Status of resistance to imidacloprid and thiamethoxam in the Colorado potato beetle in Michigan and Wisconsin

Colorado potato beetle, Leptinotarsa decemlineata (Say), is among the most serious insect pests of potatoes and is well known for its ability to develop resistance to insecticides. In recent years, US potato growers have relied primarily on the neonicotinoid insecticides, imidacloprid and thiamethoxam, for control. However, resistance has become a significant problem. In 2008 and 2009, Colorado potato beetle populations from across the US, with an emphasis on Michigan and Wisconsin, were tested for resistance to these two insecticides using topical assays. LD50 values for imidaclopirid were 10-100 times the susceptible value for numerous populations in the eastern US and parts of the Midwest. Resistance in Michigan and Wisconsin is appearing in more locations and LD50 levels are increasing. LD50 values for thiamethoxam are still less than 20 times the susceptible value for most populations. Availability of numerous new brands and combination products containing neonicotinoid insecticides makes management of resistance in Colorado potato beetle even more complicated for growers.