Euwallacea fornicatus
Common name: PSHB, Ambrosia beetle (broad group name), invasive shot hole borer
Family: Curculionidae (subfamily Scolytinae)
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The polyphagous shot hole borer is an ambrosia beetle native to Southeast Asia. It has since become invasive in several parts of the world, including California, Israel, Australia, and South Africa. In South Africa, it was first identified in the KwaZulu-Natal National Botanical Garden, and has since spread to eight of the country's nine provinces, making this the largest recorded PSHB outbreak anywhere in the world. It affects trees across agricultural, commercial, and urban settings alike.
The polyphagous shot hole borer is an ambrosia beetle native to Southeast Asia. It has since become invasive in several parts of the world, including California, Israel, Australia, and South Africa. In South Africa, it was first identified in the KwaZulu-Natal National Botanical Garden, and has since spread to eight of the country's nine provinces, making this the largest recorded PSHB outbreak anywhere in the world. It affects trees across agricultural, commercial, and urban settings alike.
PSHB is extremely small and easily overlooked in the field. As an ambrosia beetle, it does not feed on wood directly; instead, female beetles bore into the trunk and branches of a host tree and cultivate a symbiotic fungus, Fusarium euwallaceae, inside the tunnels (galleries) they excavate, which serves as food for the developing brood. Once established, this fungus spreads through the tree's water- and nutrient-conducting tissue. Females release an aggregation pheromone based on a compound called quercivorol, which draws more females to an already-colonised tree, allowing beetle numbers to build rapidly and overwhelm the tree's natural defences.
PSHB is highly polyphagous, having been recorded on more than 80 tree species in South Africa alone, spanning agricultural crops (including avocado), commercial forestry and amenity trees, and numerous indigenous and exotic urban trees.
Entry and exit holes bored by the beetle are often accompanied by staining, gumming, or sap and sugar exudation on the bark, along with wilting and dieback of affected branches. It is primarily the associated Fusarium fungus, rather than the beetle's tunnelling itself, that kills the tree. The fungus blocks the tree's vascular tissue, cutting off the movement of water and nutrients. Once a tree is significantly colonised, decline is often irreversible, and there is currently no fully effective cure once infection has taken hold, making early detection and prevention the most valuable tools available to growers and municipalities.
Because PSHB is so small and its damage can be subtle in its early stages, monitoring using the beetle's own aggregation pheromone is a key detection tool. Traps baited with quercivorol should be placed outside, rather than inside, an orchard or stand of susceptible trees, since the lure is a powerful attractant that can otherwise draw more beetles into an area. Beyond monitoring, management relies on prompt removal and destruction (chipping or solarising) of severely infested material, and, where infection is still limited, systemic trunk injection treatments applied by a qualified arborist.
Insect Science solution: Insect Science has synthesised quercivorol, a kairomone released by the symbiotic fungus associated with PSHB, into a dedicated P.S.H.B. PheroLure®, which maintains effective trap catches for an extended period, even under summer conditions.
Insect Science is also researching repellent signals associated with heavily infested trees, with a view to developing a combined "push-pull" approach that attracts beetles away from valuable trees while repelling them from infested ones.
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