Walnut twig beetle landing rates differ between host and nonhost hardwood trees under the influence of aggregation pheromone in a northern California riparian forest

1. Host selection behaviour of the walnut twig beetle (WTB) among hardwood trees was investigated in a riparian forest in northern California by monitoring the landing rate of the beetle with sticky traps on branches baited with 3-methyl-2-buten-1-ol, the male-produced aggregation pheromone.
2. The assay was conducted over 7 days (22 May to 29 May 2017) and compared landing rates on branches of six nonhost species paired with northern California black walnut, Juglans hindsii (the host).
3. A total of 2242/1192 WTB were collected on branches of host/nonhost pairs, and more WTB landed on J. hindsii than on nonhosts in 42 of 58 instances. Female landing rate generally exceeded male landing rate, which underscores the influence of the male-produced synthetic pheromone in this system.
4. Landing rates of WTB males, females, and the combined sexes on boxelder, Acer negundo, and valley oak, Quercus lobata, did not differ significantly from the landing rates on J. hindsii, suggesting that these two nonhost riparian hardwoods do not repel WTB (in the context of the aggregation pheromone).
5. Significantly fewer WTB landed on Oregon ash, Fraxinus latifolia, river red gum, Eucalyptus camaldulensis, Fremont cottonwood, Populus fremontii, and red willow, Salix laevigata, than on J. hindsii, which suggests that these four nonhosts may repel one or both sexes of WTB in the context of the aggregation pheromone. Future analysis of the volatiles from these four hardwood species may lead to the discovery of semiochemical repellents for WTB.