Development of a bioassay system for monitoring susceptibility in Thrips tabaci

A system is described for collecting adult and larva of Thrips tabaci from onion foliage into insecticide-treated vials to evaluate susceptibility to insecticides. The thrips insecticide bioassay system (TIBS) allows one to treat vials and store them for 3 weeks before thrips are collected. Depending on the population density in the field, collection of the insects for the test required from 3-6 h for one person. Assays are read after 24h. This system was used in 1997 and 1998 in commercial onion fields in Honduras and Nicaragua, and TIBS was sensitive enough to detect differences to the insecticides tested, to thrips life stages and to different generations within an onion-growing season. Data collected suggest that there were not serious problems with thrips insecticide resistance, with the possible exception of cypermethrin in Nicaragua which had a resistance ratio (RR) value of 26.9 for adult thrips. The largest RR values were observed at the end of the growing season, and this may be caused by the season-long selection by insecticide sprays. The mortality of adults and larvae followed the same general pattern, but the ratio between larvae and adults differed for each chemical group.