The Euwallacea gang
This case study is a bit different to the others written up so far. It involves a symbiotic relationship between a complex of beetles and a fungus- Euwallacea fornicatus and Fusarium euwallacea, hence the name of the case study. And while there is little reporting on damage to a particular ecosystem, large numbers of trees species, both wild and cultivated are being effected.
An interesting case of the complexities that can arise in ecology.
The Misplaced
Appearance
The beetle component of this partnership is a type of ambrosia beetle and is a complex of very similar types which may, in fact, be four separate species which are indistinguishable from each other in terms of appearance. For the sake of simplicity, let's just call them PSHB which stands for Polyphagous Shot Hole Borer. Polyphagous is referring to a wide range of target plants as outlined below. They originate in parts of South East Asia.
The beetles are dark coloured. As insects, they have a head, thorax and abdomen. There are 6 legs attached to the thorax, and wings under a hard casing covering the abdomen. Note that the wings are only functional in the females.
Size: females average 2.5mm long, males 1.6mm long
The borers are inconspicuous due to their size, but also because they spend the vast majority of their lives inside tunnels that they create in the branches and trunks of trees. They are most likely to come to notice due to the small holes they leave in the surface of infected trees.
The fungus component is member of the Fusarium genus which contains many species that cause wilting and dieback in plants. It reproduces via spores. Fungal growth is influenced by temperature, with optimum growth of F. euwallaceae occurring around 27°C.
Sexual Reproduction
An adult female selects a site to begin boring a new tunnel. The eggs are laid as soon the tunnelling stops. A single female initiating a new tunnel in a tree lays an average of 32 eggs, with the first adults developing in as little as 22 days at 24°C. The eggs hatch into a larvae (grub) which molt twice as they grow. Larvae then goes through pupation, altering body tissues to take on the adult form.
The fungal spores hitch a ride on the female beetles when they move to new locations.
Ecology
Infested wood generally contains multiple colonies from numerous females and population sizes can be tremendously large and self-sustaining over relatively long periods of time. The beetles spend almost all their life in the tunnels that they create in the tree branches but that does not mean they are totally untouched by environmental conditions. Both the body length of the insect and the life cycle duration is influenced by temperature and the variety and condition of the host trees.
Beetles try to infect a range of trees in a new environment.
- In some trees, the fungus does not take.
- In others, the fungus might settle in but in such a way as to not allow the beetle to reproduce.
- In the best case for the Euwallacea gang, the fungus thrives and the beetle is able to complete life cycles to produce more beetles.
Nutrition
The borers change the woody tissue of host trees into frass , but that does not mean that the beetle is eating the wood. It is the fungus, Fusarium euwallacea that eats the wood. E. fornicatus eats the fungus itself, both at the larval stage and as an adult.
The beetle, the fungus, the tunnel and the frass
Dispersal
Active flight by the females is one of the main means of movement to previously uninfected areas. Adult females can fly up to 400 m, but usually will attack hosts in a range of 35m. frass
However, human intervention has resulted in the increasing global movement of commodities, which has increased significantly the transport of this and related species in timber and wood packaging material. This means that countries across the globe have been invaded by these borers.
The Displaced
The range of host trees which may be infected by these borers is thought to be in the vicinity of 500 different species of trees. They don't call them polyphagous without reason. Trees under cultivation and trees in the wild have both been killed or injured by infections.
A list of known host species can be found at https://gd.eppo.int/taxon/FUSAEW/datasheet
The Consequences
Given a wide choice of hosts species and a lack of both competitors and predators, PSHBs have the potential to spread quickly causing significant disruption in a new location.
These invasions have been studied in urban and agricultural environments but there have not been extensive studies in natural ecosystems.
One study, that has been carried out in California, recorded the impact of shot hole beetles in a river valley community. ( The impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California )
Findings of the study include the following
- extensive mortality of trees (especially members of certain families)
- infestation rates were highest in units that were nearest water
- the infestation has altered the structure of the forest canopy, which is likely to promote the growth of invasive plant species that were relatively inconspicuous in the forests prior to the beetle attack
Global Distribution
The fungus is reliant on the beetle for transport, so this list also represents the beetle distribution outside of its native range.
- Argentina
- Mexico
- Israel
- South Africa
- the United States of America (California and Hawaii)
- Australia (Western Australia)
There have also been sporadic outbreaks in greenhouses in several European countries. These infestations are transient as the infected plants are destroyed immediately upon discovery.
Often the first outbreaks are around port areas, indicating that human mediated dispersal via ships and planes play a significant role.
Case study - California, USA
Case study, South Africa
Since its first detection in 2012, PSHB has spread to all but one province in the country. The accompanying map was constructed in 2021 and published in The Polyphagous Shot Hole Borer (PSHB) beetle: current status of a perfect invader in South Africa
Potential transport vectors for PSHB include airplanes, trains, trucks, cars, and boats. PSHB was most likely introduced into South Africa via ships as the earliest known record for it is close to two major ports. Subsequent spread throughout the country was probably aided by the movement of infested wood (e.g. firewood).
Controlling the invaders
The best methods for controlling this invasive so far involve inspection and quarantine. Monitoring of trees for signs of early invasion should be carried out. Inspection of wood products being transported within a country are needed and strict quarantine regulations should be put in place.
One example of this approach is the reaction of finding the beetle in Australia. The presence of the beetle in Australia has prompted the species to be listed on The National Priority List of Exotic Environmental Pests, Weeds and Diseases and strict quarantine conditions have been put in place to restrict the spread of the beetle/fungus team.
Where invasion has taken place, removal and burning of infected wood is regarded as the best option. There are no universally agreed-upon, effective chemical or biological remedies currently available.
The term 'euwallacea" is derived from the name of the scientist Alfred Russel Wallace who was an English naturalist, explorer and illustrator active across Asia and South America in the 19th century.
He proposed the idea of natural selection independently of Charles Darwin, and was extremely instrumental in the establishing the field of biogeography.