Stability of insecticide resistance in Frankliniella occidentalis to acrinathrin, formetanate and methiocarb
1 The stability of the resistance in the Western Flower Thrips, Frankliniella occidentalis (Pergande) to selective insecticides (acrinathrin, formetanate and methiocarb) was studied in both laboratory and field conditions.
2 In laboratory strains reared over a sufficient number of generations in insecticide free-conditions, levels of resistance were stable.
3 Laboratory strains selected for resistance showed moderate to very high levels of resistance. In the strain ACR9, very highly resistant to acrinathrin [resistance factor (RF) > 1000-fold], resistance was maintained with little change after 5 and 8 months without acrinathrin exposure. In the MET9 strain, resistance to methiocarb (RF = 40) was maintained after 5 months without selection pressure (RF = 33). However, 3 months later, there was a significant increase in the susceptibility to this insecticide (RF = 10). The formetanate resistance in the FOR7 strain showed a no significant decline during the 8 months without selection (from RF = 31 to RF = 17).
4 The field-collected strains from sweet pepper greenhouses (PTF4, PP2 and PM5) showed a marked fall in the LC50 to acrinathrin and methiocarb, and a slight decrease to formetanate, after being reared in the laboratory in the absence of selection pressure for 8 months.
5 In greenhouses, a significant fall in resistance to the selected insecticides was produced when insecticide pressure was withdrawn, at the end of the season, probably caused by the immigration of thrips from nearby crops or weeds, and the dilution of resistance through interbreeding with susceptible individuals.
6 Factors influencing stability or reversion of insecticide resistance are discussed.