International Year of Plant Health (Part 2): How do the trees die?

22 July 2020

Dr Trudy Paap explains that "The polyphagous shothole borer (PSHB) is a 2 mm long ambrosia beetle that is native to Southeast Asia."
"The female beetle carries three fungal species, one of which is Fusarium euwallaceae. The beetle bores through the bark into the sapwood of the tree and inoculates the fungus into the living wood. The fungus grows in the galleries (tunnels) created by the beetle and serve as a ‘vegetable garden’ for the beetle larvae, but in susceptible trees the fungus can spread through the sapwood causing disease or even death of the tree," she said.
 
The PSHB boring through a tree, creating tunnels or galleries

The PSHB and its galleries or tunnels, where it leaves a fungus for its larvae

In its native environment in Southeast Asia, it seems as if the beetle and fungus do not cause serious damage. "This is usually the case for insects and microorganisms in their natural environments. As a result of co-evolution between pests and their hosts, together with pressures from competition, predation, and parasitism by natural enemies, damaging outbreaks by pests in their natural environments are rare," said Dr Paap.

When these organisms are moved to novel environments outside of their natural range (usually unintentionally by humans through trade or travel), they have the potential to encounter novel hosts lacking resistance. Worldwide there are many examples of alien forest pests and pathogens arriving and establishing in new environments, resulting in disastrous outbreaks impacting commercial, natural, and urban forests.

Until December 2018 the PSHB was known as Euwallacea nr fornicatus. However, Gomez et al (2019) showed with DNA sequences that Euwallacea fornicatus is actually a species complex including four closely related, but distinct species.

These four species of shot hole borer are very similar in external appearance, and can only be distinguished by specialists with the aid of a microscope, or with DNA sequences. The four species carry different fungal species, and have different host ranges and geographical distributions (Gomez et al 2018).

With South African borders being open for trade to Southeast Asia, there remains a risk that one of the other species can be introduced. Dr Paap explains that "Co-occurring species increase the chances for interbreeding, which may enhance the adaptability of the beetles to new hosts and new environments, posing an even greater threat. We thus need to confirm the identity of the PSHB from each new location or host tree reported in South Africa, with a DNA sequence of the fungus, the beetle, or both, before the host or location is added to the official list."

It is important to distinguish between different types of hosts. Reproductive host trees are trees that the beetle infests and where it successfully establishes a breeding gallery in which the fungus grows, eggs are laid, and larvae develop into mature adults, thus completing the beetle’s life cycle. The majority of reproductive hosts eventually succumb to the disease symptoms caused by the fungus.

A selection of shots showing a keurboom affected by pshb

The South African Keurboom is an example of an indigenous tree that is a reproductive host. Notice how the PSHB leaves lesions and noticeably affects the tree as it bores its way through, leaving the fungus for its larvae to feed on.

Non-reproductive host trees are trees where the beetle attacks, penetrates, and inoculates the fungus that then starts growing in the sapwood. However, the beetle either leaves or dies without reproducing in these trees. We are yet to observe non-reproductive hosts succumb to the disease and die.

SA monkey plum tree affected by pshb  a monkey plum tree affected by pshb

The monkey plum tree is a South African example of a non-reproductive host.

A problem when compiling these lists is that sometimes the PSHB can infest a stressed tree (eg, as the result of drought, too much water, root damage, etc). Such a stressed tree might then become a reproductive host, whereas healthy, non-stressed individuals of the same species are barely affected. When trees are assessed for Fusarium disease, and whether or not they are reproductive hosts, other stress factors on the tree should always be considered.

A more complete list of examples of reproductive and non-reproductive host trees can be found  in Part 6 of this feature. 

The PSHB and how it affects trees by boring galleries through them and spreading a fungus it carries to feed its larvae.