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.
*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