Insecticide Resistance in European and African Strains of Western Flower Thrips (Thysanoptera: Thripidae) Tested in a New Residue-on-Glass Test

A residue-on-glass exposure technique was developed to determine the level of insecticide resistance in different strains of the western flower thrips, Frankliniella occidentalis (Pergande). In this bioassay, the contact toxicity of pesticides was tested on adult female thrips because they are of the key sex and stage in colonization of crops. The bioassay was done with a series of ventilated glass cells, the insides of which were sprayed in a Potter tower. The pesticides applied were in aqueous dilutions and 2.0 ± 0.2 ml/cm2 of the pesticide solution was deposited on the inside surfaces. The food in the cells consisted of unsprayed tulip, Tulipa sp. L., pollen and a water container was fitted to the side of each cell to serve as a liquid source. Mortality was assessed after 24 h. Relative resistance to acephate, endosulfan, and methiocarb was tested for five thrips strains from greenhouses as compared with a susceptible field strain from California. Significant resistance at the LC50 was found for all five greenhouse strains compared with the susceptible strain. The LC50 of acephate ranged from 6.0 to 1,407 ppm (AI); the LC50 of endosulfan ranged from 156 to 1,246 ppm (AI); and the LC50 of methiocarb ranged from 2.47 to 23.90 ppm (AI) for the six thrips strains. These results confirm observations from commercial greenhouses of substantial resistance against several groups of insecticides among European and African strains of F. occidentalis. The resistance mechanism seems to be quite stable; one of the resistant strains had been in pesticide-free culture for 4 yr (≍100 generations). Furthermore, the possibility of cross-resistance seems great; a resistance ratio of up to 9.7 has been observed against methiocarb, an insecticide that has never been used previously against the thrips strains collected for this experiment. The large variation in insecticide resistance levels among different greenhouse strains emphasizes the need for proper resistance level descriptions of F. occidentalis test strains before larger insecticide efficacy screening trials are conducted.