions apply when spraying. Talus has vapor action, which is great for killing mealybugs and scale insects that are hiding in tight places where spray solution typically cannot penetrate.

Be patient. It takes the treated nymphs a few days to die. Some IGRs do not affect reproduction so adults can continue to lay viable eggs, which will hatch into nymphs that need another application before dying.

Understand the compatibility between IGRs and biological control. In general, IGRs are compatible with predatory and parasitic insects and mites, particularly with adults. IGRs have short residual longevity, meaning that biological control agents can be reintroduced within days of application.

IGRs are effective and versatile. IGRs are effective in breaking the life cycle of pests, and they should be part of every pest management program to keep those hard-earned profits in your pockets.

J.C. Chong, Ph.D. is the Technical Development Manager, Ornamental at SePRO.