Susceptibility of rice insect pests and their natural enemies to commonly used insecticides

Abstract

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 LD50/ LD90, 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. N. lugens collected from both sites showed high resistance to permethrin. L. oratorius population at Angunakolapalassa was resistant to carbosulfan compared to Batalagoda population. L. oratorius, M. discolor and Tetragnatha sp. populations at Angunakolapalassa were susceptible to permethrin compared to Batalagoda populations and Tetragnatha sp. population at Angunakolapalassa was susceptible to chlorpyrifos. Others from both study areas showed similar resistance levels. In general, most of the species tested from Batalagoda and Angunakolapalassa had lower tolerance for malathion while some species showed higher tolerance for DDT.