Silvicultural Disturbances Affect on Root-feeding Bark Beetle Populations and the Incidence of Ophiostomatoid Fungal Species Contributing to Southern Pine Decline

Abstract

Root-feeding beetles and weevils are known to be vectors of ophiostomatoid fungi which contribute to Southern Pine Decline (SPD) in the southeastern United States. This study examined population changes of Hylastes spp. in response to either mechanical thinning or harvesting in Pinus taeda L. stands and the factors associated with the incidence of ophiostomatoid fungi. In addition, the study also quantified ophiostomatiod fungal response to mechanical thinning in central Alabama and Georgia. Three different insect traps were used during the two-and-half-year study. Pinus taeda roots were excavated and assayed for ophiostomatoid fungal infections from both pre- and post-treatments in thinned and control plots. Of the 46,865 total insects captured, 22,495 were Hylastes spp. Populations of the Hylastes spp. significantly increased after thinning treatments at study sites. Although Hylastes spp. decreased in response to harvesting in some plots, their populations recovered to pre-treatment levels and were stable over the study duration. The dominant fungus recovered was Leptographium procerum (Kendr.) Wingf. followed by other species including L. terebrantis Barras & Perry, Grosmannia alacris T.A. Doung, Z.W. de Beer & M.J. Wingf. sp. nov., G. huntii (Rob.-Jeffr.) Zipfel, Z.W. de Beer & M.J.Wingf., and Ophiostoma ips (Rumbold) Nannf. Grosmannia alacris and O. ips were recovered from tree roots in plots with severe decline symptoms. Sites with mechanical thinning had increased incidence of ophiostomatoid fungal species that may serve as a source to infest the remaining trees in the stand leading to SPD.

In general, thinning and harvesting are recommended as bark beetle management strategies. However, in the current study, recent mechanical thinning significantly increased pathogen-vectoring Hylastes spp. and ophiostomatoid fungi which contribute to SPD. Thus, future research should consider either how to thin or how to control the insect vectors to reduce possibility of SPD infestation in P. taeda stands.