- Provides long-term control of fleas -

Summary: Methoprene is an insect growth regulator (IGR), a very low toxicity chemical that prevents immature flea larvae, and other insects, from completing development and reaching the adult stage. Methoprene is long-lasting, inexpensive and relatively safe to use.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

What is methoprene**?

Methoprene is a chemical that interferes with the development of insects and some closely related arthropods. There are a number of similar chemicals that act this way and they are all called insect growth regulators (IGRs) because they stop normal insect growth and development. So, for example, if a larval flea (right) is exposed to methoprene it will not complete development to an adult flea and will eventually die. Manufacturers often refer to this as "breaking the flea's life cycle".

** also sometimes called by the trade name Precor

methoprene stops the development of flea larvae

Flea larva. Methoprene is a chemical that prevents this stage from developing into an adult flea.

Methoprene is practically non-toxic to mammals, including us, so is generally safe to use. It is widely used for flea, mosquito and fly control either alone or in combination with another insecticide. Related chemicals like hydroprene are used in baits for nuisance ant control. On a negative note, if IGRs get into marine (salt water) ecosystems such as bays or estuaries these chemicals can also interfere with the growth and development of marine arthropods such as crabs and lobsters.

Why insect growth regulators work

Roughly 75% of fleas in an active infestation are in the larval stage -- the stage you never see. Since exposure to these chemicals prevents larvae from becoming adult fleas, an IGR alone could eventually get rid of an infestation. But because IGRs don't control adult fleas you would have to wait until the adults died off on their own. This is why IGRs are sometimes combined with a conventional insecticide to quickly control the adult, biting fleas.

Using methoprene for flea control

Methoprene should be used to treat pet bedding, carpets, and other areas where pets sleep since this is where flea larvae are found. Methoprene can be used in addition to topically-applied flea and tick control medications like Frontline Top Spot (see How To Safely Control Fleas In Homes and On Pets for step by step flea control). Methoprene is very long-lasting and should only be needed a few times per year. Follow package instructions carefully. Do not apply methoprene to pets -- these products are intended for treatment of the pet's living space.

IGRs are generally available in both hand-pump sprayers and pressurized aerosols. I prefer hand-pump sprayers because they are more economical and easier to control than aerosol sprays. Pump sprayers also produce a larger droplet size which is less likely to float in the air and get up your nose during treatment! A hand-pumped formulation of methoprene is available here from, our affiliate.

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