Current and possible future impacts of the tree-killing polyphagous shot hole borer beetle in a Southern Afrotemperate forest complex in South Africa
Date
2021-12
Authors
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Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The Polyphagous Shot Hole Borer beetle (PSHB) (Euwallacea fornicatus) is a recently
introduced pest in South Africa that can kill susceptible host trees rapidly. Since its first
detection in 2012, PSHB has spread to all but one province in the country. Its ecological and
economic impact is most profound in urban areas, but it has been found infesting important
forestry, agricultural and native forest species. Many native trees in urban settings are
susceptible to infestation, but its presence in natural ecosystems is unstudied. In this thesis the
presence and determinants for PSHB colonization were determined for the very early stages of
its invasion in a native Southern Afrotemperate forest complex. More than 1600 individual
trees representing 68 species was monitored over a one-year period. Breeding populations of
PSHB was detected in six native species and these may be especially vulnerable to population
reductions. The tree-pathogenic fungus Fusarium euwallaceae (a PSHB mutualist) was
isolated from an additional eleven tree species, without signs of beetle breeding. These species
may still be vulnerable to fusarium dieback disease. PSHB colonization was more apparent at
monitoring sites in close proximity to highly infested urban areas, especially when containing
dense stands of reproductive breeding hosts. However, PSHB was also detected at distant sites
where tourists are allowed to use firewood procured from areas outside of these protected
forests. Larger trees were generally infested first and by more individuals than smaller trees
and when surrounded by other infested trees. This is of particular concern given that these
forests contain some of the largest native trees in South Africa. The beetle had a preference for
eight tree species, most of which were identified as reproductive hosts. Infestation severity
increased more rapidly for individuals in pioneer forest than in mature forest. Direct
anthropogenic impact had no discernible effect on levels of infestation but did aid colonization
of distant sites. Projections based on the severity of infestations indicated that PSHB will have
lasting economic and social impacts such as decreased carbon storage value and decreased
value of provisioning, regulating and cultural services of these forests. Total carbon storage in
trees across the entire forest complex was calculated as ca. 14.48 Mt at a value of ca. $ 241
780 818 (R 3.55Bn). Future infestation of these forests by PSHB may lead to a loss of ca. 1.2
Mt of the stored carbon, with economic losses of ca. $ 20 540 841 (R 301 266 352) expected.
This study therefore confirms that Southern Afrotemperate forests are highly susceptible to
invasion by PSHB and that severe impacts on ecosystem function and services can be expected.
Mitigation efforts should therefore be a high priority.
AFRIKAANSE OPSOMMING: Die polifagies Stompneus kewer (PSHB) is ‘n onlangse insekpes in Suid-Afrika wat vinnig hoogs vatbare gasheer bome kan vernietig. Sedert sy ontdekking in 2012, het die kewer na al die provinsies behalwe een in die land versprei. Sy ekologiese en ekonomiese impak is nadelig in stedelike gebiede, maar was ook gevind om belangrike bosbou, landbou en inheemse woud spesies te besmet. Baie inheemse bome in stedelike nedersettings is vatbaar tot infestasie, maar die kewer se teenwoordigheid in natuurlike ekosisteme is nog onbekend. In hierdie tesis was die teenwoordigheid en bepalende faktore vir PSHB kolonisasie vir die vroeë fases van infestasie in ‘n natuurlike Suidelike Afrotemperate woud kompleks bepaal. Meer as 1600 individuele bome wat 68 spesies verteenwoordig was oor ‘n tydperk van een jaar gemonitor. Broeiende bevolkings van PSHB was in ses inheemse boom spesies ontdek en hierdie spesies mag dalk veral vatbaar wees tot bevolking afname. Die boom-patogeen fungus Fusarium euwallaceae (‘n PSHB mutualist) was geïsoleer van ‘n addisionele elf boom spesies, sonder tekens van kewer teling. Hierdie spesies mag dalk nog steeds vatbaar wees tot fusarium terug- sterf siekte. PSHB kolonisasie was meer prominent by monitor plotte wat naby aan hoogs infesteerde stedelike gebiede was, veral wanneer daar ‘n digte hoeveelheid van besmette gashere was. PSHB was ook ontdek by plotte ver weg van groot infestasies, waar daar egter baie toeriste toegelaat word om aangeskafde braaihout van infestasie areas te verbruik in die beskermde woude. Groter bome teenoor kleiner bome was oor die algemeen eerste geinfesteer en wanneer dit omring was deur ander besmette bome. Dit is ‘n besonderse bekommernis, gegee dat hierdie woude van die grootste inheemse bome in Suid-Afrika bevat. Die kewer het ‘n voorkeur vir agt boom spesies vir infestasie en meestal van die is geïdentifiseer as gasheer plante waarin die kewer broei. Die kewer bevolking het vinnig vermeerder vir gasheer individue in pionier/woudrand woude as in klimaks/volwasse woude. Direkte menslike impak het geen onderskeibare effek op infestasie gehad nie, behalwe vir die bydra tot kolonisasie in woud landskappe ver wag van groot infestasies. Projeksies gebaseer op die geweldigheid van infestasies het aangedui dat PSHB ekonomiese en sosiale impakte veroorsaak wat die koolstof stoor waarde en die waardes van voorsiening, regulering en kulturele dienste in die woude sal verminder. Die berekende totale koolstof stoor in bome wat strek regdeur die hele woud kompleks was ca. 14.48 Mt met ‘n waarde van ca. $ 241 780 818 (R 3.55Bn). Toekomstige infestasie van hierdie woude deur PSHB mag dalk lei tot ‘n neerlaag van ca. 1.2 Mt van gestoorde koolstof met ‘n verwagte ekonomiese skade van ca. $ 20 540 841 (R 301 266 352). Hierdie studie erken dat Suidelike Afrotemperate woude hoogs vatbaar is tot infestasie deur PSHB en dat hewige impakte op ekosisteem funksie en dienste gevolglik verwag kan word. Verligting pogings is daarom ‘n prioriteit.
