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

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% and 96%, 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.