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A minute exotic beetle that carries disease-causing fungi is threatening tree life in South Africa. This beetle bores holes into mostly the trunks of healthy, living trees where it cultivates a fungal garden to feed on. The beetle itself is small, and it does not feed on the host tree, but it rather feeds on the fungi that it cultivates in the tunnels it bores throughout the tree. Therefore, the beetle on its own holds little threat to the trees. The fungi, however, are tree disease-causing organisms (mostly belonging to the genus Fusarium) and a wide range of trees will become ill and may die when infected with these fungi.
The beetle-fungus pair was formally recorded for the first time in KwaZulu Natal in South Africa in 2017, but it is likely to have been here for a while before the first formal record. During the past few years that it has been present in South Africa, it has caused significant damage – mostly to thousands of urban trees that died in Johannesburg, but also to numerous other tree hosts spread widely throughout the whole of South Africa.
A before and after picture from a street in Chula Vista, California, aptly conveys the effect that the beetle’s presence has had (Figure 1).
The beetle responsible for the upset is called the polyphagous shot hole borer or PSHB (scientific name: Euwallacea whitfordiodendrus). Polyphagous means “feed on many types of host “and it refers to the wide range of hosts that may be attacked by the beetle.
For several reasons, the risk of invasion by the PSHB is high, and the subsequent risk of damage to trees is also high.
The beetle is a super invader for the following reasons:
- The beetle is minute, and its living habit is concealed
- These beetles are a mere 1.8-2.5mm long. That is similar to the size of a small ant (Figure 2). Other than being barely visible, they live inside the tree, which means that the beetle itself mostly goes unnoticed.
Figure 2. A female polyphagous shot hole borer from a Jacaranda tree infestation in California.
Photo credit: Aguilar Plant Care
Beetle reproduction is highly effective
The female beetles drill tunnels in the wood in which she will lay up to 30 eggs, of which the hatch rate is greater than 80%. A sibling mating ability means that the siblings reproduce with each other, and adult females will all be fully fertilised when they fly off from the tree in which they were raised to find a new tree host.
A new population of the beetle can thus be established by a single female. The rate of reproduction depends on the environment, but between 2-12 generations have been observed per year. In South Africa, about six generations have been noted. Given six generations, a single female beetle will give rise to millions of female offspring in a year. (Should 12 fertile adult female offspring arise per generation the progeny will be almost 3 million after six generations, with a less conservative estimate of 20 fertile female offspring, the number rises to 64 million females per year).
The host is not a restricting factor
Since the beetle feeds on the fungus that grows in the cavities that the beetle bore out in the wood and not the wood itself the tree host specificity is low. Worldwide, beetles that belong to the PSHB group (there are four similar beetles in what is called a “species complex”) collectively infests 412 species from 75 families, and the list is ever growing. In South Africa, the PSHB that occurs here (Euwallacea whitfordiodendrus) has been confirmed from 59 hosts. The host list is maintained by the Forestry, and Agricultural Biotechnology Institute at the University of Pretoria and a complete list of hosts can be found at this website https://fabinet.up.ac.za/index.php/research/7 (this can also be obtained as the first search result by searching for the words “FABI and PSHB”).
Reproductive vs non-reproductive hosts
The hosts of PSHB can be divided into reproductive and non-reproductive hosts. As the name suggests, in reproductive hosts, the beetle reproduces, while in the non-reproductive hosts the beetle can infest and live in the host, but it cannot reproduce. Reproductive hosts will be infected by the fungus and will be damaged by it. Fungal infection and disease in non-reproductive trees are not always a given. In South Africa, 21 of the recorded hosts are reproductive hosts, and the remainder is non-reproductive hosts.
Thanks to humans, it spreads fast!
The natural spread of the beetle is not as fast as it relies on the distance that the female beetles can fly (while the males are flightless). It is generally held that the beetles can fly as far as 500m. Human-assisted the spread of the beetles, however, is rapid as the beetles are spread via infested wood. Wooden objects such as packing crates and pallets may harbour the beetle and spread via infested nursery material is also possible.
In California, the beetle has spread from a single county to six counties in two years. The range of hosts also expanded during that time from urban forests and landscape trees to native forests and commercial avocado orchards.
In South Africa, wood is widely used for making fires, and the human-assisted the spread of the beetle is unprecedented. Since its first introduction, it has been recorded in most South African provinces. Despite a seeming calm from national authorities, the magnitude of the impact of the beetle can be very high – and since trees may take many years before they die, the catastrophic effects may only be visible ten years from now.
If humans remain uninformed about the risks of PSHB, the spread of the beetle and resultant destruction of tree life will continue indefinitely.
Climate is not a restricting factor
Male beetles spend its entire life inside the tree, and female beetles spend most of their lives in the tree. Thus, the climate is not as much of a restrictive factor as it is for other species.
