Efficacy of pesticide mixtures on the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae)

Western flower thrips (WFT), Frankliniella occidentalis is a commonly encountered and economically important insect pest of greenhouses. Greenhouse producers typically apply pesticides as mixtures to mitigate WFT populations; however, there is limited information available on the compatibility and efficacy of commonly used pesticide mixtures. This study assessed nine two-way pesticide mixtures commonly used in greenhouses with pesticides containing abamectin, acephate, azadirachtin (Azatin and Ornazin), bifenazate, bifenthrin, fenpropathrin, novaluron, pymetrozine, and spinosad. Pesticide mixture compatibility was determined using jar tests. Also, mixtures were applied to nine horticultural plant species to determine phytotoxicity; with effects assessed 7 days after treatment.

Bean-dip bioassays were performed in a laboratory using green bean, Phaseolus vulgar to determine LC₅₀ values for each individual pesticide and the mixtures to establish whether the mixtures were synergistic, antagonistic, or there was no effect. Western flower thrips mortality was assessed after 24 hours, and LC₅₀ values were calculated. Furthermore, greenhouse experiments were performed to evaluate the efficacy (based on percent mortality) of the pesticide mixtures against WFT. Results from the jar tests indicated that all the mixtures were visibly compatible, and none of the mixtures were phytotoxic to any of the plant species evaluated. Laboratory results indicated spinosad + bifenazate was strongly antagonistic against WFT. The greenhouse experiments demonstrated significantly reduced efficacy associated with both mixtures of abamectin + azadirachtin; however, each two-way mixture provided ≥75% WFT mortality in the greenhouse experiments.