Negative effect of Phostrol on Colorado potato beetles (Leptinotarsa decemlineata (Say))

Colorado potato beetle, Leptinotarsa decemlineata (Say), is the most important insect defoliator of potatoes. This beetle’s notorious ability to develop insecticide resistance may increase the benefit gained from under-utilized indirect control methods. Field studies in 2008 and 2009 demonstrated reduced beetle densities and reduced defoliation on potato foliage treated with a phosphorous acid-based fungicide Phostrol. In 2010, we conducted follow-up laboratory studies measuring the effects of Phostrol on mortality and time of development of the Colorado potato beetle larvae. Two field strains were fed Phostrol-dipped excised potato leaves: resistant strain with field-evolved resistance to multiple insecticides, and susceptible strain. Feeding on Phostrol-treated foliage increased larval mortality and delayed development. There was an interesting and significant interaction between beetle strain and Phostrol treatment, with the negative effects being stronger for the insecticide-resistant strain. These negative effects included increased time required for maturation to the next life stage for later instars and pupation. Increased mortality rates of late instar larvae were also observed. Negative effects on insecticide resistant beetles may have important applications in insecticide resistance management.