Mortality and reduced brood production in walnut twig beetles, Pityophthorus juglandis (Coleoptera: Curculionidae), following exposure to commercial strains of Beauveria bassiana and Metarhizium brunneum

Authors: CASTRILLO, LOUELA - Cornell University; MAYFIELD, ALBERT - Us Forest Service (FS); Griggs, Michael; CAMP, ROBERT - University Of Tennessee; MUDDER, BRYAN - Us Forest Service (FS); TAYLOR, ADAM - University Of Tennessee; VANDENBERG, JOHN - Former ARS Employee

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2017
Publication Date: 8/8/2017
Citation: Castrillo, L., Mayfield, A., Griggs, M., Camp, R., Mudder, B., Taylor, A., Vandenberg, J. 2017. Mortality and reduced brood production in walnut twig beetles, Pityophthorus juglandis (Coleoptera: Curculionidae), following exposure to commercial strains of Beauveria bassiana and Metarhizium brunneum. Biological Control. 114:79-86. https://doi.org/10.1016/j.biocontrol.2017.08.007.

Interpretive Summary: Walnut trees, especially eastern black walnuts are important sources of lumber and nut crops. These trees are now under threat due to an insect pest, the walnut twig borer, and the fungal pathogen it carries. Numerous beetle attacks and the associated fungal infection result in cankers, or localized areas of dead bark and underlying wood. The merging of numerous cankers girdle the affected stems or branches resulting in tree mortality within a few years of initial beetle attacks, hence the name thousand cankers disease. Controlling the beetle and the disease is difficult because pesticide applications are restricted on edible nut crops. An environmentally-friendly way to control the beetle is by the use of insect- killing fungi that are currently available to control other insect pests. Laboratory and field studies we conducted in 2014 and 2015 have shown that these fungi kill walnut twig beetle adults within a few days, which prevents females from laying eggs and results in smaller beetle populations. The beetles are also repelled by these fungi, and sprayed walnut logs have fewer beetle attacks. These results show that these insect-killing fungi can be used to control walnut twig beetles and help slow the development of the thousand canker disease in walnut trees.

Technical Abstract: Thousand cankers disease (TCD), caused by the walnut twig beetle (WTB), Pityophthorus juglandis, and its associated fungal symbiont, Geosmithia morbida, is a disease of economic and ecological concern on eastern black walnut, Juglans nigra. Numerous attacks and gallery formation by the WTB and subsequent development of cankers caused by the fungus result in progressive crown dieback. The disease can kill affected trees in a few to several years following initial infestation and very few management options are available for preventing or reducing impact of TCD on black walnut trees. Since advanced development of TCD requires multiple WTB generations and numerous beetle attacks, control strategies that reduce beetle attacks and brood production, without completely eliminating infestation, could still significantly benefit tree health and survival. We evaluated the use of entomopathogenic fungi Beauveria bassiana and Metarhizium brunneum against the WTB. Laboratory and field studies showed that WTB adults are susceptible to commercial strains B. bassiana GHA and M. brunneum F52. Exposure of beetles to sprayed walnut bolts resulted in reduced brood production, primarily due to the death of parental adults from fungal infection prior to egg laying. Spraying walnut bolts with B. bassiana GHA and M. brunneum F52 prior to field exposure to natural WTB populations reduced emergence of next generation adults by up to 98 percent and 96 percent, respectively, due to a combination of fewer beetle attacks and mortality among those that tunneled. These results demonstrate the potential use of entomopathogenic fungi in the integrated management of TCD in walnut trees.