Simple SummaryConserving predators of orchard pests is a critical part of pest management. To achieve this, growers can choose pesticides that are minimally harmful to these predators. While this information is commonly available for insecticides, there is little information about how herbicides affect pest predators. Knowing which herbicides are harmful to predators is particularly important as growers move away from using glyphosate due to consumer safety concerns. Adjuvants are chemicals that are sometimes added to pesticides to increase their efficacy, and their effects on predators are also poorly described. In apple orchards in Washington State, U.S.A., two species of predatory mites are known to be critical for maintaining pest mite populations below damaging levels. We tested seven pesticides and five adjuvants for effects on these important mite predators in laboratory assays. We found that three herbicides (glufosinate, paraquat, and oxyfluorfen) either killed the adult predators or reduced their reproduction. The adjuvants were minimally harmful to the predator mites. Because glufosinate and paraquat are likely replacements for glyphosate, pest mite outbreaks in orchards could result from their increased use.AbstractThe phytoseiid mitesGalendromus occidentalisandAmblydromella caudiglansare 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 onA. caudiglansandG. 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 inA. caudiglansand 56% mortality inG. occidentalis. Sublethal effects were significant for both species when exposed to oxyfluorfen. Adjuvants did not cause non-target effects inA. caudiglans. The non-ionic surfactant and methylated seed oil increased mortality and decreased reproduction inG. 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.