Author
 |
Castle, Steven |
|
PRABHAKER, NILIMA - University Of California |
|
Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/15/2013 Publication Date: 6/1/2013 Citation: Castle, S.J., Prabhaker, N. 2013. Monitoring changes in bemisia tabaci susceptibility to neonicotinoid insecticides in Arizona and California. Journal of Economic Entomology. 106(3):1404-1413. Interpretive Summary: Bemisia tabaci has been a principal pest of vegetable and field crops for many years in the American southwest and is also considered one of the most serious agricultural and horticultural pests worldwide. Beginning in 1993, persistent use of imidacloprid on fall/winter vegetables and melons and again on spring melons has exposed large segments of regional populations to imidacloprid. Additional neonicotinoid insecticides including acetamiprid, dinotefuran, and thiamethoxam have since been registered on many of the same crops as imidacloprid, including cotton. Since all neonicotinoids have the same mode of action that involves the nicotinic acetylcholine receptor at nerve synapses, the potential for resistance to neonicotinoid insecticides is enhanced with the increased usage of neonicotinoids overall. The present study was initiated to evaluate the susceptibilities of field populations of Bemisia tabaci to all four neonicotinoids. A high degree of variability in bioassay test results was observed for each insecticide across spring, summer and fall seasons. However, the highest levels of resistance as determined by LC50 values occurred with imidacloprid, the insecticide that has been used most by far. In contrast, dinotefuran was consistently toxic to most field populations, but also to imidacloprid-resistant lab strains. Characterization of how each neonicotinoid insecticide performs against a variety of field populations of B. tabaci will contribute to more proficient use of each insecticide, a decrease in selection pressure and improved resistance management of the neonicotinoid insecticides. Technical Abstract: Laboratory bioassays were carried out on field-collected and laboratory strains of Bemisia tabaci to evaluate relative toxicities of four neonicotinoid insecticides: acetamiprid, dinotefuran, imidacloprid and thiamethoxam. Susceptibility to all four neonicotinoids in leaf-uptake bioassays varied considerably across populations with highest levels during mid-summer and lowest levels during late fall. Using a variability ratio of highest LC50 to lowest LC50 in field-collected whitefly adults from Arizona and California, the greatest range of responses across all tests within compounds was to imidacloprid at ratios of 555 and 142, respectively. In contrast, variability ratios to thiamethoxam in the same tests were only 7.5 and 10, respectively. Within-test comparisons among the 4 compounds generated variability ratios as high as 52, indicating differential responses to neonicotinoid compounds and possibly incomplete cross-resistance. Further evidence of differential responses among neonicotinoids was observed in multiple tests of dinotefuran against imidacloprid-resistant lab strains that produced a variability ratio as high as 120 when the LC50s of dinotefuran and imidacloprid were compared. Field strains cultured without insecticide exposure in a greenhouse and assayed periodically produced significantly higher LC50s to all four neonicotinoids compared to the field strain, suggesting that environmental factors can have a dominant influence on bioassay outcomes. |
