Lower toxicity impact of common insecticides used for more than three decades as exemplified by the relative susceptibility of adults of Diaphorina citri Kuw. (Homoptera: Psyllidae)

Relative toxicity of fifteen insecticides to citrus psylla, Diphorina citri Kuw., adults was determined in the laboratory. Carbaryl, phosphamidon, phosvel, formothion, phenthoate, fenitrothion, fenthion, morphothion and tetrachloroviuphos were found to be 406.4, 84.9, 36.5, 8.5, 8.2, 8.0, 5.0, 4.4 and 4.1 times, respectively more toxic than endosulfan. On the other hand, five of them, viz., malathion, diazinon, pyrethrin, lindane and nicotine sulphate proved to he 0.7, 0.6, 0.5, 0.2 and 0.05 times less toxic, respectively. Although there was an increase in the LC₅₀ values of nicotine sulphate, endosulfan and lindane since 1959, the actual median lethal concentrations being 0.4083, 0.01914 and 0.1182 per cent, respectively during 1992; the shift in the level of susceptibility of D. citri to these insecticides was not pronounced. This is exemplified by low relative susceptibility values, i.e., 3.59, 2.40 and 1.97, respectively. The toxicity of malathion was more or less the same even after a lapse of nearly 33 years. This suggests that those commonly used insecticides did not cause appreciable resistance in D. citri after such a long gap.