AFRIKAANSE OPSOMMING: Die polifagies Stompneus kewer (PSHB) is ‘n onlangse insekpes in Suid-Afrika wat vinnig hoogs vatbare gasheer bome kan vernietig. Sedert sy ontdekking in 2012, het die kewer na al die provinsies behalwe een in die land versprei. Sy ekologiese en ekonomiese impak is nadelig in stedelike gebiede, maar was ook gevind om belangrike bosbou, landbou en inheemse woud spesies te besmet. Baie inheemse bome in stedelike nedersettings is vatbaar tot infestasie, maar die kewer se teenwoordigheid in natuurlike ekosisteme is nog onbekend. In hierdie tesis was die teenwoordigheid en bepalende faktore vir PSHB kolonisasie vir die vroeë fases van infestasie in ‘n natuurlike Suidelike Afrotemperate woud kompleks bepaal. Meer as 1600 individuele bome wat 68 spesies verteenwoordig was oor ‘n tydperk van een jaar gemonitor. Broeiende bevolkings van PSHB was in ses inheemse boom spesies ontdek en hierdie spesies mag dalk veral vatbaar wees tot bevolking afname. Die boom-patogeen fungus Fusarium euwallaceae (‘n PSHB mutualist) was geïsoleer van ‘n addisionele elf boom spesies, sonder tekens van kewer teling. Hierdie spesies mag dalk nog steeds vatbaar wees tot fusarium terug- sterf siekte. PSHB kolonisasie was meer prominent by monitor plotte wat naby aan hoogs infesteerde stedelike gebiede was, veral wanneer daar ‘n digte hoeveelheid van besmette gashere was. PSHB was ook ontdek by plotte ver weg van groot infestasies, waar daar egter baie toeriste toegelaat word om aangeskafde braaihout van infestasie areas te verbruik in die beskermde woude. Groter bome teenoor kleiner bome was oor die algemeen eerste geinfesteer en wanneer dit omring was deur ander besmette bome. Dit is ‘n besonderse bekommernis, gegee dat hierdie woude van die grootste inheemse bome in Suid-Afrika bevat. Die kewer het ‘n voorkeur vir agt boom spesies vir infestasie en meestal van die is geïdentifiseer as gasheer plante waarin die kewer broei. Die kewer bevolking het vinnig vermeerder vir gasheer individue in pionier/woudrand woude as in klimaks/volwasse woude. Direkte menslike impak het geen onderskeibare effek op infestasie gehad nie, behalwe vir die bydra tot kolonisasie in woud landskappe ver wag van groot infestasies. Projeksies gebaseer op die geweldigheid van infestasies het aangedui dat PSHB ekonomiese en sosiale impakte veroorsaak wat die koolstof stoor waarde en die waardes van voorsiening, regulering en kulturele dienste in die woude sal verminder. Die berekende totale koolstof stoor in bome wat strek regdeur die hele woud kompleks was ca. 14.48 Mt met ‘n waarde van ca. $ 241 780 818 (R 3.55Bn). Toekomstige infestasie van hierdie woude deur PSHB mag dalk lei tot ‘n neerlaag van ca. 1.2 Mt van gestoorde koolstof met ‘n verwagte ekonomiese skade van ca. $ 20 540 841 (R 301 266 352). Hierdie studie erken dat Suidelike Afrotemperate woude hoogs vatbaar is tot infestasie deur PSHB en dat hewige impakte op ekosisteem funksie en dienste gevolglik verwag kan word. Verligting pogings is daarom ‘n prioriteit.
Description
Thesis (MScConsEcol)--Stellenbosch University, 2021.
Keywords
Polyphagous shot hole borer -- South Africa, Euwallacea fornicatus -- Species, Fusarium euwallaceae -- Ecology, Forests and forestry -- Economic aspects -- South Africa, Invasion biology, Carbon -- Storage -- Mathematical model, Trees -- Diseases and pests -- South Africa, Southern Afrotemperate forest, Forest insects, UCTD