Assessment of semiochemical repellents for protecting walnut trees from walnut twig beetle (Coleoptera: Curculionidae) attack in a commercial orchard setting in California

Abstract

The walnut twig beetle, Pityophthorus juglandisBlackman, the vector of thousand cankers disease (TCD), poses a significant threat to North American walnut (Juglandaceae Juglans) trees. Despite discovery of TCD-related tree mortality over a decade ago, management options are lacking. This study represents the culmination of several years of investigating the chemical ecology of P. juglandisin hopes of developing a semiochemical repellent to disrupt the beetle’s host colonization and aggregation behaviors. Numbers of P. juglandislanding on semiochemical-treated Juglans regiaL. trees in a commercial walnut orchard were compared based on captures on sticky traps. Two repellent combinations were tested: R-(+)-limonene and trans-conophthorin (LimeCon), and R-(+)-limonene, trans-conophthorin, and R-(+)-verbenone (LCV). Both repellents reduced P. juglandisaggregation (captures) equally; thus, we proceeded with the LimeCon combination to reduce potential treatment cost. Subsequent trials included a 2× dose (Dual) of LimeCon. Both LimeCon and Dual significantly reduced the number of P. juglandiscaught compared with the baited control, however, only for the lower of two trap positions. Beetle landings were modeled by trap distance from repellent placement on each tree. Beetle responses to the pheromone lure were surprisingly localized and did not bring the whole tree under attack. LimeCon, LCV, and Dual treatments averaged fewer than a single beetle caught for all trap distances; however, performance of the repellents beyond 150 cm is not clear due to the localized landing response of P. juglandisto pheromone lures. Further testing is required to fully analyze the zone of inhibition of the LimeCon repellent.

Citation

Audley, Jackson P; Bostock, Richard M; Seybold, Steven J. 2021. Assessment of semiochemical repellents for protecting walnut trees from walnut twig beetle (Coleoptera: Curculionidae) attack in a commercial orchard setting in California. Journal of Economic Entomology. 114(3): 1180-1188. https://doi.org/10.1093/jee/toab054.