Insecticide Dip Treatments to Prevent Walnut Twig Beetle Colonization of Black Walnut Logs

The health, sustainability, and commercial viability of eastern black walnut (Juglans nigra) are currently under threat from thousand cankers disease. The disease is caused by an invasive bark beetle species, the walnut twig beetle (Pityophthorus juglandis), and its associated fungal pathogen (Geosmithia morbida). Range expansion of the beetle and pathogen has likely been facilitated by transport of infested walnut forest products. Preventing colonization of these products is crucial to limiting further spread of thousand cankers disease. This study evaluated three insecticides for their ability to induce walnut twig beetle mortality and prevent colonization of black walnut bolts, 3 to 5 cm in diameter, after dip treatment applications. Treatments included 0.003 percent azadirachtin, 15 percent disodium octaborate tetrahydrate (DOT), 0.5 percent permethrin, and water in Trial 1, and 0.013 percent azadirachtin, 30 percent DOT, 0.5 percent permethrin, and water in Trial 2. A total of 40 beetles, 4 beetles per sample, were exposed to treated samples and observed for 120 hours in each trial. Permethrin was the only treatment to achieve 100 percent mortality and prevent all colonization activity. The 30 percent DOT treatment increased mortality compared with the control; however, it did not reduce the mean number of attacks or mean gallery length. Azadirachtin was not effective at either concentration. Results suggest that insecticide dip treatments can prevent walnut twig beetles from colonizing cut black walnut logs. Treatments could be used in conjunction with phytosanitation to help prevent further spread of thousand cankers disease while allowing for the continued transport of bark-on walnut forest products. Eastern black walnut (Juglans nigra) and several other Juglans species in North America are currently under threat from an emergent, often fatal disease known as thousand cankers disease (Tisserat et al. 2009, Seybold et al. 2011). The disease is caused by the phloem-feeding walnut twig beetle (Pityophthorus juglandis) and an associated fungal pathogen (Geosmithia morbida; Tisserat et al. 2009, Kolařı́k et al. 2011). Adult walnut twig beetles introduce the pathogen upon entering the phloem of a host tree. G. morbida is not a systemic pathogen and numerous beetle attacks are required to induce mortality (Tisserat et al. 2009). Mortality generally occurs in the branches of the canopy first and progresses down the main stem until the host tree dies. Dieback is the result of cankers and beetle gallery formation coalescing in the phloem, girdling branches, and ultimately the main stem (Tisserat et al. 2009). Neither the beetle nor the pathogen is native to the eastern United States. Rather, the walnut twig beetle is historically known from the American Southwest, originally associated with Arizona walnut (Juglans major; Cranshaw 2011, Seybold et al. 2012). Arizona walnut is hypothesized to be the primary host of the beetle and pathogen in its native range (Moltzan 2011, Utley et al. 2013), and tree mortality due to thousand cankers disease on Arizona walnut has not been reported. Over the past 2 decades, the range of the walnut twig beetle has greatly expanded. The beetle has been collected primarily from declining black walnut across the western states where it is not native, but has been extensively outplanted (Cranshaw 2011, Tisserat et al. 2011, Seybold et al. 2012). In 2010, walnut twig beetles were first discovered within the native range of black walnut on infested trees in Knoxville, Tennessee (Grant et al. 2011). Since that time, the beetle and pathogen have been recovered in Pennsylvania, Virginia (Seybold et al. 2012), The authors are, respectively, Graduate Student and Assistant Professor, Forestry, Wildlife, and Fisheries Dept., Univ. of Tennessee, Knoxville ([email protected] [corresponding author], [email protected]); Professor, Dept. of Plant Sci., Univ. of Tennessee, Knoxville ([email protected]); Research Entomologist Project Leader, USDA Forest Serv., Southern Research Sta., Asheville, North Carolina ([email protected]); and Entomologist, USDA Animal and Plant Health Inspection Serv., Plant Protection and Quarantine, Buzzards Bay, Massachusetts (scott.w. [email protected]). This paper was received for publication in July 2015. Article no. 15-00038. Forest Products Society 2016. Forest Prod. J. 66(3/4):233–240. doi:10.13073/FPJ-D-15-000038 FOREST PRODUCTS JOURNAL Vol. 66, No. 3/4 233 North Carolina (Hadziabdic et al. 2013, Wiggins et al. 2014), Ohio (Fisher et al. 2013), Maryland (Maryland Department of Agriculture 2013), and Indiana (Marshall 2015). Both the beetle and pathogen were also recovered from a walnut grove in Italy (Montecchio et al. 2014), constituting the first international occurrence of thousand cankers disease. Given the tremendous distance and disjoint nature of these introductions, anthropogenic influence is likely involved in the transport of the beetle. Introductions may be the result of the commercial transport of infested walnut material in which the bark was left intact (e.g., logs, unedged lumber, or firewood; Newton and Fowler 2009, Turcotte et al. 2013). Continued spread of thousand cankers disease threatens one of North America’s most economically valuable hardwood species in eastern black walnut (Shifley 2004, Newton and Fowler 2009, Moltzan 2011). Total value of standing black walnut timber for lumber and veneer production is estimated at more than $500 billion (Newton and Fowler 2009). Black walnut wood is commonly used for gun stocks, cabinetry, and several other finished wood products and is prized for its strong and durable wood, especially the distinctive, dark-brown heartwood (Williams 