Use of imidacloprid-treated row mixtures for Colorado potato beetle (Coleoptera: Chrysomelidae) management

The "high dose" strategy of applying imidacloprid as an "at-planting" soil treatment uniformly throughout a potato held may result in rapid pest adaptation, as it has for other systemic insecticides. Alternatively, treated and untreated plants in a field provide a refuge to pests so that some portion of the population is not subjected to selection. In 1993- and 1994-field experiments involving potato cultivars with contrasting maturity periods, plots of mixtures of imidacloprid-treated and untreated rows were compared with plots of untreated and 100%, treated rows to determine their effects on the Colorado potato beetle, Leptinotarsa decemlineata (Say). First-generation populations and feeding injury were significantly reduced by mixtures of imidacloprid treated rows, when post-diapause adults were released along the edge of plots to simulate a typical colonization pattern. Densities of life stages and percentage defoliation decreased as the proportion of imidacloprid-treated rows increased. The effectiveness of imidacloprid was greater than the additive effect based on the proportional contribution of the treated rows. Beetle populations decreased as the distance increased inward from the outer rows, because invading beetles were increasingly suppressed as they encountered rows of imidacloprid-treated plants. This suggests that the treated row at the edge of the field in a row mixture acted as a toxic barrier to colonizing beetles. Even when colonizing adults were introduced uniformly in row mixture plots, the treated rows trapped many dispersing adults and suppressed feeding injury on adjacent untreated rows. Mixtures of treated and untreated rows deployed alone or in combination with a perimeter treatment are discussed as ways to reduce the amount and cost of insecticide used create refuges to minimize resistance development, and reduce environmental risks.