More reasons for concern:
- We don’t understand the real risk well enough
- The beetle is not a pest in its native range. In the native range, trees are mostly resistant, natural enemies keep the population of the beetle low, and the beetle mainly infests trees that are dying. It is only known to be a pest in the invaded range. It was first recorded as an invading pest in California in 2003 and in Israel in 2009. The full host range in South Africa and the exact extent to which hosts may be affected remains uncertain. These unknowns contribute to the likelihood that the risk of invasion could be higher than anticipated. Globally, this beetle has caused significant damage to natural, urban and agricultural trees.
The importance of PSHB to agriculture
Several agricultural crops cultivated in South Africa are hosts (Table 1), although mostly non-reproductive hosts, of the beetle. Avocados are highly susceptible reproductive hosts, and notable, avocado production in Israel and California has been significantly hindered by this pest. In South Africa, backyard avocado trees have been found to be infested in Sandton and Knysna. It has not yet been reported from commercial avocado orchards in South Africa. An infestation of a commercial pecan orchard in the Northern Cape has been reported as well as a single pecan tree in Nelspruit.
Table 1. Agricultural tree hosts of PSHB that are cultivated in South Africa
|Reproductive host||Non-reproductive hosts|
*Figs were not found to be reproductive hosts in South Africa, but it is a reproductive host in other parts of the world
**Agricultural host crops from which the beetle has not formally been recorded in South Africa yet, but which are non-reproductive hosts
High risk for range expansion – evidence from experience and potential scenario
Oak trees have been declining and dying in Knysna. These trees were cut down, and logs were moved by the public over the wider Knysna area unbeknown that it harboured PSHB. This has caused a significant expansion on the range in which PSHB is found in the Knysna area.
In agriculture, wood chip mulch placed under trees in orchards have in recent years become standard practice as a measure to manage soil moisture. Notably, farmers have been encouraged to chop down invasive tree species to use for wood chip mulch. Several invasive tree species, however, are reproductive hosts of the beetle (Table 2). It is not implausible that an infested tree might be processed to wood chips and that infested wood chips may be placed under non-reproductive host trees in an orchard. This will expose the full orchard to potential infestation by the beetle and the subsequent risk of infection by the Fusarium fungus. Fusarium species are notoriously difficult to control and the fate of the trees, if susceptible to the fungus, will inevitably be to become ill and potentially die.
Table 2. The scientific and common names of trees that are exotic invasive species in South Africa that are also reproductive hosts of PSHB
|Scientific name||Common name|
|Acacia mearnsii||Black wattle|
|Acer buergerianum||Trident (Chinese) maple|
|Gleditsia triacanthos||Honey locust|
|Ricinus communis||Castor bean|
Symptoms of infestation
Infested trees may appear wilted or dying branches may be visible. Small “shot-hole” lesions at the point of entry/exit – up to 0.85mm in diameter might be visible (Figure 3). These entry/exit points could be surrounded by sugary exudates (Figure 3), gumming (Figure 4), blotches of resin, wood powder or wet stains. The bark surrounding the entry/exit point may become discoloured. This staining is caused by the spread of the Fusarium fungus along with the vascular system within the tree (Figure 5).
Monitor for the beetle
Suppliers of wood mulch and other raw wood products such as pallets and packaging and nurseries should routinely be monitored for the potential presence of PSHB. At the same time, the public should do all in their power to help survey and monitor public and urban areas, including those along with ports of entry and along highways. Strictly speaking, a national emergency action plan should be implemented, yet formal response up to now has been slow, and public participation is needed.
Preventative chemical control is most useful since the eradication of the beetle after the infestation is difficult. As yet, no products for chemical control of the beetle has been registered in South Africa although trial work for product registration is underway.
If plants are infested, removal and chipping of the wood is the best way to get rid of the beetle. The wood should be chipped to be smaller than 2cm, be solarised by placing it in the sun under a plastic sheet, or it could be burnt. Burning of infested wood is the most effective measure of control. The chipped wood material can also be kept wet in order to encourage decomposition.
Extreme caution should be taken when eradicating infested plants, since female beetles housed inside the tree may take flight. Therefore, it is best not to destroy infested material yourself. Rather contact with the correct institutions, service providers or authorities to support you.
Public awareness of the pest and participation in preventing further spread is crucial.
WHAT YOU SHOULD DO RIGHT NOW
Inspect all trees and wood in your vicinity for the presence of the beetle or symptoms of a potential infestation. Be 100% sure that wood is not infested before you transport it. Thus, if you buy firewood, be sure to check it is not infested before you take it to your home – remember the health of the trees in your garden is at stake, you might be introducing tree destroyers.
Report the infestation:
Throughout South Africa
Mobile App: www.TreeSurvey.co.za
Cape Town and Somerset West
Web Form: www.capetowninvasives.co.za
New regions and new host trees
Arrange for the wood to be destroyed by expert individuals that know how to handle infested wood
Inform others. The only way to keep South Africa’s trees healthy is by informing everybody of the risks that PSHB holds.
Contact the authors
Ida Wilson is crop health consultant at Agrimotion Consulting. Contact her at firstname.lastname@example.org
Hilton Fryer is the founder of Heuristic Guru, a data science and data analytics consultancy. Contact him on Hilton@pshb.co.za[/membership]
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