Walnut Twig Beetle Landing Rates Differ Between Host and Nonhost Hardwood Trees Under the Inﬂuence of Aggregation Pheromone in a Northern California Riparian Forest

Agricultural and Forest Entomology (2021), 23, 111–120 DOI: 10.1111/afe.12410 Walnut twig beetle landing rates differ between host and nonhost hardwood trees under the inﬂuence of aggregation pheromone in a northern California riparian forest ∗ † ‡ ∗ Crystal S. Homicz , Jackson P. Audley , Yigen Chen , Richard M. Bostock§, Catherine A. Tauber ¶ and ∗∗∗ Steven J. Seybold ∗ Department of Entomology and Nematology, University of California, Davis, CA, 95616, U.S.A., †Oak Ridge Institute for Science and Education, USDA Forest Service, Pacifc Southwest Research Station, Davis, CA, 95618, U.S.A., ‡Foundation Research, Analytics and Business Applications, E. & J. Gallo Winery, Modesto, CA, 95354, U.S.A., §Department of Plant Pathology, University of California, Davis, CA, 95616, U.S.A., ¶Department of Entomology, Cornell University, Ithaca, NY, 14853, U.S.A. and ∗∗ USDA Forest Service, Pacifc Southwest Research Station, Davis, CA, 95618, U.S.A. Abstract 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 infuence 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 signifcantly 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 Signifcantly 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. Keywords Chrysoperla, Geosmithia morbida, host selection behaviour, Juglans hindsii, northern California black walnut, Pityophthorus juglandis, Scolytidae, thousand cankers disease, walnut pest, walnut twig beetle. Introduction phloem occurs with each beetle attack causing thousand cankers disease (TCD), which results in the decline and mortality of wal- The walnut twig beetle (WTB), Pityophthorus juglandis Black- nut trees (Seybold et al., 2013b, 2019). man (Coleoptera: Scolytidae), vectors a phytopathogenic fun- Walnut twig beetle and G. morbida were frst described in gus, Geosmithia morbida Kolarík,̌ Freeland, Utley, & Tisserat the western U.S.A., and are thought to be native on Arizona (Ascomycota: Hypocreales: Bionectriaceae) to the phloem of walnut, Juglans major, in the southwestern U.S.A. and northern walnut and wingnut trees. A canker (e.g. tissue necrosis) in the Mexico (Bright, 1981; Kolaríǩ et al., 2011; Zerillo et al., 2014; Rugman-Jones et al., 2015; Seybold et al., 2016). Walnut twig Correspondence: Crystal S. Homicz. Tel.: (530) 524-2885; fax: (530) beetle was frst collected in New Mexico in 1896, followed by 752-1537; e-mail: [email protected] records in Arizona, California, and Chihuahua, Mexico (1907, © 2020 The Royal Entomological Society 112 C. S. Homicz et al. 1959, and 1960, respectively) (Bright, 1981; Rugman-Jones have been studied widely, however little is known of WTB’s et al., 2015; Seybold et al., 2016, 2019). The frst published response to nonhost trees. The host colonization behaviour of incidence of TCD occurred in New Mexico in 2001; however, the WTB may be explained by one of two prevailing hypotheses disease complex was likely active in the early 1990s in Oregon that explain bark beetle host colonization: (i) Walnut twig beetle and Utah based on reports of walnut tree mortality in those U.S. may actively respond to long-range volatile cues and signals states (reviewed in Tisserat et al., 2011). Following these events, while in fight to locate an appropriate host tree; (ii) WTB mortality due to TCD was noted in 2003 in Colorado (Tisserat may be infuenced by gustatory cues and shorter-range olfactory et al., 2011), whereas TCD was frst detected in California signals/cues once it has landed on a tree to select host which in 2008 in Yolo Co. and Solano Co. (Seybold et al., 2019). indicates use of random landing (Raffa et al., 2016). It has been However, the disease had likely been present much longer in shown that WTB likely uses long-range signals and/or cues while California (Flint et al., 2010). Following these descriptions, in fight to select susceptible hosts (Hishinuma, 2017; Audley TCD was detected in the eastern U.S.A., with reports from et al., 2020; Lona et al., 2020). While feeding, the male beetle Tennessee, Virginia, Pennsylvania, Ohio, Maryland, Indiana, and releases an aggregation pheromone (3-methyl-2-buten-1-ol) to North Carolina (Tisserat et al., 2011; Seybold et al., 2013b). attract female and additional male beetles to the tree (Seybold Most recently (2013) TCD was discovered in fve regions in Italy et al., 2015). Male WTB then mate with several females and the (Faccoli et al., 2016; Moricca et al., 2018). cohort completes its life cycle in the phloem of the selected host Thousand cankers disease threatens walnut trees grown for tree. urban shade cover, agriculture, and wood production, as well In this study, we examined how WTB responded in fight to as trees in native North American riparian stands that provide tree volatile cues, in the presence of an aggregation pheromone benefts to wildlife (Hefty et al., 2018; Seybold et al., 2019). lure, to distinguish between J. hindsii (host trees) and nonhost In California, two native tree species, southern California black hardwood trees. Additionally, we were interested in testing the walnut, Juglans californica, and northern California black infuence of sex and branch aspect on behavioural responses walnut, Juglans hindsii, have particularly limited distributions as well. The sex ratio can be revealing of the infuence of (Jepson, 1917; Griffn & Critchfeld, 1972). These native wal- semiochemical constituents. For example, we typically observe nut tree species are found in riparian ecosystems and food approximately twice as many female beetles responding to the plains throughout California’s foothills and valleys. J. hindsii aggregation pheromone lure in trapping assays (Chen & Sey- is listed as critically imperilled by the California Native Plant bold, 2014; Seybold et al., 2015). Deviations from a 2:1 female Society (CNPS, 2018), therefore, conservation of the species to male ratio were interpreted as potentially different sensitiv- is a high priority. Even though their range has been signif- ities to volatile profles. We also assessed WTB preference for cantly reduced, they still occur throughout the state with similar branch aspect (top, bottom, left, right) because past studies have canopy and undergrowth structure as their historical counterparts found higher landing rates on the bottom side of the branch (Katibah, 1984). (Hishinuma, 2017). Bark beetles have been shown to use their antennae to Understanding host selection behaviour of WTB will likely sense semiochemicals (volatile chemical compounds that inform potential management strategies for this invasive bark elicit behavioural responses in receiving organisms) such as beetle and disease complex. This study also reports on the pheromones and kairomones to locate trees while in fight possible attraction of potential WTB predators and natural for colonization, feeding, and reproduction (Silverstein, 1981; enemies of foliar walnut pests to the pheromone lure. Studying Wood, 1982; Raffa et al., 1993; Borden, 1997; Raffa, 2001; these behavioural phenomena in a native riparian area provides Raffa et al., 2016; Seybold et al., 2018). Bark beetles follow a an opportunity to discover ecologically sourced volatiles that four-step process for host colonization: (i) the dispersal phase, repel the WTB. Knowledge of the action of these volatiles where (ii) selection phase, (iii) concentration phase, and (iv) establish- the WTB’s hosts and natural enemies are present, may form the ment phase (Wood, 1982). The beetles begin the dispersal phase basis for the development of a semiochemical repellent that can by responding to cues from the host trees and/or pheromones be used within an integrated pest management setting to protect after emerging from the brood tree. They then select a potential walnut trees in urban landscapes, native habitat, and commercial host tree by responding to stimuli from the host while in fight nut orchards. and/or after landing. Next, they initiate colonization by boring through the bark and into the phloem. During the concentration phase, they release aggregation pheromone to attract other Materials and methods conspecifcs. The establishment phase occurs when a tree has Site description been suffciently colonized by bark beetles and fungal inoculum. Past studies have shown that host colonization can

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