Of the 23 species of agricultural pest known to resist insecticides in China,4 are cotton pests, 4 rice pests and 5 are pests of brassicae. In the green rice leafhopper, malathion resistance is caused by increased carboxylesterase (CarE) activity, which plays a more important role in the resistance to dimethoate than the mixed-function oxidases (mfos). The in-vitro and in-vivo results are in agreement with studies of synergism of malathion and dimethoate by TPP and EBP. These synergists delay the development of resistance, and EBP when added to malathion has limited the development of resistance to malathion in the green rice leafhopper. In the cotton aphid, resistance to organophosphates involves several factors: acetylcholinesterase (AChE) insensitivity, high CarE activity, slight ( x 2) increase in glutathione S-transferases (GSH-ases), mfo activity as well as reduced penetration. In vitro, the I,, of the insensitive AChE is x 1 4 that of S aphids, and anaphthyl-acetate CarE hydrolysing activity is 70 times greater in R than in S aphids. Insecticide mixtures, alternation or rotation. can delay build-up of

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