Toxic effects of spinosad on predatory insects

Abstract

Spinosad (Dow AgroSciences) is a mixture of tetracyclic-macrolide compounds produced by a soil actinomycete and has been classified as a bioinsecticide. Spinosad is highly active against Lepidoptera but is reported to be practically nontoxic to insect natural enemies. We assessed the impact of Spinosad in a granular maize-flour formulation on a selection of insect predators over periods of 2–14 days. In all cases, the quantities of Spinosad used were less than the maximum recommended rates given on the product label. Adults of Aleochara bilineata Gyllenhal (Coleoptera:Staphylinidae) suffered a high prevalence of mortality following consumption of 1000 or 2000 ppm Spinosad active ingredient (a.i.), but little mortality at 200 ppm. Larvae of Chrysoperla carnea (Stephens) (Neuroptera:Chrysopidae) did not consume the granular formulation and suffered little overall mortality. After 14 days of exposure, the earwig, Doru taeniatum (Dohrn) (Dermaptera:Forficulidae), suffered 48% mortality in the 1.2 ppm Spinosad treatment increasing to 98% in the 1200 ppm Spinosad treatment compared to 20% in controls. Earwigs suffered 86% mortality/intoxication 72 h after feeding on Spinosad-contaminated Spodoptera frugiperda J. E. Smith (Lepidoptera:Noctuidae) larvae. A field trial was performed to compare applications of commercial granular chlorpyrifos and Spinosad in maize-flour granules (200 and 2000 ppm a.i.; 4.8–48 g a.i./ha, respectively) or as an aqueous spray (160 ppm a.i.; 48 g a.i./ha) on earwigs held inside gauze bags. Mortality of earwigs on control plants was less than 15% at 2 days postapplication compared to 33% on plants treated with granular chlorpyrifos, 83% on plants sprayed with 160 ppm Spinosad, and 91–95% on plants treated with 200–2000 ppm Spinosad granules, respectively. Further mortality in the 24-h period postsampling ranged from less than 5% in control treatments, to 9% in the chlorpyrifos treatment, and to 55–65% in the Spinosad spray and granule treatments. We conclude that Spinosad cannot be considered to have an environmental safety profile similar to most established biological insecticides.