Methoprene and control of stored-product insects

Estimated direct and indirect losses of grains and grain-based products caused by stored-product insects range from about 10% in temperate regions to almost 50% in humid tropical areas. Pest management strategies in bulk grains include the use of fumigants such as phosphine and sulfuryl fluoride, and grain protectants, which are sprayed directly on commodities as they are loaded into storage. Fumigants, aerosols, and contact sprays are also used as structural treatments in mills, processing plants, and food warehouses. Some older organophosphate protectants and contact sprays have been phased out worldwide and have been replaced by safer insecticides, including pyrethroids and insect growth regulators (IGRs). These IGRs include juvenile hormone analogues (JHAs), ecdysteroids and chitin synthesis inhibitors, and are considered safe due to their insect specificity. Methoprene is the JHA that has been used most extensively in stored-product pest management. The formulations of methoprene originally introduced into the stored-product market in the 1980s contained the racemic mixture with both R- and S- forms, but now only the purified S-methoprene isomer is used. Methoprene has received broad attention and has been tested over decades for its direct lethal effects, but many recent studies focus more on sub-lethal effects. Although methoprene has been used for more than four decades, there has not been a recent and comprehensive synopsis or review of this IGR on stored-product insects. This review addresses the history and present use of methoprene with special emphasis on stored-product protection.