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

The cotton aphid (Aphis gossyp'i Glover) is one of the most important pests infesting cotton in the cotton areas of North China. Since 1953 organophosphorus insecticides such as parathion and systox have been used to control the aphids for keeping up good yield of cotton. After several years. the concentration and the amount of spray were increasing in the field. In the early 1980's highly effective pyrethroid insecticides such as decis and sumicidin were largely imported into China. When first used 2. 5% decis emulsion was diluted in the ratio from 1110000 to 1812000. However in 1985 the resistance of cotton aphid to pyrethroids increased by 171 times in general, 3230 times in some cotton fields. Thus it has prompted us to investigate the mechanism of resistance to insecticides and to search for the strategy to control the resistant aphids.Experiments showed that the use of synergists including SV, (0, 0-diethyl, 0-phenyl p h w phorothionate) and PB has given evidence indicating mixed function oxidases (MFO) . a-NA esterases and a-NA carboxylesterase are involved in the formation of resistance. The results also showed that the sensitivity of AChE to paraoxon in resistant aphids was lower than that in susceptible aphids.Experiments showed that SV1 was particularly synergistic to organophosphorus or pyrethroid insecticides and had played an excellent role in overcoming the resistance of cotton aphids to insecticides.