Paap, a postdoctoral fellow at a biotechnology
institute at the University of Pretoria.
During a survey for diseases in the KwaZulu-
Natal Botanical Gardens in Pietermaritzburg,
Paap found a lane of infested plane trees.
The identity of the beetle was subsequently
confirmed and the tiny beetle – they are each
about 2mm long – has been found at work in
gardens and roadsides in Johannesburg, about
500 km from Pietermaritzburg.
holes. On some trees he had also spotted
small, elevated lesions on the bark resembling
shotgun wounds.
Hill said that trees had already started dying
with these symptoms in 2015, but the cause
was unknown. Microscopic and DNA tests in
the institute’s laboratories confirmed that the
polyphagous shothole borer and its fungus
had arrived in Sandton.
The beetle isn’t alone. It carries several fungal
species with it when it infests living trees. One
of these, Fusarium euwallacea, seems to be a
permanent associate of the beetle. This fungus
can eventually kill a beetle-infested tree. The tree species affected in the Sandton area
include non-native ornamental trees such as
Japanese and Chinese maple, London plane,
kapok, and liquid amber. Several paper bark
trees, native to South Africa, were also heavily
infested and dying.
The beetle and the fungus have devastated
trees in California in the US as well as in
Israel. Insecticides aren’t effective because
the beetles bore deep into the wood. The only
known method of managing the spread is to
cut down infested trees and burn them. But
research is underway to find more effective
methods. The polyphagous shothole borer doesn’t
appear to have done much damage to trees
in Southeast Asia, its place of origin. That’s
probably because tree species evolved with
the beetle and the fungus and have developed
resistance towards them. It might also be
because there are natural enemies controlling
populations of the beetle in its native habitat.
A threat to native forests and fruit trees
In late January my colleagues and I at the
Forestry and Agricultural Biotechnology
Institute were contacted by Niel Hill, an urban
forestry consultant in Johannesburg. He was
concerned about several dying trees in the
Sandton area. Symptoms varied on different
tree species from patches of white powdered
wood (called frass), to blotches of oozing resin,
on the bark surrounding the beetles’ entrance But it’s a different story in California in the US
and in Israel. The beetle and its fungus were
introduced in these countries during the past
15 years and have caused serious damage,
especially on avocado trees.
Paap’s work has confirmed that the South
African beetle and fungus are the same
genotypes as those found in Israel and
California.
A close-up of the polyphagous shot hole borer beetle. Image: George Municipality
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