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.

Meta-Analysis of Herbicide Non-Target Effects on Pest Natural Enemies

“…Data from 78 papers representing 801 total observations were extracted from the published literature (1960-2020) and three (at the time of analysis) unpublished studies by Schmidt-Jeffris et al Two of the unpublished papers have since been published [36,37], and the unpublished data are noted as such in the database. No papers matching our search criteria were published prior to 1960.…”

“…However, research examining pesticide non-target effects on bees has found that differences between formulations with the same AI can alter conclusions of non-target effects testing [47]. Surfactants and other adjuvants, either included in the pesticide formulation or tank mixed to improve herbicide performance, can increase AI toxicity or even have direct, non-synergistic impacts on natural enemies [37,46,48,49]. In the case of glyphosate, much of its reported toxicity to bees is attributed to the inert, surfactant ingredient ethoxylated tallowamine [47].…”

“…Paraquat is the main burn-down alternative to glyphosate in field crops [20], although it is less effective [19] and is also one of the most toxic herbicides to humans and other vertebrates [52]. In tree fruit, paraquat and glufosinate are the most likely glyphosate replacements for managing intra-and inter-row weeds post-emergence [37]. In the European Union (EU), only quizalofop-ethyl is available as a post-emergent herbicide alternative to glyphosate in tree fruit, and cycloxydim, 2,4-D, and dicamba are the most suitable replacements in herbaceous crops [27].…”