Factors that influence the flight activity, abundance and infestation severity of the polyphagous shot hole borer beetle (PSHB, Euwallacea fornicatus) in an urban-agricultural fringe setting

Abstract

The polyphagous shot hole borer (PSHB, Euwallacea fornicatus) has invaded multiple countries, including South Africa. Along with its fungal mutualist, PSHB has caused the death of a wide range of tree species. Unmitigated costs of invasion in South Africa are estimated to be high. This study consisted of a 26-month PSHB monitoring programme using baited traps in an urban-agricultural fringe setting in the Western Cape province, focusing on revealing the factors most important to flight activity, beetle abundance, and infestation severity. It also tested the validity of felling infested trees as a management option to reduce beetle numbers (propagule pressure). More than half of the 94 study plots evaluated had PSHB-infested trees. Beetle flight activity was highest in summer and negligible in winter, with flight activity being positively correlated with temperature, beetle developmental degree days and beetle flight hours. The surrounding abundance of infested hosts was positively correlated with beetle abundance and infestation levels, though “saturation” in the infestation of a focal host tree may be a deterrent to dispersing beetles. Trees in plots that experience water stress (seasonal flooding) had higher dispersing beetle abundance and infestation levels, suggesting that tree stress might play a role in attracting beetles or making trees more vulnerable to infestation. Beetle activity during tree felling echoed the overall seasonal trend, and higher numbers of dispersing beetles on the day of tree felling in summer suggest that felling activities should best be conducted in winter. For three host tree species, infestation severity increased with decreasing tree size, which may be due to larger trees having thicker bark or other better defence systems. This study provides insight into the ecology of PSHB in this region of South Africa, which is important for developing monitoring and management strategies.