Spinosyn resistance and cross-resistance - A 25 year review and analysis

The spinosyns were introduced more than 25 years ago (spinosad - Qalcova™ active, 1997) and later followed by spinetoram (Jemvelva™ active, 2007), for the control of a wide range of pest insects, especially lepidopterans and thrips. Although IRM guidelines were initiated at the time of introduction, there have been some cases of overuse, resulting in resistance development in several pest insect species. Additionally, laboratory selection for spinosyn resistance has been a tool to investigate the mode of action of the spinosyns. As part of an ongoing effort to understand the scope of resistance to the spinosyns, >400 studies covering >1100 cases were analyzed where resistance to the spinosyns was examined (field or laboratory studies) or spinosyns were examined for cross-resistance to other insecticides. To date there have been 400 cases where spinosyn-selected resistance was investigated, with half in field strains of which only a few were associated with sub-optimal control. Most cases of field selected spinosyn resistance involved thrips (46 %), lepidopterans (22 %) or dipterans (18 %). Importantly, only a small percentage of the field selected spinosyn resistance cases show high levels of resistance (>1000-fold), with many (∼30 %) exhibiting either no or limited levels of resistance. There was no correlation in spinosyn-selected insects for cross-resistance to other insecticide classes (e.g. neonicotinoids, avermectins diamides, pyrethroids). Analysis of 400+ studies indicates that cross-resistance to the spinosyns from other insecticides is also unlikely, potentially driven by the unique chemistry and novel mode of action of the spinosyns. Thus, the overall impact of spinosyn resistance is far less than might be implied from the large and expanding body of studies over the last 25 years.