SUMMARY The resistance to pirimicarb of populations of Aphis gossypii Glover from several glasshouses in England was studied in the laboratory by topical applications of insecticide and by electrophoresis and isoelectric focusing of aphid esterases. When compared with a known susceptible population, the aphids were found to be resistant to pirimicarb. Esterase patterns produced from resistant populations by poly‐acrylamide gel and starch gel electrophoresis and by isoelectric focusing consistently differed in both numbers of bands and migration rates from the susceptible type. Resistant A. gossypii were found infesting only chrysanthemums and would not transfer to cucumber, the host of the susceptible population.

Strains of the aphid Aphis gossypii, taken from cucumber in UK glasshouses were tested for resistance to pirimicarb, diazinon and heptenophos. Compared to a susceptible strain, resistance factors to diazinon of up to 659 were found in populations collected between 1987 and 1989. Compared to the same susceptible strain, diazinon-resistant populations also showed a reduced response to heptenophos. All field populations tested against pirimicarb were resistant. Host plant preference tests showed that all strains (including stock cultures) tested could readily breed on cotton. However strains originally collected from chrysanthemum did not transfer to cucumber and likewise strains from cucumber would not colonise chrysanthemum. Polyacrylamide gel electrophoresis of four of the aphid strains showed differences in migration rates and intensity of staining of esterase bands. Intensity of staining was correlated with the degree of diazinon resistance.

Between 1980 and 1984, a total of 2710 individuals of Myzus persicae (Sulzer) collected in England and Wales was tested by electrophoresis and classified as being susceptible (S), moderately resistant (R1) or strongly resistant (R2) to insecticides. Aphids sampled from oilseed rape from the Midlands and western and eastern England had the lowest frequencies of S and the highest frequencies of R1 aphids. Over all crops, eastern England had a low frequency of S and a high frequency of R1 aphids. R2 aphids were more common in northern England than in other areas of England and Wales but only on potatoes. There was no evidence of any increase in the incidence of resistant aphids (both R1 and R2) since a previous survey in 1976. Possible reasons for the distribution patterns of the three aphid types are discussed.

The F A 0 standard dip-test was used to test resistance of glasshouse populations of Aphis gossypii from chrysanthemums to the organophosphorus insecticides heptenophos and diazinon. Compared with a susceptible population collected in 1981, resistance factors to diazinon of up to 538 were found in populations collected after 1984. Resistance t o heptenophos was less than that to diazinon but some populations collected after 1984 were 34 times as resistant as the susceptible strain. Possible reasons for the sudden increase in resistance in A. gossypii and current prospects for control of resistant populations are discussed.

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