Potential Impacts of Orchard Pesticides on Tetranychus Urticae: A Predator-Prey Perspective

This laboratory study investigated the lethal and sublethal effects of topical exposure to 13 common orchard insecticides and 3 fungicides against spider mite Tetranychus urticae (Koch) and compared the results to previously published bioassays conducted on its most important predator in Washington orchards, the phytoseiid mite Galendromus occidentalis (Nesbitt). 

In many cropping systems, like tree fruits, spider mites are secondary pests, only a management issue when pesticides are applied to control other pests. This phenomenon is typically attributed to the non-target effects of many pesticides on spider mite natural enemies.  In addition, some studies have identified a phenomenon known as hormoligosis, where a pesticide reduces development time and increases fecundity of a pest, rather than suppressing it. Spider mites are also notorious for developing resistance to pesticides. Hence, understanding non-target effects of pesticides on both spider mites and their key natural enemies may allow producers to choose pesticides that do not selectively favor a pest over its natural enemies.

The researchers examined female mortality and several reproductive parameters, including fecundity (number eggs produced/female), egg hatch (number eggs hatched/female), and live larvae (number live larvae produced/female). The tested concentrations were 2X, 1X, and 0.1X the maximum labelled field rate applied at 935 L/ha. 

In all cases, materials were either less harmful to T. urticae or were equally harmful to both species. Pesticides that were minimally harmful to T. urticae, but highly harmful to G. occidentalis included neonicotinoids (acetamiprid, thiacloprid, imidacloprid) and novaluron. Diamides (chlorantraniliprole, cyantraniliprole, and flubendiamide) had minimal effect on both species. Some pesticides (lambda-cyhalothrin, spinetoram, and spirotetramat) were highly toxic to both predator and prey.

The accumulated lethal and sublethal effects of each pesticide were best described by the percentage reduction in live larvae. Using this parameter, the authors calculated a life table selectivity index (LTSI) which allows, by pesticide, a relative comparison of a given pesticide’s toxicity to spider mite vs. its natural enemy. All of the pesticides tested in this study had a negative LTSI, indicating that all were more harmful to the predator than the prey, but the degree of contrast varied by chemical. Materials with strong negative LTSI scores have higher potential to disrupt biological control by G. occidentalis. The seven chemicals with the most negative LTSI scores, in order, were: acetamiprid, novaluron, imidacloprid, carbaryl, sulfur, thiacloprid, and mancozeb-copper. However, the authors caution that even materials with slightly negative LTSI scores (such as lambda-cyhalothrin, spirotetramat, and spinetoram) should be used with caution, given the ability of spider mites to develop resistance more quickly than their natural enemies.

Schmidt-Jeffris, Rebecca A. and Elizabeth H. Beers
Crop Protection
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