Nontarget Impacts of Herbicides on Spiders in Orchards

Spiders are key predators in many agroecosystems, including orchards. Despite the importance of spiders in biological control, pesticide nontarget 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 nontarget 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. Only oxyfluorfen caused significant spider mortality. P. 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. 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.