1990, Kirkman et al. 2007, Moltzan 2011). The highestquality logs are sold as veneer logs and veneer-quality walnut is exceptionally valuable (Moltzan 2011). In response to the spread of thousand cankers disease, several states have established quarantines restricting the movement of walnut logs and wood products (Newton and Fowler 2009, Moltzan 2011). In Tennessee, regulated articles include: ‘‘the walnut twig beetle; Geosmithia morbida; all plants and parts of plants in the genus Juglans including nursery stock, budwood, scionwood, green lumber, and other material living, dead, cut, or fallen, including logs, stumps, roots, branches, mulch and composted and un-composted chips; and any hardwood firewood’’ (Tennessee Department of Agriculture [TDA] 2014). Other states that have enacted quarantines have similar lists of restricted materials. To remove any of these items from counties in which thousand cankers disease has been confirmed, a phytosanitation certification or a compliance agreement from the transporter must first be obtained (Newton and Fowler 2009, TDA 2014). Although effective phytosanitation methods, such as steam heat (Mayfield et al. 2014), have been reported for walnut twig beetle and G. morbida, recent evidence suggests that the beetle can colonize the bark of treated logs (Audley 2015). In experiments assessing the efficacy of several cultural and chemical treatments for eradication of the walnut twig beetle from felled black walnut logs, Peachey et al. (2011) found beetle emergence up to 21 months posttreatment from both treated and untreated logs. Beetle emergence was also reported from logs from which no pretreatment emergence was noted. This represents further evidence to support the notion that walnut twig beetles can colonize the bark of treated walnut logs. Haack and Petrice (2009) also reported colonization of heat-treated logs and boards of lumber with bark left intact by species of bark and ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) and species of long horned beetles (Coleoptera: Cerambycidae). Their findings provided significant evidence to support a revision in the International Standards for PhytosanitaryMeasures (ISPM) 15 guidelines requiring complete debarking before phytosanitation for wood packing materials (Haack and Brockerhoff 2011). The requirement for debarking logs or lumber can be burdensome to the walnut wood industry. Producing lumber that is completely bark free reduces the volume of top-quality boards as a result of overedging (National Hardwood Lumber Association 2003), and veneer log buyers require the bark to remain intact on the log to protect the wood from drying and fungal discoloration before processing at the mill. The potential for walnut twig beetle colonization of phytosanitized wood coupled with industry’s needs to transport intact logs creates an environment in which further precautions may be required to ensure that no walnut twig beetles are transported on treated wood while not overly restricting industry. One approach could be the use of insecticides applied to the surface of logs. Insecticides are a commonly used management technique for protecting individual trees from bark beetle attack (Fettig et al. 2006) and have been shown to be effective against attack on cut logs and lumber (Strom and Roton 2009, Fettig et al. 2011). Several active ingredients are commercially available in insecticide products labeled for treatment of logs and wood products that may reduce or prevent colonization of phytosanitized walnut logs by adult walnut twig beetles. Azadirachtin is a terpenoid extract from the neem tree (Azadirachta indica) that provides broad-spectrum insecticidal activity and low mammalian toxicity (Schmutterer 1990, Naumann et al. 1994, Newberry et al. 2013). The chemical structure of azadirachtin is similar to that of ecdysone, an insect hormone that regulates metamorphosis. Azadirachtin acts as an ecdysone blocker, preventing successful development of insect larval stages; thus its primary mode of action is as a growth regulator (Schmutterer 1990, Newberry et al. 2013). Studies with azadirachtin have also demonstrated population regulation through feeding deterrence, mating disruption, adult sterilization, or repellency of all life stages of several insect pests (Schmutterer 1990, 1995; Naumann et al. 1994; Xie et al. 1995; Newberry et al. 2013). Efficacy trials using azadirachtin to manage barkand wood-boring beetles have yielded mixed results. In a study testing systemic injections of azadirachtin on emerald ash borer (Agrilus planipennis) in ash (Fraxinus spp.) trees, 100 percent of larvae failed to complete development at doses 13.6 mg/cm diameter breast height; however, all doses tested were not effective in controlling adults (McKenzie et al. 2010). Asian longhorned beetle (Anoplophora glabripennis) larval mortality reached 60 percent when fed a diet with an azadirachtin concentration of 50 parts per million (Poland et al. 2006). Systemic treatments were also effective against larvae of mountain pine beetle (Dendroctonus ponderosae) in treated lodgepole pine (Pinus contorta var. latifolia; Naumann et al. 1994, Naumann and Rankin 1999). However, no reduction in the attack rate of adult mountain pine beetles on lodgepole pine treated with a topical azadirachtin emulsion was found (Duthie-Holt and Borden 1999). Borates are commonly used wood preservatives (Taylor and Lloyd 2009) and demonstrate broad-spectrum insecticidal and fungicidal efficacy, with low mammalian toxicity, with labeled uses for agricultural and domestic applications (Lloyd 1998). Slahor et al. (2005) demonstrated borate to be a cost-effective preservative for treating pallet lumber, especially when combined with a heat treatment, thus lending support to the idea of practical application in an