Esterase activity and associated insecticide resistance in the damson-hop aphid,Phorodon humuli(Schrank) (Hemiptera: Aphididae)

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Abstract
Phorodon humuli(Schr.) was obtained from both wild and cultivated hop plants, cultured, and bioassayed using seven different foliar spray insecticides, and the results were assesssed by probit analysis to determine LC50s and LC95s. The wild aphids and their progeny were highly susceptible to the insecticides, whereas those from cultivated hops were generally resistant to them. The level of resistance was related to the source of the original aphid population and the insecticide used. Clones of insecticide-resistant and susceptible aphids were maintained under identical conditions, and individual adult apterae were assayed for total esterase and esterase isoenzyme activities. Total esterase activity was appreciably higher in resistant aphids than in susceptible ones. Resistant aphids were associated with high esterase isoenzyme activity of complex bands II–V, while in susceptible aphids these bands were virtually missing. An insecticide-resistant clone ofP. humuliwas selected for low esterase activity and a susceptible clone was selected for high esterase activity, over eight generations. Selection pressure was then reversed for a further eight generations. The resistant clone lost its esterase activity by generation six and it remained low thereafter, whereas that of the control stock remained high. By contrast, the esterase activity of the susceptible clone remained unaltered by selection. The resistant clone also lost its resistance to both methomyl and methidathion.