Methoprene is used against insects. It comes in two different forms called s-methoprene and r-methoprene, and s-methoprene is the one that behaves like an important hormone in insects. It can be used against fleas, flies, moths, beetles, and other insects. Methoprene was first registered for use in the United States in 1975 and s-methoprene was later registered in 1985.

Methoprene is available in over 500 pesticide products, most of which contain s-methoprene. It is commonly added to insecticide products because it affects the insects that survive exposure to the other pesticides. It comes in several formulations including dusts, granules, ready-to-use liquids, pressurized sprays, and in flea collars. Methoprene is commonly found in flea treatments for dogs and cats, in cattle feed to control flies, and in mosquito control products. It is also used in insect baits, and home insect sprays.

Methoprene is an insect growth regulator. By acting like an insect hormone, it interferes with insect growth and development. It can prevent normal molting, egg-laying, egg-hatching, and development from the immature phase (i.e. caterpillar) to the adult phase (i.e. moth). This prevents the insects from reproducing.

You can be exposed to methoprene by eating it, getting it on your skin, or by breathing it in. Methoprene is allowed to be used on a wide variety of foods, including stored grains, corn, mushrooms, peanuts, cattle, and cereals. You may also be exposed to methoprene when applying products in your home or on your pets.

Methoprene is rapidly broken down in the body and excreted. In studies with rats that were fed methoprene, they found methoprene in the urine, feces, and breath. Three quarters of the dose was eliminated within 5 days. Small amounts of methoprene can be stored in the body. Researchers found most of the stored residues in the blood, liver, kidney and lungs.

Based on long-term studies in animals, methoprene is not expected to contribute to the development of cancer.

Yes. In some studies where pregnant laboratory animals were fed high doses of methoprene in their diets for several weeks, the animals gained less weight, and fewer pups were born. However, no effects on the fetus were observed in other studies with similar or lower doses.

No studies could be found that evaluated methoprene’s potential to cause endocrine disruption in humans. However, in studies where young mice were dosed with methoprene for several days, no estrogenic effects were observed.

While children may be especially sensitive to pesticides compared to adults, there are currently no studies showing that children have increased sensitivity specifically to methoprene.

Methoprene is moderately toxic to some fish and low in toxicity to others. Methoprene can accumulate in fish tissues. It is slightly toxic to crustaceans such as shrimp and crayfish, and very highly toxic to freshwater invertebrates.

Methoprene is relatively non-toxic to birds. It also appears to be low in toxicity to adult bees, although bee larvae may be more sensitive.