Acute toxicity: Methoprene is slightly toxic by skin exposure.

Organ toxicity: The target organ primarily affected by methoprene after long-term exposure is the liver.

Effects on birds: Methoprene is slightly toxic to birds [1,158]. The reported 5- to 8-day LC50 values for Altosid, a methoprene formulation, are greater than 10,000 ppm in mallard ducks and bobwhite quail, and the acute oral LD50 for Altosid is greater than 4640 ppm in chickens [1,158]. In mallards an acute oral LD50 of greater than 2000 mg/kg was determined [158]. Nonlethal effects that may affect survival of the birds did appear at acute oral doses of 500 mg/kg. These effects appeared as soon as 2 hours after treatment and persisted for up to 2 days and included slowness, reluctance to move, sitting, withdrawal, and incoordination [63]. These effects may decrease bird survival by making them temporarily more susceptible to predation. No effects were observed in the reproduction of bobwhite quail and mallard ducks at 30 ppm constant feeding of Altosid [158].

Effects on aquatic organisms: Methoprene is slightly to moderately toxic to fish [157]. The reported 96-hour LC50 values for the methoprene formulation Altosid were 4.6 mg/L in bluegill sunfish, 4.4 mg/L in trout, and greater than 100 mg/L in channel catfish and largemouth bass [1,8]. Methoprene residues may have a slight potential for bioconcentration in bluegill sunfish and crayfish [155]. Methoprene is very highly toxic to some species of freshwater, estuarine, and marine invertebrates, while the acute LC50 values are greater than 100 mg/L in freshwater shrimp, and it is greater than 0.1 mg/L in estuarine mud crabs [159]. Altosid had very little effect, if any, on exposed non-target aquatic organisms including waterfleas, damselflies, snails, tadpoles, and mosquito fish [159].

Breakdown in soil and groundwater: Methoprene is of low persistence in the soil environment; reported field half-lives are up to 10 days [155]. In sandy loam, its half-life was calculated to be about 10 days [155]. When Altosid was applied at an extremely high application rate of 1 pound per acre, its half-life was less than 10 days [155]. In soil, microbial degradation is rapid and appears to be the major route of its disappearance from soil [155,157]. Methoprene also readily undergoes degradation by sunlight [157]. Methoprene is rapidly and tightly sorbed to most soils [155]. It is slightly soluble in water [1]. These properties, along with its low environmental persistence make it unlikely to be significantly mobile. In field leaching studies, it was observed only in the top few inches of the soil, even after repeated washings with water [155,159].

Breakdown in water: Methoprene degrades rapidly in water [8]. Studies have demonstrated half-lives in pond water of about 30 and 40 hours at initial concentrations of 0.001 mg/L and 0.01 mg/L, respectively [49]. At normal temperatures and levels of sunlight, technical Altosid is rapidly degraded, mainly by aquatic microorganisms and sunlight [159,49].

Breakdown in vegetation: Altosid is biodegradable and nonpersistent, even in plants treated at very high rates. It has a half-life of less than 2 days in alfalfa when applied at a rate of 1 pound per acre [159]. In rice, the half-life is less than 1 day [49]. In wheat, its half-life was estimated to be 3 to 7 weeks, depending on the level of moisture in the plant [155]. Plants grown in treated soil are not expected to contain methoprene residues.