Polyphagous shot-hole borer

Is it coming to an orchard near you? By Anna Mouton.

Shot-hole borers are diminutive beetles that cause oversized trouble. One member of this group — the polyphagous shot-hole borer — has invaded South Africa and was recently detected in the Western Cape suburb of Somerset West. Polyphagous shot-hole borers attack trees ranging from acacias to avocados. Are they a threat to deciduous fruit production? A new project funded by Hortgro aims to find out.

Down on the fungi farm

Polyphagous shot-hole borers belong to a group called ambrosia beetles. These beetles make tunnels in wood and then cultivate fungi in the tunnels. The fungi are their only food source — shot-hole borers do not eat wood. The deterioration and death of infested trees are caused by a combination of the pathogenicity of the fungi and the boring activities of the beetles.

“The fungus needs the beetle to get into the tree,” says Prof. Francois Roets of the Department of Conservation Ecology and Entomology at Stellenbosch University. He explains that the beetles carry the fungus in special internal structures and plant it wherever they start a new tunnel system.

“They also maintain their fungal gardens. If you remove the beetles, then the gardens get weedy — other fungi grow in them. Eventually they collapse.”

Polyphagous shot-hole borers have complicated lives. A single fertilised female will start a new tunnel system and lay eggs. Her offspring mate with one another and some of the young females then leave to start their own tunnels. Males are flightless and remain behind.

What happens if an unfertilised female happens to bore into a tree? According to Roets, she may be able to provide her own mates by laying eggs that hatch into male offspring. “So one beetle is enough to begin an epidemic.”

Roets relates that a single female can travel up to a kilometre — even though she is only a couple of millimetres long. But, he stresses, movement by people is the biggest problem and the most likely means by which the beetles reached Somerset West.

Will borers eat your lunch?

Polyphagous shot-hole borers are already a headache for avocado growers in California and Israel. Will they prove troublesome to our local deciduous fruit producers? Roets hopes to answer this question by studying the beetles and their fungi in the laboratory and the field.

Surveys have found polyphagous shot-hole borers in isolated fruit trees — including apple and stone fruit — in gardens. That doesn’t guarantee that the beetles will attack trees in a commercial setting.

“This is probably the strangest pest we’ve ever had to deal with,” says Roets. “It attacks so many trees and the conditions under which it does so differ for each tree. So it’s impossible to generalise about what will happen to a host.”

Roets plans to study the interaction of the beetles with fruit trees in the laboratory. This will indicate whether the beetles can complete their life cycle in pome and stone fruit trees. He will also do experimental infections with the fungus to determine how it affects the performance of trees.

“We can’t take the beetle to places where it doesn’t occur, but we can take the fungus because it needs the beetle to spread,” states Roets. He will infect trees by drilling artificial tunnels and then planting the fungus in them. The growth of the fungus will be measured and its impact on plant performance monitored.

Dealing with polyphagous shot-hole borers

“The future is to manage this problem,” says Roets. “We’re not going to get rid of it — the beetle is here to stay.”

There are currently no effective insecticides and fungicides registered for polyphagous shot-hole borers in South Africa. Research shows that certain chemicals can reduce beetle numbers but will not eliminate them or prevent new infections.

“The challenge is to get a chemical agent to the beetle,” says Roets. “Once they’re established, you can’t reach them. The tree often closes the tunnels behind the beetles, and they’re sealed inside.”

Roets believes that polyphagous shot-hole borer will have a severe impact on ornamental trees, but he is optimistic about the future for commercial orchards. “There’s always a way to control a problem in a production environment. The more we learn, the more we’ll know how to apply treatments so that we minimise damage. We won’t eliminate the beetle, but we’ll prevent it from having a crippling impact.”

A note on nomenclature

Polyphagous shot-hole borers belong the genus Euwallacea. There are no indigenous species of Euwallacea in South Africa. Until recently, the invasive shot-hole borer found here was called Euwallacea whitfordiodendrus. But now taxonomists have reconsidered, and the name has changed to Euwallacea fornicatus.

You might think Euwallacea fornicatus earned its species name because of its lifestyle. Not so. The word fornicate in biology means arched or hoodlike. A glance at the profile of the polyphagous shot-hole borer shows the aptness of this description.

What are people saying about this research?

Graeme Krige | Fruitmax Agri

“We absolutely welcome and support the local research. It is imperative to determine, firstly, which fruit crops are hosts under local conditions, secondly, whether these hosts will be negatively affected by the fungus associated with polyphagous shot-hole borer, and thirdly, to identify and propose control measures where necessary. Researchers studying the borer under local conditions are in the best position to provide these answers.

“Hortgro and Hortgro Science were excellent in their communication to industry and growers during the time that polyphagous shot-hole borer was in the news. An industry Fresh Notes, a field guide and a circular on the borer were sent to all growers in June 2019. Thank you to them.”

Image: The polyphagous shot-hole borer — Euwallacea fornicatus.

Supplied by Jiri Hulcr | University of Florida