An innovative strategy for control of the chestnut weevil Curculio elephas (Coleoptera: Curculionidae) using Metarhizium brunneum

The chestnut weevil (Curculio elephas) is a major pest of chestnut. Female weevils oviposit in the fruit and emergent larvae eat the kernel, making the nut unmarketable. In Turkey, the fruit is gathered into piles called gomu and allowed to ripen over a 1-2 month period. Late instars emerge from infested nuts to pupate in the soil underlying the gomu. Since the pest is concentrated, the opportunity exists for targeted control with biological control agents such as entomopathogenic fungi (EPF) and nematodes (EPN). The efficacy of the EPF Metarhizium brunneum strains ARSEF 4556 and V275 used alone and in combination with the EPN Heterorhabditis bacteriophora on the mortality of last instar chestnut weevil was assessed. Both M. brunneum ARSEF 4556 and V275 were highly virulent causing 86% and 94% mortality, respectively. Although H. bacteriophora killed the larvae (68% mortality) more quickly, the fungal strains were more efficacious 12 days post-inoculation. Application of ARSEF 4556 and V275 as a top dressing (spreading the fungus uniformly over the soil surface) in simulated gomu resulted in 78% and 80% larval mortality in laboratory pot tests and 83% and 70% mortality in outdoor trials, respectively. Premix applications (mixing the fungus in the top few centimeters of soil) of ARSEF 4556 and V275 resulted in 83% and 85% larval mortality indoors, respectively, yet outdoor trials caused >80% larval mortality. These EPF strains were also evaluated against larvae of the chestnut tortrix, Cydia splendana, which also feeds on nut kernels, and emerges at the same time as the chestnut weevil larvae to pupate in the soil. Both ARSEF and V275 caused 96.6% mortality inlast instar C. splendana. In contrast, the most virulent EPN (Steinernema feltiae) caused only 80% mortality. Co-application of this nematode with M. brunneum resulted in 100% mortality with the interactions being additive.