The susceptibility to a carbamate insecticide (pirimicarb) of Acyrthosiphon pisum reared on a partially-resistant Pisum sativum L. genotype (Onward) and a susceptible one (Hurst Greenshaft) grown in two nitrogen regimes, was assessed by topical application and leaf-dip bioassays, using either detached or intact leaves. Similar trends in insecticide susceptibility were found between treatments (genotypes or nitrogen levels) in all application methods. However, differences in susceptibility were less apparent when aphids were exposed on insecticide-treated intact leaves than when on detached ones. In all bioassays, an interaction was found between nitrogen level and genotype for insecticide susceptibility of aphids. The fourth instar nymphs and adult aphids reared on \\\\\\\\\\\\\\\'Onward\\\\\\\\\\\\\\\' were significantly more susceptible to pirimicarb than those reared on \\\\\\\\\\\\\\\'Hurst Greenshaft\\\\\\\\\\\\\\\' when plants were supplied with nitrogen solution (15 mM N), but not when supplied with nitrogen-free solution (0 mM N). On N-fertilised plants, the differences in the insecticide susceptibility of aphids between the two genotypes were more pronounced in older plants. Nitrogen fertilisation increased the LD50 of pirimicarb to adults by 87% when aphids were reared on the susceptible \\\\\\\\\\\\\\\'Hurst Greenshaft\\\\\\\\\\\\\\\' genotype, but by only 7% on the partially-resistant \\\\\\\\\\\\\\\'Onward\\\\\\\\\\\\\\\' genotype. The results were discussed in terms of the relationship between aphid weight and the level of insecticide susceptibility, and the implications for practical insecticide resistance management.

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