Insecticide resistance in damson-hop aphid, Phorodon humuli in commercial hop gardens in Kent

From 1966 to 1976, samples of Phorodon humuli were collected annually from five commerical hop gardens in Kent and from other hop gardens where problems in control occurred. A susceptible stock was obtained from wild hop growing in northern England in 1969. The samples were cultured in isolation on potted hops and bioassayed against insecticides in common use. The level of resistance to demeton-S-methyl was c. 10X in 1966 after 10 yr use, and more than doubled from 1968–1974 apparently due to the spread of a more resistant type; there was a further increase to c. 50X in 1975–1976. There was also resistance of C. 20–30X to omethoate, 2–7X to methidathion and 4–8X to methomyl. Assays and field results show an increased resistance to methidathion and less certainly to methomyl after 5 yr use. There was no clear change in response to endosulfan. The LC₅₀'s estimated from a single dose and a mean probit slope were found to agree satisfactorily with the LC₅₀'s calculated from serial doses and so should be adequate for monitoring trends in resistance.