Comparative susceptibility of Bemisia tabaci to imidacloprid in field- and laboratory-based bioassays

Bemisia tabaci biotype B is a resistance-prone pest of protected and open agriculture. Systemic uptake bioassays used in resistance monitoring programs have provided important information on susceptibility to neonicotinoid insecticides, but have remained decoupled from field performance. Simultaneous bioassays conducted in field and laboratory settings were compared and related to concentrations of imidacloprid in plant tissue for clearer interpretation of resistance monitoring data.

Mean mortalities of adult whiteflies confined on cantaloupe leaves field-treated with three rates of imidacloprid did not exceed 40% in two trials. In contrast, laboratory bioassays conducted on different subsets of the same whitefly populations yielded concentration-response curves suggestive of susceptibility to imidacloprid in five populations (LC50 values from 1.02 to 6.4) relative to a sixth population (LC50  = 13.8). In the field, densities of eggs and nymphs were significantly lower on the imidacloprid-treated cantaloupes compared with the untreated control, but the margin of control was greater in 2006 than in 2007. The potential impact of imidacloprid on whitefly eggs was explored in a greenhouse test that showed egg mortality occurring in both early (one-day-old) and late (three-day-old) eggs on cotton leaves systemically treated with imidacloprid. Quantification of imidacloprid residues in cotton leaves used routinely in systemic uptake bioassays revealed concentrations that greatly exceeded concentrations found in the field-treated cantaloupe leaves, at least at the three highest solution concentrations used for uptake.

Systemic uptake bioassays have been widely used for monitoring B. tabaci resistance to imidacloprid, but without knowledge of imidacloprid concentrations that occur in test leaves relative to field concentrations. Higher mortality observed in systemic uptake bioassays relative to field-treated cantaloupes in this study suggests that field rates of imidacloprid are only partially effective against B. tabaci adults, in contrast to systemic uptake bioassays that showed susceptibility to imidacloprid. The discrepancy between field- and laboratory-based mortalities is probably due to extraordinarily high concentrations of imidacloprid that can occur in leaves of systemic uptake bioassays, potentially skewing perception of susceptibility to imidacloprid. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.