Abstract
The Colorado potato beetle, Leptinotarsa decemlineata (Say), is a major agricultural pest in the Central Sands region of Wisconsin. Imidacloprid, a neonicotinoid insecticide, has commonly been used for control of L. decemlineata since its registration in 1995. In the last 10 years, many field populations of L. decemlineata have begun to show increasing imidacloprid resistance. We studied resistance phenotype as a phenomenon that reduces neonicotinoid efficacy and has practical consequences for potato pest management. Although we have not observed complete field failure following the use of these products, multiple studies have demonstrated that the lethal concentration to kill 50% of the test organisms (LC50) in different field populations of L. decemlineata varies greatly which may suggest that resistance of L. decemlineata is heritable and involves genetic changes. An important challenge in understanding resistance is assessing the genetic mechanisms associated with resistance and classifying up-regulated genes that may be involved in combating an insecticide insult. In this study we uncovered trends in imidacloprid phenotypic response that have developed in the region by estimating the LC50 values among different field populations against a range of imidacloprid doses. The LC50 values collected in 2008–2011, and more recently in 2013 and 2014, show that some field locations remain susceptible to imidacloprid, while nearby fields (<100km) have developed high levels of resistance. We also sought to uncover potential mechanisms of resistance at each field location. We compiled a transcriptome for populations, characterized as phenotypically ‘susceptible’ and ‘resistant’, by isolating mRNA from adult beetles and analyzing gene expression level differences. Strong differences were observed in constituently up and down-regulated genes among different field populations. Most significantly, the up-regulation of 3 cytochrome p450s and a glutathione synthetase related protein in multiple resistant populations provide a mechanistic explanation of resistance evolution in L. decemlineata.
Highlights
The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is a key agricultural pest infesting commercial potato (Solanum tuberosum), as well as tomatoes (S. lypcopersicum), eggplants (S. melongena) and peppers (S. annuum) [1]
We identified significantly up- and down–regulated genes by adjusting levels according to a false discovery rate (FDR) of 0.059 between the populations
The analysis revealed 290 up-regulated and annotated transcripts that correlated to known genes that were classified into 349 Gene Ontology (GO) terms and grouped into 65 GO_slim classes in the systemic-1 population
Summary
The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), is a key agricultural pest infesting commercial potato (Solanum tuberosum), as well as tomatoes (S. lypcopersicum), eggplants (S. melongena) and peppers (S. annuum) [1]. L. decemlineata is thought to have expanded from the southwestern United States and Mexico to a global range encompassing the United States, Mexico, Europe, and Asia, inclusive of over 16 million km, and switching from its native host plant buffalobur (Solanum rostratum) to agricultural crops [1,2]. Potato fields throughout the United States are regularly treated with a combination of at-plant, systemic and foliar applications of neonicotinoid insecticides throughout the growing season to control L. decemlineata. Due to its unique structure, imidacloprid has high specificity to insect nAChR and low specificity to vertebrate nAChR, leading to low mammalian toxicity [3] and permitting a variety of uses on many crop and non-crop plant species
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