Invasion of an Afrotemperate forest complex by the polyphagous shot hole borer beetle

Abstract

The polyphagous shot hole borer (PSHB), Euwallacea fornicatus Eichhoff (Coleoptera: Curculionidae, Scolytinae), is a significant tree-killing pest recently introduced into South Africa. Many native trees in urban settings are susceptible to infestation, but the presence of PSHB in natural ecosystems is unstudied. The presence and drivers of PSHB colonization in 1682 trees of 68 species were evaluated in 51 plots across a native Afrotemperate forest complex in South Africa. Breeding colonies of PSHB were found in six native species (breeding hosts). An additional 11 species did not contain PSHB colonies but hosted its mutualistic fungus Fusarium euwallaceae Freeman et al. (Hypocreales: Nectriaceae). Invasibility increased when plots were closer to the urban infestation border, further away from surface water, and when containing a larger number of breeding hosts. Invasibility decreased with an increase in tree species richness. Polyphagous shot hole borers were found in climax forest distant to urban areas at sites frequented by tourists. The severity of infestation of trees increased with an increase in host diameter, breeding host abundance, and infested tree abundance. Probability of infestation increased with an increase in the number of infested trees. Infested trees were not spatially clumped. Instead, PSHB preferentially selected eight of the 17 native host species. And the data suggest that larger trees of these species may be more susceptible to PSHB. Eight species were infested at random and two were infested seemingly accidentally. Infestations increased more rapidly on larger trees and on those surrounded by a high abundance of breeding hosts. This study confirms that Afrotemperate forests are highly susceptible to invasion by PSHB. Direct anthropogenic impact had no discernible effect on infestations, but humans aided spread of PSHB to distant sites. Halting movement of contaminated wood is important. Management of PSHB should focus on highly infested areas and trees as these increase the likelihood of further and more severe infestations.