Cey. J. Sci. (Bio. Sci.) Vol. 30,2002,55-66 55 INSECTICIDE RESISTANCE IN THE BRUCHID CALLOSOBRUCHUS MACULATUS, A STORAGE PEST OF LEGUMES M.M.S. Bogamuwa, K.C. Weerakoon and S.H.P.P. Karunaratne* Department of Zoology, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka. ABSTRACT Status and the mechanisms of insecticide resistance in a local population of the bruchid Callosobruchus maculatus, a storage pest of legumes, were investigated. Adults were subjected to insecticide bioassays with carbosulfan (carbamate), dimethoate and malathion (organophosphate), permethrin (pyrethroid) and DDT (organochlorine) by topical application. Mortalities were recorded after 24 hours and the LD50 (|ig/mg) values for dimethoate, permethrin .and carbosulfan were 0.028,1.838 and 0.284 respectively. Resistance percentages (percentage survivals) for fixed dosages of DDT (1.75 ug/mg) and malathion (1.4 ug/mg) were 65% and 77.5% respectively. C. maculatus had a higher tolerance to all the insecticides tested. Activity of insecticide metabolizing enzymes (carboxylesterases, malathion carboxylesterases, glutathione-S-transferases) and monooxygenase concentrations were investigated. Results showed that all these enzymes except oxidases may be involved in in vivo detoxification of insecticides. Propoxur inhibition of insect acetylcholinesterases suggests the absence of an insensitive target-site resistance mechanism to organophosphates and carbamates. Key words Bruchid, Insecticide resistance INTRODUCTION The southern cowpea weevil Callosobruchus maculates Fabricius, a cosmopolitan pest of legumes, is widespread throughout most of the tropics and subtropics. Bruchids reduce the harvested yield of grain legumes by feeding on seeds. The principal host of C. maculatus is the cowpea, Vigna unguiculata, which is an important food source in tropical countries (Ofuya, 1987). Seed damage by C. maculatus reduces seed weight and viability, resulting in poor marketability. Damaged seeds have a poor germination capacity and are generally unfit for human consumption. Control measures mostly include treatment with synthetic insecticides. However, the development of insecticide resistance has become a major problem * Corresponding Author's E-mail: shppkaru@yahoo.com