|PRABHAKER, NILIMA - University Of California|
Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2013
Publication Date: 12/24/2013
Citation: Castle, S.J., Merten, P., Prabhaker, N. 2013. Comparative susceptibility of bemisia tabaci to imidacloprid in field- and laboratory-based bioassays. Pest Management Science. 70:1538-1546.
Interpretive Summary: The sweetpotato whitefly is a serious pest of field, vegetable, and ornamental crops worldwide. Insecticides, especially those of a specific class, the neonicotinoids, are critically important to management of this pest. However, intensive or long-term use can result in loss of control from insecticide-resistance. Therefore, continued availability of effective insecticides is dependent on timely and accurate monitoring for the development of resistant whitefly populations. Resistance monitoring is usually accomplished through systemic uptake bioassays, where the insects are exposed to the insecticide after it is taken up into plant leaves. Although these assays have been widely useful, their relevance to field conditions has not been demonstrated. We found that for one commonly used neonicotinoid (imidacloprid) applied to cantaloupe, insecticide concentrations within plant leaves in bioassays were highly variable but much higher than in treated plants in the field. This finding suggests the systemic uptake bioassays may overestimate the effectiveness of this insecticide. However, we found that immature whiteflies were still controlled under field conditions, possibly because they are more susceptible to imidacloprid than are adult whiteflies. These findings demonstrate that inclusion of field assays for insecticide-resistance, along with laboratory bioassays, will enhance efforts to detect and monitor resistance in this important pest.
Technical Abstract: 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 (LC50s 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 to the untreated control, but the margin of control was greater in 2006 than 2007. The potential impact of imidacloprid on whitefly eggs was explored in a greenhouse test that showed egg mortality occurring in both early (1 day) and late (3 day) 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 suggest 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 likely due to extraordinarily high concentrations of imidacloprid that can occur in leaves of systemic uptake bioassays, potentially skewing perception of susceptibility to imidacloprid.