The whitefly, Bemisia tabaci (Gennadius), is among the most damaging of all pests in cropping ecosystems. Among the different biotypes found in this complex species, B biotype is one of the most widespread in agricultural systems. Within a few years of its initial introduction imidacloprid became the standard product for control of whitefly. This study evaluates the effect of frequent applications of imidacloprid to a population of whitefly. B biotype whitefly collected from a commercial greenhouse in Michigan, was cultured and put under imidacloprid selection to determine the rate of development of resistance to this insecticide. Before imidacloprid treatments started this population already had a a reduced susceptibility to imidacloprid with a resistance ratio RR of 11-fold compared with that of a reference strain that we obtained from University of California, Riverside. When adults of this resistant population were exposed to an increasing concentration of imidacloprid, the resistance level increased to 218-fold in the F11 generation, indicating a rapid development of resistance after repeated exposure to high doses of imidacloprid. The realized heritability h2 of the resistance was also determined. The overall mean estimate of realized heritability for imidacloprid resistance in this B biotype population was 0.10, suggesting that phenotypic variation in imidacloprid resistance in this population was accounted for by an additive genetic variation. The response of the resistant population to treatment with other neonicotinoids was then assessed. The imidacloprid resistant population exhibited a low level of cross resistance to thiamethoxam (RR=9fold) and to acetamiprid (7-fold). To have a complete picture of our assessment, the effect of exposure of whitefly to a very low concentration of this insecticide was also determined. Three different populations were used:a reference strain, a resistant and a rebuilt reference strain. The exposure of immatures of the resistant strain to a low priming concentration of imidacloprid (0.0012 mg a.i. per pot), followed by a subsequent exposure to imidacloprid as adults elicited an increase in fecundity when compared with non-primed females (77.0 eggs/female compared with a 66.4 eggs/female). Resistant females pre-exposed to low priming concentration (0.0012 mg a.i per pot) were also found to exhibit greater egg production than resistant females primed with a high concentration of imidacloprid (12 mg a.i. per pot ) (54.4 and 46.0 eggs per female). F1 female progeny of the reference strain from parents primed with imidacloprid at 0.0012 mg a.i. per pot produced more eggs than F1 female progeny from non-primed parents (19.6 and 13.6 eggs/female). The test was repeated on the rebuilt reference strain using two priming concentrations (0.0012 and 0.00012 mg a.i. per pot). The test on the parental generation of this rebuilt reference strain showed that females primed with the two low concentrations of imidacloprid 0.0012 and 0.00012 mg a.i. per pot have greater egg production than the non-primed females for the rebuilt reference strain (191.8, 186.7 and 169.3 eggs per female, respectively). However, no difference was found in the fecundity of the F1 progeny of females for this rebuilt reference strain. All the results suggest that treatment with a low amount of imidacloprid applied during an early immature stage of development has an effect on adult whitefly fecundity. The result of this study has provided more insight into the strategy for whitefly management and into the effects of residual amount of insecticide in the organism.
Read
