Herbicides Harm Key Orchard Predatory Mites

Abstract

The phytoseiid mites Galendromus occidentalis and Amblydromella caudiglans are critical for conservation biological control of pest mites in Washington State, U.S.A. apples. While the non-target effects of insecticides on phytoseiids are well described, research on herbicide effects is limited. Using laboratory bioassays, we examined lethal (female mortality) and sublethal (fecundity, egg hatch, larval survival) effects of seven herbicides and five adjuvants on A. caudiglans and G. occidentalis. The effects of mixing herbicides with recommended adjuvants were also tested to determine if the addition of an adjuvant increased herbicide toxicity. Glufosinate was the least selective herbicide tested, causing 100% mortality in both species. Paraquat caused 100% mortality in A. caudiglans and 56% mortality in G. occidentalis. Sublethal effects were significant for both species when exposed to oxyfluorfen. Adjuvants did not cause non-target effects in A. caudiglans. The non-ionic surfactant and methylated seed oil increased mortality and decreased reproduction in G. occidentalis. The high toxicity of glufosinate and paraquat for both predators is concerning; these are the primary "burn down" herbicide alternatives to glyphosate, which is decreasing in use due to consumer toxicity concerns. Field studies are needed to determine the extent to which herbicides disrupt orchard biological control, focusing on glufosinate, paraquat, and oxyfluorfen. Consumer preferences will need to be balanced with natural enemy conservation.

Keywords: Amblydromella caudiglans; Galendromus occidentalis; adjuvant; biological control; herbicides; integrated pest management; pesticide selectivity; phytoseiid.

Figure 1

Summary of lethal and sublethal effects based on corrected values, showing percent increase (mortality) or percent decrease (all other) in comparison to the control. Values are categorized as low (x < 25%, white), moderate (25% ≥ x < 75%, light gray), and high (x ≥ 75%, black) [37]. When a treatment resulted in less mortality or greater fecundity, egg hatch, or live larvae than the control, the corrected value was set to “0”.