Effects of soil-applied imidacloprid on behavior of Planococcus ficus

Planococcus ficus threatens grapevine production worldwide. Because mealybugs are protected from contact insecticides by residing under bark, systemic insecticides including the neonicotinoid imidacloprid are frequently used for control. The response of mealybugs to vines treated with soil-applied imidacloprid was assessed in a series of no-choice and choice tests. No-choice tests indicated that crawlers could initially establish on plants treated at all imidacloprid concentrations tested. While all mealybugs on plants treated with the 2 highest concentrations of imidacloprid died prior to maturation, mealybugs completed development and produced viable offspring on plants treated with the lowest concentration. To evaluate the effects of imidacloprid on mature mealybugs, no-choice tests were conducted using mealybug cohorts aged 3 or 4 wk. A single mealybug was allowed to establish on an untreated grapevine. After the mealybug was established, the vine was treated with one of 4 concentrations of soil-applied imidacloprid, and the response was evaluated after 6 d. Treatment concentration had minor effects on the survival of mature mealybugs, with the fecundity of females exposed to the highest concentrations of imidacloprid reduced compared to controls. Choice tests using mealybugs from cohorts aged 3 and 4 wk indicated no preference for treated versus untreated plants. Results indicated that crawlers are more susceptible to imidacloprid than mature mealybugs. Because imidacloprid must be consumed to kill insects, mature mealybugs appear to respond to treatment by feeding less and relying on stored energetic resources.