Assessing Eucryptorrhynchus brandti as a Potential Carrier for Verticillium albo-atrum from Infected Ailanthus altissima

Significant mortality of the invasive tree of heaven (TOH), Ailanthus altissima (Mill.) Swingle, was first observed in Pennsylvania in 2002 to be caused by an apparently host-specific strain of Verticillium albo-atrum Reinke & Berthold, a soil-borne, vascular wilt fungus. A limited survey conducted in Virginia revealed two sites where TOH stands were infected with V. albo-atrum. A virulence test confirmed that fungal isolates from both states were found to be highly pathogenic on TOH, killing all inoculated seedlings in 9 weeks. After overwintering, 11% (n = 37) of TOH root sections tested positive for V. albo-atrum, although the origin of the colonies was not identified. The pathogenicity of this pathogen suggests that it could be used together with host-specific insects for the biological control of TOH. A host-specific herbivorous weevil from China, Eucryptorrhynchus brandti Harold (Coleoptera: Curculionidae) that has been extensively studied as another potential biological control agent for TOH is currently pending quarantine release. Quarantine experiments were conducted to test different forms of transmission with E. brandti and V. albo-atrum simultaneously. In one experiment, 75% (n = 32) of adult E. brandti transmitted V. albo-atrum to TOH seedlings after walking on an actively growing culture and feeding on infected plant material. In another study after feeding on infected TOH stems for 24, 48 and 72 h, respectively, 16.7% (n = 120), 15.0% and 12.5% of adult E. brandti ingested and passed viable V. albo-atrum propagules into feces. Surviving weevils (83%, n = 20) overwintering in infested potting mix carried viable V. albo-atrum propagules externally. In addition, all weevil progeny that emerged from infected TOH billets appeared to be as healthy as weevils reared from non-infected billets and wild parents from China. Results from these laboratory studies indicate E. brandti has the ability to spread V. albo-atrum from tree to tree in a laboratory setting.