Beware of this tiny beetle

The invasive polyphagous shot-hole borer threatens fruit trees. By Engela Duvenage.

Polyphagous shot-hole borers began killing trees in gardens and forests nationwide in the past five years. Prof. Francois Roets of the Department of Conservation Ecology and Entomology at Stellenbosch University has now confirmed the presence of the beetle in fruit orchards around Somerset West.

The tiny beetles bore into trees but do not feed on wood. Instead, they feed on a specific fungus that they establish in their tunnels. The fungus invades wood and kills the tree by blocking its vascular system.

“Basically, polyphagous shot-hole borers infest any tree that the fungus is happy in. We already know they can breed in 50 of the 130 tree species in South Africa we’ve found them in,” said Roets. “Those 50 are in serious danger — they can die. They include avocado, wattle, and some indigenous trees.”

His research group is part of national containment efforts and received funding from Hortgro and Winetech to study the possible threat to the Western Cape fruit and wine sector.

They currently largely focus on Somerset West, where the beetle was first noticed in one small area in 2019 — the first record of polyphagous shot-hole borer in the Cape. “One year later it’s spread to half the town, despite control measures being in place,” said Roets, “and it’s been noticed in outlying areas of Stellenbosch this year.”

Roets thinks that this movement is almost certainly human-induced, as green waste from Somerset West is taken to dumping sites in Stellenbosch and elsewhere.

Borers, fungus, and fruit trees

Roets and co-workers have been studying the potential effects of borers on fruit trees. “We started working on the fungus, to make sure we can contain it because it cannot spread without the beetle.”

Trials in Cripps Pink apple trees showed that the fungus grows well in apple trees.

“We’ve previously seen old polyphagous shot-hole borer infested trees in Somerset West gardens killed,” reported Roets. “In recent months the beetle and fungus were noticed in apple orchards adjacent to town. We’re unsure yet if they can breed in apple trees.”

MSc student Mignon de Jager has established that the beetle can breed in almond, nectarine, and apricot trees, at least in gardens. “These stone-fruit trees are all highly susceptible to the fungus,” commented Roets.

The fungus grew equally quickly in the wood of two plum cultivars, Sungold and Fortune. It fared better in Angeleno plums than Alpine nectarines. Roets speculated that this could mean that plums are at higher risk than nectarines.

In May, the first borers were detected in plum orchards. “We’re still unsure if they’ve successfully formed a colony within those trees,” said Roets.

The researchers are most worried about the future of pears.

“Beetles have started moving into pear orchards, of which they seem particularly fond,” said Roets. “Pears show very high infestation rates compared to other fruit trees. We’ve seen orchard trees with up to 150 holes that beetles made, all within a metre off the ground. If only ten result in colonies, these trees will likely die rapidly.

“We’re unsure why polyphagous shot-hole borers attack pears so easily. We’re unsure if because of their already high numbers in the area, they opportunistically move to pear trees, or if this will be a trend generally seen in pear production areas.”

He added that the fungus seems to do better in pears than in any other host trees in South Africa.

The beetle and fungus were also noted in May in a commercial vineyard but studies on vines infected with the fungus suggest that the beetle would not be able to survive in vines.

Research brings good and bad news

Some good news is that De Jager developed a molecular screening tool to easily detect the fungus. The test is highly reproducible, can be done on large sample numbers, and can be used for various tree species. Results are available within two days.

“It’s an important step,” said Roets. “You need a rapid and cost-effective test to screen large numbers of trees for agricultural purposes.” Testing is performed on a piece of wood — even dry wood — from a potentially infected tree. There is no need to first extract or grow a fungal sample.

A second genetic variation or haplotype of the beetle has been detected by Dr Anandi Bierman of the Centre of Invasion Biology at Stellenbosch University. This suggests that polyphagous shot-hole borers were introduced to South Africa more than once.

Roets explained that this new beetle variant appears to be associated with a new, not yet described, fungus. “We’ve no idea if it will increase, decrease, or change the beetle’s host range. Researchers at the University of Pretoria are investigating it.”

More bad news comes from studies by MSc student Madeleine Pienaar on the thermotolerance of the borers.

“Polyphagous shot-hole borers can move and potentially become problematic in an extremely wide temperature band, between 20 °C and 30 °C,” said Roets. “They’re also mobile under and above those temperatures, ranging from 8–38 °C.”

Roets worries not only about primary infestations, but also that wounds created by beetles could increase secondary infections. Furthermore, orchard windbreaks such as beefwoods and poplar trees could potentially be an accessible breeding ground for polyphagous shot-hole borers.

Meanwhile, researchers are unsure if chemical controls work. Studies are ongoing. “We can do more tests and screening in orchard settings than in urban trees,” said Roets. “It’s also easier to test chemicals in orchards should we find something to potentially control it.”

Image: Prof. Francois Roets, Stellenbosch University. Supplied by Peartree Photography.