Reduced Susceptibility of Homalodisca vitripennis (Hemiptera: Cicadellidae) to Commonly Applied Insecticides
Pest management for the glassy-winged sharpshooter, Homalodisca
vitripennis Germar (Hemiptera: Cicadellidae), in Kern County, California
relies on the application of insecticides. These treatments have
contributed to low H. vitripennis field counts since applications were
initiated in 2001. However, densities have been high in recent years
despite continued management, prompting efforts to evaluate the
susceptibility of current populations to insecticides. H. vitripennis
adults were subjected to bioassays with five commonly applied
insecticides, and the results were compared to baseline toxicities
determined in 2002. Two neonicotinoids, imidacloprid and thiamethoxam,
were evaluated using systemic uptake bioassays. Contact toxicities of
the neonicotinoid acetamiprid and pyrethroids bifenthrin and
fenpropathrin were estimated using leaf dip bioassays. Dose-mortality
responses were analyzed by probit analysis. For each compound, there was
no significant difference in annual LC50 values determined over 2 yr.
Compared to baseline toxicities, acetamiprid and bifenthrin were found
to be significantly less toxic to H. vitripennis.The LC50 values of
these two compounds increased sevenfold and 152-fold, respectively.
Tests with the neonicotinoids revealed a trend of decreasing
susceptibility levels within each season followed by reversion back to
early season LC50 estimates in the following year. In addition, data
showed seasonal and site variation in susceptibility to imidacloprid,
possibly due to differential applications in nearby fields.