The polyphagous shothole borer (PSHB) and its fungal symbiont Fusarium euwallaceae are killing trees in South Africa as well as in California.
Erythrina humeana in the Manie van der Schijff Botanical Garden, Pretoria
The pest complex’s presence was detected in August 2017 through an international sentinel tree program – the first detection of a tree pest under the program. Under the ‘sentinel plantings’ program, staff at botanical gardens and arboreta monitor their holdings – often exotic species growing outside of their natural range – and alert program partners when they detect damage caused by insects or pathogen not previously known to pose a risk. The International Plant Sentinel Network (IPSN) was launched in 2013. Botanical gardens and arboreta in South Africa joined the international effort in 2016 (Paap et al. 2918 – see list of sources at the end of this blog).
PSHB-caused tree mortality was initially detected in the KwaZulu-Natal National Botanical Gardens in Pietermaritzburg in August 2017. Affected trees were London Plane (Platanus x acerifolia) (Paap et al. 2018).
A beetle collected in 2012 in Durban, 50 km away from Pietermaritzburg, has now been determined to belong to the Euwallacea fornicatus species complex – indicating that the invasive insect and fungal species have been established in South Africa for several years (Paap et al. 2018). [Interestingly, 2012 is also the year that Dr. Akif Eskalen detected PSHB in a backyard avocado in southern California – setting off the detection, research, and slow-the-spread efforts now under way there.]
locations of PSHB detections in South Africa; map from http://polyphagous-shot-hole-borer.co.za/
South African authorities were immediately concerned because the beetle-fungus complex attacks such a broad range of trees (species in 58 plant families). Hosts include several species native to southern Africa – including cabbage tree (Cussonia spicata), common calpurnia (Calpurnia aurea), monkey plum (Diospyros lycioides), two species of coraltree (Erythrina humeana and E. lysistemon), huilboerboon (Schotia brachypetala), honey flower (Melianthus major), two alders (Cunonia capensis and Nuxia floribunda), and red orchid bush (Bauhinia galpinii). Also at risk are several commercial crop trees such as avocado (Persea americana), macadamia nut (Macadamia integrifolia), pecan (Carya illinoinensis), peach (Prunus persica), orange (Citrus sinensis) and grapevine (Vitis vinifera) and several ornamentals, including maple, holly, wisteria, oak and Camellia (Paap et al. 2018).
South Africa is home to a highly unique flora. Indeed, the “Cape Floral Kingdom” is the smallest of the six floral regions on Earth. For more about South Africa’s botanical importance, go here or here.
Rapid spread of the beetle-fungus complex appears likely because one of the most important reproductive hosts, castor bean (Ricinus communis) is a widespread woody weed in the KwaZulu-Natal region (Paap et al. 2018).
By July 2018, it was clear that PSHB was established in several parts of the country (see map). In George — a city along the southern coast, due east of Capetown, the beetle and fungus are affecting a wide range of indigenous and exotic trees in the botanical garden and the region‚ including box elder‚ Chinese and Japanese maple‚ oak‚ plane trees‚ Kapok trees‚ paper bark acacia‚ wild plum‚ dwarf corral and common corral (Chambers 2018).
In Johannesburg, a concerned citizen tracking the pest complex’ spread thinks that the beetle-fungus combination has already infested well over 100,000 of Johannesburg’s trees and is on track to damage or kill millions more (there are an estimated 6 – 10 million trees in Johannesburg, nearly all exotic) (Weltz 2018).
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SOURCES
Chambers, D. “A 2mm beetle is laying waste to George’s trees” Sunday Times. 30 May 2018 https://www.timeslive.co.za/news/sci-tech/2018-05-30-a-2mm-beetle-is-laying-waste-to-georges-trees/
Johannesburg Urban Forest Alliance. The Shot Hole Borer Beetle is destroying our Urban Forest http://www.jufa.org.za/pshb.html
Paap, T., Z.W. de Beer, D. Migliorini, W.J. Nel, M.J. Wingfield. 2018. Australasian Plant Pathology https://doi.org/10.1007/s13313-018-0545-0 https://link.springer.com/article/10.1007/s13313-018-0545-0
Weltz, A. Beetle Mania The Nasty Insect that is Killing the Trees of Johannesburg. Yale Environment 360; Published at the Yale School of Forestry and Environmental Studies. https://e360.yale.edu/features/beetle-mania-the-nasty-insect-that-is-killing-the-trees-of-johannesburg