Globally, tree die-offs have been increasingly reported since the 1970’s (Raffa et al. 2008, Allen et al. 2010) and are affecting various tree species (Fisher 1997, Fensham & Holman 1999, Suarez et al. 2004, Tsopelas et al.2004, Foden et al. 2007, Hogg et al. 2008). Die-offs have occurred in a variety of ecosystems indicating that this phenomenon is not limited to one type of environment (Allen et al. 2010). Tree mortality is increasingly being linked to the occurrence of drier conditions, or more variable rainfall patterns, linked to climate change (Wardle & Allen 1983, Dezzeo et al. 1997, Liang et al. 2003, Lwanga 2003, Clark 2004, Jurskis 2005, Bigler et al. 2006, Allen 2009). Drier conditions can lead directly to mortality through effects on tree physiological functions, or indirectly through effects on tree vigor and defences, which allow insects and pathogens to kill trees more easily and even develop widespread outbreaks (Konkin & Hopkins 2009, Allen et al. 2010).
Modern climate change is affecting southern Africa to a greater degree than most other parts of the planet and is expected to increase greatly in its impacts over the next few decades (Pollack et al. 1998, Hulme et al. 2001, Schulze et al. 2005, Du Plessis et al. 2003, Boko et al. 2007, Midgley & Thuiller 2011). It is predicted that there will be an increase in winter and summer temperatures by three to seven degrees Celsius by the year 2100 (Boko et al. 2007, Houniet et al. 2009). This is supported by Jury (2013), showing observed (since 1900) and projected data illustrating the increase in temperature and decrease in rainfall for southern Africa with a 0.02 degrees Celsius/year increase being projected for the future. Minimum temperatures will more likely have a higher increase than maximum temperatures leading to more significant changes in temperature during winter than summer (Du Plessiset al. 2003). It is also predicted that rainfall will increase or decrease, depending on region, by 20% during the 21st century (Boko et al. 2007, Houniet et al. 2009). With climate change leading to higher temperatures and unpredictable rainfall certain pathogens will gain a greater infection range in South Africa. This is of great concern to not only the forestry industry in South Africa but also the native environment (van Staden et al. 2004).
Increasing numbers of reports are being made of the decline and death of native trees in Southern Africa (Malan 2006, Roux et al. 2009). In some cases these deaths have been attributed to microbial pathogens (Von Broembsen & Kruger 1985, Coetzee et al. 2003), while in other cases climate change has been suggested as the primary driver in the death of these trees (Foden et al. 2007, van der Linde et al. 2012). Reports of native tree death include Euphorbia ingens (Naboom) trees in the Limpopo Province, Acacia erioloba (camel thorn) in the Northern Cape and Namibia, Adansonia digitata (Baobab) in various areas in Southern Africa and Pterocarpus angolensis (Kiaat) in Zimbabwe and South Africa.
Ambrosia beetles and some fungal associates have been found to be associated with the decline of Euphorbia ingens (Roux et al. 2009, van der Lindeet al. 2012). In recent years, a number of Ambrosia beetles (Coleoptera; Curculionidae, Scolytinae) and their associated fungi have been identified as the causes of important tree diseases (Kühnholz et al. 2001). Most Ambrosia beetles produce populations within dead or severely stressed trees (Kühnholz et al. 2001). Recent reports have shown that increasing numbers of these beetles, together with their fungal pathogens, are attacking and killing healthy trees, leading to substantial tree mortality (Kovach & Gorsuch 1985, Kamata et al. 2002, Kühnholz et al. 2001). For example, in Japan increasing levels of oak die-back, caused by Platypus quercivora and its fungal symbiont in the genus Raffaelea have been reported (Kubono & Ito 2002), while in the USA laurel wilt disease has been attributed to Xyloborus glabratus and its fungal symbiont Raffaellea lauricola (Fraedrich et al. 2008) as well as “thousand cankers disease” attributed toPityophtorus juglandis and its fungal symbiont Geosmithia morbida killing Juglansnigra (black walnut) (Kolatik et al. 2011).
Several hypotheses have been proposed to explain the increasing number of reports of Ambrosia beetles attacking healthy trees. Anthropogenic activities and warmer climates have been proposed to alter the three way interaction between Ambrosia beetles, the symbiont fungi and the tree hosts (Hepting 1963, Krcmar-Novaic et al.2000, Künholz et al. 2001, Six 2009, Six et al. 2011). The introduction of non-native trees and plant material has led to the introduction of non-native Ambrosia beetles, with their symbiont fungi, into new environments (Krcmar-Novaic et al. 2000). Introduced fungi, with their Ambrosia beetles, are responsible for killing healthy native hosts (Fraedrich et al. 2008), not only due to host susceptibility, but also due to altered pathogenicity in the fungi. The increased pathogenicity of the fungi are hypothesized to be as a result of climatic changes which in some cases coincides with altered flight patterns of the beetles attacking trees at their most susceptible periods (Kühnholz et al. 2001).
It is clear that Ambrosia beetles and their associated fungi require more intensive study. This is particularly true in Africa, where although a number of Ambrosia beetles have been reported from native trees (Beaver et al.2005, Jordal et al. 2001, Jordal 2006), no studies have been done on the fungal associatesof these insects. The lack of information on the fungal associates of Ambrosia beetles in Africa, including Southern Africa, presents a serious stumbling block in understanding the mortality of E. ingens in South Africa. Changing land management through human population growth as well as climate change are furthermore guaranteed to result in an increase of stress on trees, predisposing them to opportunistic pests and pathogens, such as Ambrosia beetles (Six et al. 2011).
My PhD study aims to address our lack of understanding regarding Ambrosia beetle diversity in Southern Africa, particularly the lack of information on their fungal associates. This will be done by focussing on E.ingens trees. This tree genus has been selected for a number of reasons. Firstly, reports of the mass mortality of E.ingenstrees in recent years (Roux et al. 2008, 2009; van der Linde et al. 2012) have resulted in the discovery of a number of Ambrosia beetles and their fungal associates (Roux et al. 2008, 2009; van der Lindeet al. 2012). Secondly, a number of previously unknown Ambrosia beetles have recently been described from Euphorbiaspecies in South Africa (Jordal & Hewitt 2004). Lastly, modern climate change might be influencing the three way interaction between the beetles, fungi and host, leading to the sudden die-offs of E. ingens. This model can be used to understand the effect of modern climate change, in South Africa, on the native environment (from an insect, fungus and host perspective) and lay the foundation for future related studies.
(a) Large scale mortality of E. ingens trees at Mokopane. (b) Dead E. ingens trees at Last Post. (c) Typical spotting and lesions on the external area of the succulent branches. (d) Internal rotting and feeding of the Megasis moth.