LAUSR

BACKGROUND Organic pest management eschews synthetic pesticides and insecticide resistance is rarely studied in organically managed systems. Spinosad is a biologically based insecticide used widely by both organic and conventional growers. Colorado potato beetle, Leptinotarsa decemlineata, is infamous for its ability to evolve resistance to insecticides. Spinosad resistance was surveyed in conventionally managed fields in Eastern NY in 2006. In response to grower reports of spinosad failure on two organic farms in 2009, resistance to spinosad was assayed in both conventionally and organically managed fields the following year, and growers were surveyed for their prior spinosad use. RESULTS In 2006 spinosad resistance measured by LD50 varied 9.8 fold among the 8 conventional fields sampled and a laboratory susceptible strain. In 2010 the resistance ratios of LD50 's relative to a laboratory susceptible strain ranged from 17.5 to 40.6 in conventionally managed fields, and from 128.7 to 5750.3 in organically managed fields, a dramatic increase from 2006 with higher resistance ratios in organically managed fields. Organic growers reported much heavier use of spinosad in years prior to 2010. CONCLUSION This is the first report of high-level resistance to spinosad in Coleopterans. Strength of selection due to the number of years used and applications per season appear to have been the primary factors driving the evolution of resistance to spinosad, highlighting the need for resistance management in organic production, where fewer alternative active ingredients for resistance management are available. This article is protected by copyright. All rights reserved.