Polyphagous Shothole Borer

Polyphagous Shothole Borer
Euwallacea fornicatus
Fusarium euwallaceae; Graphium euwallaceae
Wood boring
Vascular wilt

The female beetles bore holes into the trunk and/or branches of a tree, the holes are perfectly round and about 1 mm in diameter. Symptoms of beetle infestation vary between tree species but external symptoms of infestations may include wet patches and resin exudates, sugar fountains, or accumulation of frass (mixture of sawdust and excreta) and noodles (compacted frass) at the entrance holes and around the base of the tree, as well as “shotgun” like lesions. Removal of the bark around the entrance holes may reveal a brown- to pinkish stain formed by the fungal symbiont, Fusarium euwallaceae, spreading from the gallery into the vascular tissues in a streaking fashion below the bark. Additional symptoms such as branch dieback and tree decline can be observed later as F. euwallaceae colonises and blocks the xylem vessels of the tree, preventing water flow. Removal of affected limbs or whole trees reveal staining of the fungal symbiont and tunnel systems of the beetles throughout the tree.

The PSHB is a small beetle, no larger in size than a match head. Female beetles are 1.8 – 2.5 mm in length, and are able to fly. Males are 1.5–1.67 mm in length, and lack wings. Beetles can vary in colour from brown to black, the antennae and legs vary from golden to dark brown. Larvae are white, no more than a couple of millimetres in size and can be found within galleries in the sapwood of a host.

The word polyphagous in the common name refers to the ability of PSHB to infest many different host tree species. Trees in which the beetle can fully complete its lifecycle are known as reproductive hosts. When an adult female beetle locates a suitable reproductive host, she will tunnel into the sapwood, inoculating the fungal symbionts, including F. euwallaceae, and establish a brood gallery in which to lay her eggs. Within this gallery the larvae will develop and fully mature, feeding primarily on the structures produced by F. euwallaceae. Male beetles usually remain in natal galleries and are rarely found to venture outside of the host.

The PSHB has a haplodiploid mating system in which haploid males develop from unfertilized eggs. If a mated female initiates a colony then both male and female offspring may be produced (ca. 7% of beetles may be male). These male and female offspring (brothers and sisters) can mate within the galleries, resulting in mated female progeny that can leave the natal gallery and start new colonies. However, virgin females that construct galleries can lay haploid eggs that will result in all-male offspring. The female can then mate with her male offspring to produce diploid eggs in subsequent generations. The result is that a single female beetle, whether mated or not, can start new infestations with no indication that the populations will suffer from inbreeding. Under optimal conditions, the life cycle can be completed in 22 days.

In non-reproductive hosts, PSHB will “test” the tree to determine if it is a suitable host but will not establish a gallery.

2018
Eastern Cape, Free State, Gauteng,KwaZulu-Natal,Northern Cape,North West and Western Cape
Fusarium euwallaceae likely aids PSHB to infest a multitude of different tree species. In South Africa, signs of PSHB infestations have now been confirmed from 114 tree species, of which at least 36 are suitable for sustaining breeding beetle populations (i.e. reproductive hosts). PSHB infestations have been observed on a small number of roadside wattle trees – but to date, no trees in commercial plantations have been infested. Based on observations from commercial forestry in SE Asia, however, there is concern around the threat posed to Acacia spp. of importance to the South African forestry industry. The full list of confirmed hosts is available at: https://www.fabinet.up.ac.za/pshb
Southeast Asia

Gallery

Image Title
Female (A-B) and male (C-D) PSHB beetles
Beetle gallery
"Sugar fountain" on avocado
Vascular staining of Acacia mearnsii, following colonisation by Fusarium euwallaceae
PSHB entry holes on Acacia mearnsii
"Shotgun" like lesions on London plane
Extensive gallery formation and fungal staining in castor bean stem

Cooperband MF, Stouthamer R, Carrillo D, Eskalen A, Thibault T, Cossé AA, Castrillo LA, Vandenberg JD, Rugman-Jones PF. 2016. Biology of two members of the Euwallacea fornicatus species complex (Coleoptera: Curculionidae: Scolytinae), recently invasive in the U.S.A., reared on an ambrosia beetle artificial diet. Agricultural and Forest Entomology 18:223–237.

Paap T, de Beer ZW, Migliorini D, Nel W, Wingfield MJ. 2018.The polyphagous shot hole borer (PSHB) and its fungal symbiont Fusarium euwallaceae: a new invasion in South Africa. Australasian Plant Pathology 47:231-237.

Smith SM, Gomez DF, Beaver RA, Hulcr J, Cognato AI. 2019. Reassessment of the species in the Euwallacea fornicatus (Coleoptera: Curculionidae: Scolytinae) complex after the rediscovery of the “lost” type specimen. Insects 10:1–11.

Stouthamer R, Rugman-Jones P, Thu PQ, Eskalen A, Thibault T, Hulcr J, Wang LJ, Jordal BH, Chen CY, Cooperband M, Lin CS, Kamata N, Lu SS, Masuya H, Mendel Z, Rabaglia R, Sanguansub S, Shih HH, Sittichaya W, Zong S. 2017. Tracing the origin of a cryptic invader: phylogeography of the Euwallacea fornicatus (Coleoptera: Curculionidae: Scolytinae) species complex. Agricultural and Forest Entomology 19:366–375.