The impact of the polyphagous shothole borer on pecan trees in South Africa

Abstract

The weevil family Curculionidae in the Coleoptera is one of the largest families within the animal kingdom, including more than 80000 species. Among these are approximately 3400 species of ambrosia beetles classified in 11 tribes, with the majority belonging to the Scolytinae. One of these ambrosia beetles is the Polyphagous Shot Hole Borer (PSHB), Euwallacea fornicatus, that forms part of the Euwallacea fornicatus species complex. The importance of the PSHB is that it carries three fungal symbionts of which one, Fusarium euwallaceae, is a serious pathogen causing Fusarium dieback of a variety of tree species. The PSHB originates from Southeast Asia from where it has been introduced to Israel, the USA, and more recently, South Africa, where it has become invasive. The establishment of invasive populations poses a severe threat to trees in urban and natural forests as was seen in California, and now also South Africa. In terms of agricultural crop trees, it was especially the avocado industries that were impacted in Israel and the USA. In South Africa, the first agricultural crop on which an outbreak of PSHB was observed, was pecan (Carya illinoinensis). The beetle was discovered infesting hundreds of pecan trees in the Jan Kempdorp area in the Northern Cape in February 2017 and was confirmed to be PSHB in early 2018. Since the early 2000’s, several studies regarding the biology of PSHB have been conducted in the USA and Israel, but very little is known about the life cycle and behaviour of the beetle in South Africa, especially not in a dry, extreme climate like the Northern Cape. In addition, not much is known about the pathogenicity of the fungal symbionts of the beetle on pecan trees, as the only other known record of PSHB attacking pecan is from a few trees in a botanical garden in the USA. The outbreak of PSHB within pecan orchards thus caused great concern within the rapidly expanding pecan industry in South Africa. A request by the South African Pecan Producer’s Association (SAPPA) to investigate the PSHB outbreak and assess the risk that the PSHB poses to the pecan industry, formed the premise of this study. 2 The first chapter of the dissertation consists of a comprehensive review of literature pertaining to the ambrosia beetles and their recent emergence as pests in various parts of the world, with an emphasis on the biology and impact of the PSHB, including possible management strategies. Based on the results from other studies, it became clear that there was a lack of information regarding the beetle and its symbiont’s impact on pecan trees, whether these trees could act as reproductive hosts for the beetle, and the rate and mechanisms of dispersal and spread of the PSHB, especially in commercial orchards. Chapter two focuses on the first report of PSHB and its fungal symbiont Fusarium euwallaceae on pecan trees in South Africa, confirming the identity of both Euwallacea fornicatus and Fusarium euwallaceae with DNA sequences. Chapter three focuses on the spatial distribution and spread of PSHB in the Jan Kempdorp area, by using pheromone baited insect traps, and its impact on orchards. We tested the hypothesis that infested English Oak trees in the vicinity of the pecan orchards, rather than pecan trees itself, were the reproductive hosts that served as epicenter for the spread of PSHB in the area. Panel traps with quercivorol lures were used to monitor activity of the beetles over time and space.