Biochemical assays were carried out within one week of storage to determine the activity levels of major enzyme groups (i.e carboxylesterases, glutathione S-transferases and monooxygenases), which are involved in insecticide metabolism. Native polyacrylamide gels were run with crude homogenates to resolve carboxylesterase isozymes. Sensitivity of the organophosphate and carbamate target site acetylcholineesterase was tested by inhibiting the enzyme with propoxur.N. lugens, S. furcifera and N. virescens showed high insecticide resistance, especially to organophosphates and pyrethroids due to higher activity levels of metabolic enzymes. All the species were susceptible to malathion mainly because of the absence of malathion carboxylesterase mechanism. Resistance to permethrin in the populations of N. lugens, C. lividipennis, M. discolour, O. indica and Tetragnatha sp. may be due to their higher mono-oxygenase activities. Presence of insensitive acetylcholinesterases could be detected among the populations of N.lugens, C. spectra, C. lividipennis and Tetragnatha sp. It appears that multiple resistance mechanisms are present in the populations tested. Higher susceptibility levels of L. oratorius and C. spectra could be attributed to the absence of resistance mechanisms in these two species.

Insect pests of rice are a major constraint in paddy cultivation in Sri Lanka. Control of insect pests heavily depends on the use of insecticides, for which the major threat has been the development of resistance. It is important to understand the present status of insecticide resistance in rice insects to identify the most suitable insecticides which are most effective against pests and least effective agaist their natural enemies.Insecticide resistance and the underlying resistance mechanisms were studied in five rice insect pests (brown planthopper Nilaparvata lugens; green leafhopper Nephotettix virescens; paddy bug Leptocorisa oratorius; white planthopper Cofana spectra and white-back planthopper Sogatella furcifera), and four of their predators (lady-bird beetle Micraspis discolor, ground beetle Ophionea indica; mired bug Cytorhinus lividipennis and spider Tetragnatha sp.). Insects were collected from the rice fields at Batalagoda, Kurunegala district (intermediate zone) and Angunakolapalassa, Hambanthota district (dry zone) of Sri Lanka from 2001 to 2003. They were subjected to insecticide bioassays with dimethoate, chlorpyrifos, permethrin, carbosulfan and fenobucarb by topical application. Log-probit mortality lines and LD 50 /LD 9j) values were obtained. To determine the resistance/susceptibility against malathion, an organophosphate which has been heavily used in mosquito control programmes, and against DDT, an organochlorine which was the major insecticide used from early 1950s to mid 1970s, bioassays were conducted with malathion and DDT as well.