Nontarget impacts of herbicides on spiders in orchards

Authors: Schmidt, Rebecca; Moretti, Erica; BERGERON, PAUL - Washington State University; Zilnik, Gabriel

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2021
Publication Date: 11/26/2021
Citation: Schmidt-Jeffris, R.A., Moretti, E.A., Bergeron, P.E., Zilnik, G.L. 2021. Nontarget impacts of herbicides on spiders in orchards. Journal of Economic Entomology. 115(1):65-73. https://doi.org/10.1093/jee/toab228.
DOI: https://doi.org/10.1093/jee/toab228

Interpretive Summary: Spiders are important predators of many pests in agriculture worldwide. They are especially important in orchards, where they eat key pests such as codling moth, aphids, and pear psylla. Because spiders move between the orchard floor and the tree canopy, they are exposed to herbicides used to control weeds. Currently, there have been few studies on how herbicides impact predators and on how pesticides in general impact spiders. This makes it difficult for growers to choose herbicides that are the least harmful to their spider populations. Researchers at the USDA-ARS in Wapato, WA, with Washington State University, performed laboratory studies to determine which herbicides were the least harmful to three species of spiders found in Washington orchards. Effects of herbicides on spider death, number of prey eaten, and movement were tested. The spiders tested were less sensitive to herbicides than insects tested in previous studies. The running crab spider was less sensitive than the two jumping spiders. Only oxyfluorfen caused high levels of spider death, but other herbicides changed spider movement levels and, in some cases, reduced number of prey eaten. These effects may reduce their ability as predators in orchards. These results will be used by growers to inform their decision-making when choosing which herbicides to use in their orchards, which may result better pest control by spiders, reducing the need for insecticides

Technical Abstract: BACKROUND: Spiders are key predators in many agroecosystems, including orchards. Despite the importance of spiders in biological control, pesticide non-target effects on this group are poorly described. This is especially true for herbicides, which spiders frequently encounter as they move between the ground cover and tree canopy. We sought to determine the non-target effects of seven herbicides used in orchards on three species of spiders that are commonly found in Washington state (USA) orchards: Pelegrina aeneola (Curtis) (Araneae: Salticidae), Philodromus cespitum (Walckenaer) (Araneae: Philodromidae), and Phanias watonus (Chamberlin & Ivie) (Araneae: Salticidae). Immature spiders were collected from orchards and used in laboratory assays. Single spiders were placed in vials with dried herbicide residues and mortality was evaluated after 1, 2, and 5 d. We also evaluated herbicide impacts on prey consumption rates and on spider movement using motion-tracking software. RESULTS: Only oxyfluorfen caused significant spider mortality. Philodromus cespitum seemed to be less acutely sensitive to oxyfluorfen than the two salticid species. Several herbicide treatments significantly increased locomotion in P. cespitum, whereas rimsulfuron numerically decreased movement of P. aeneola. Sulfonylurea herbicides (rimsulfuron, halosulfuron) decreased prey consumption of P. aeneola. CONCLUSION: Our work indicates that although spiders may be less acutely sensitive to some pesticides than beneficial insects, they can be affected by sublethal effects of herbicides. Future work should determine if herbicide applications impact spider abundance in the field and reduce biological control services. In general, more work is needed on the impacts of herbicides on natural enemies