[Pest Management] How Do Insect Growth Regulators Work?

Editor’s Note: The following articles were adapted from Techletter, a biweekly publication from Pinto & Associates, Mechanicsville, Md. To subscribe, visit www.techletter.com, or call 301/884-3020.

One type of IGR, juvenoids, interfere with the normal growth and development of insects.

Insect growth regulators (IGRs) are pesticides that don’t usually kill insects outright but instead affect the ability of insects to grow and mature normally. IGRs either block the insect’s ability to turn into an adult or cause it to change into an adult before it is physically able to reproduce. If immature insects are not able to molt into reproductive adults, the population will eventually die out. Some call IGRs “birth control” for insects.

Some IGRs are juvenoids, man-made chemical mimics of the juvenile growth hormones that occur in an insect’s body. Natural hormones control how long an insect remains in each larval or nymphal stage and when it turns into a reproductive adult.

For example, cockroach nymphs that have been exposed to juvenoid IGRs either never molt into adults or they develop into sterile adults that cannot reproduce. Cockroach adults that were exposed to IGRs as nymphs develop short, twisted wings and a darker body color. IGRs also can affect insects that are exposed later, in the adult stage, by blocking the development of viable eggs. Eggs that are exposed to IGRs may not hatch.

Juvenoid IGRs are slow to show effects and have no knockdown. You are not killing individual insects, just preventing future populations. If you use an IGR alone, it will take months to achieve control. Although no new young will be produced, most of the current crop of nymphs and adults will have to die off naturally before you will see results. That’s why a standard insecticide is usually applied along with the IGR so customers can see immediate results.

IGRs are especially useful during “clean-outs” or intensive service. Any missed cockroaches or other insects left after the treatment will be affected by the IGR and will not be able to reproduce. IGRs also can be useful against insects that have shown resistance to standard insecticides since IGRs have a different mode of action.

An added benefit to the use of some IGRs is that they stimulate cockroaches to feed. Using an IGR along with a cockroach bait can result in improved bait acceptance since the roaches are more likely to feed on the bait.

Cockroaches exposed to IGRs (top row) exhibit twisted wings and molting inhibition.

Other IGRs are chitin synthesis inhibitors. They block the development of chitin (the insect’s “skin”) so that the insect can’t form a new exoskeleton or shed the old one. These affected insects die during the molting process. Some termite baits contain chitin synthesis inhibitors. The baits are carried back to the termite colony, affecting other worker termites as well.

Common insect growth regulators are methoprene used against fleas, ants, stored product pests, mosquitoes and many others; hydroprene used against cockroaches, drain and fruit flies, bed bugs and stored product pests; pyriproxyfen used against fleas and cockroaches; and hexaflumuron, diflubenzuron and noviflumuron used against termites. IGRs are packaged as total release aerosols (foggers), crack and crevice aerosols and emulsifiable concentrates.

Insect growth regulators are non-repellent and have a long residual indoors but most degrade fairly quickly outdoors. You may need to explain to your customers just how IGRs work – that they can expect to see cockroaches or other pests after treatment, but in the long term, these insects will not be able to reproduce. Another advantage to customers is the low toxicity of IGRs to people, pets or other animals.

When should you reapply an IGR? Use monitoring traps to help determine when to retreat. Some say if 80 percent of the trapped insects are showing symptoms of IGR deformities (such as twisted wings in cockroaches), retreatment is not necessary. Less than 80 percent, it’s time to retreat.