Platypus apicalis

Platypus apicalis, known by its common name the New Zealand pinhole boring beetle, is a wood boring beetle endemic to New Zealand and found throughout the North and South Island in a range of environments.

Description

Platypus apicalis is an elongated cylindrical beetle that is coloured dark brown.^[1] The species is small, measuring less than 2mm wide. Adults specimens have a head that is wider than it is long. The jaws of the insects are oriented downward and the antennae are flattened into a club like shape.^[1] Looking at the prothorax (segment of the thorax) which is cylindrical, there is a narrow groove. The base segment of the legs are short and thick while the terminal (end) parts of the legs are slender. The elytra (insect wing cases) have small ridges on them and are largely smooth with little hair.^[1]

There are differences between males and females. At the rear of the insect, males will have tooth like projections before gradually tapering off. Females on the other hand, will have significantly more hair, an absence of projections, and a more rounded shape.^[2] There is also a distinctive difference in smell. When the beetles emerge from their host trees, male specimens will release a distinct "foul" odour that is very strong and distinguishable. The female specimens have not been found to release this odour.^[2]

At various points in its life cycle, Platypus apicalis will take on several different appearances. The eggs are small, less than 1mm in length, and round with a creamy appearance.^[1] Young larvae are flat in shape and have fleshy projections coming off their sides. In contrast, fully grown larvae are white, cylindrical and legless with yellow coloured heads. At this point the jaws begin to orient downward and ridges start to appear on the top of the prothorax.^[2]

Distribution

Natural global range

The Platypus apicalis does not extend beyond New Zealand but the family it belongs to spreads out to various countries including North America and Canada.^[3][4] The New Zealand native has developed on its own away from its family to become an individual species inside its family taxonomy.^[3]

New Zealand range

The Platypus apicalis is found mainly in beech wood forests in New Zealand but has been found to invade some exotic plantations such as Eucalyptus spp.^[3] This puts the range for this species at the West coast of the South Island and central North Island of New Zealand. The fact that they can invade other species and not just Beech wood forest leads to the conclusion that they can extend their range beyond native forests. This has been the case with forestry plantations for high quality timber.^[3] The Platypus apicalis invades these plantations because the amount of available breeding material is high, allowing them to populate rapidly and dominate sectors of the forestry industry.^[3] They damage the industry by allowing fungi into the wood, altering the colour of the finished timber, weakening the structure of the timber by boring holes into the center of the tree, and creating imperfections with their tunnels in the finished timber.^[3]

Habitat preferences

The habitat preference of this species is predominantly dead beech wood forest, including Red Beech, Silver Beech and Black beech.^[3] The beetle has no preference for hard or soft wood as it resides inside the Beech trees and some Eucalyptus species. The Platypus resides in the dead wood of the tree but when numbers grow due to sufficient breeding material, healthy trees are threatened by their invasion.^[4] They are considered pests at times to native forests but only when they threaten healthy trees.^[3] Their main pest area is the forestry plantations where they colonise and render some high quality timber useless due to imperfections and colour change in the timber.^[3]

Life cycle/Phenology

The life cycle begins with a male adult boring a tunnel into a host tree and releasing fungal spores along the way. Fungal species, particularly ambrosia fungi, serve as the primary food for the beetle.^[5][6] This tunneling process usually occurs within the early summer months of November and December.^[1] Once the tunnel is several centimetres long, the insect will release an odour that acts as an attractant to female insects. If enough male beetles do this at once, it leads to so called "mass attacks" on trees where swarms of beetles attack a single host specimen.^[5] Copulation occurs at the tunnel entrance and afterward the female goes into the tunnel and begins the process of making a nest. The male continues to enlarge the tunnel and remove excess frass (excreta).^[5] The tunnel is initially radial but eventually has a sharp right angle and moves towards the heartwood of a tree.^[1] Meanwhile the female lays the first batch of egg containing four to seven eggs. Following this, another branch of the tunnel is then started by the male and eventually a second egg batch is laid. At this point 8–10 months have passed since copulation.^[1]

While the exact time of larval hatching is not yet certain, it is known that there is a period of two years from the time the nest is laid to the time that fully grown beetles emerge from hosts. During this time the larvae go through several stages. Initially the larva simply extend the tunnels of the parents.^[1] These extensions differ from the parents in that the tunnels are concave rather than flat. During this time, the larvae grow bigger in size and begin to take on some adult features.^[1] After a period of several months, the larva excavate pupal chambers within the tree.^[1] The larvae then enter the chambers and become pupas, intermediate insect form.^[1] Finally, when two years have passed since the egg laying, adult beetles emerge from the chambers and exit the tree. This occurs during the summer months of January to March. The average number of beetles per tree is 115.^[1] The adult beetles then go out into the forest and look for a new host tree. The lifecycle is then repeated. The average lifespan of an adult pinhole beetle is estimated to be 3–4 years.^[7]

Diet and foraging

Platypus apicalis is classified as an ambrosia wood boring beetle.^[5] Beetles in the category survive by boring tunnels into the xylem (water transport tissue) and phloem (food transport tissue) of a host tree species.^[6] Ambrosia beetles have a symbiotic relationship with a category of fungus known as Ambrosia. Fungal spores are contained within the gut of Ambrosia beetles and as beetles bore into a tree, they release the spores. The fungus is then cultivated within the tree and used by the beetles as a food sources.^[6] The xylem and phloem tissue of the tree is merely a medium for the fungus, it is not a source of food for the beetle.^[1]

Predators, parasites, and diseases

Along with the Ambrosia fungi, pathogenic fungi is associated with the fully grown beetles. This fungi has been known to infect and kill the host tree when it is attacked by the adult beetles.^[7] Populations of the insect are currently controlled by agricultural cultural practices, there are no effective predators of the insect, there is a species of parasitic nematode known to situate with the insect but its effectiveness in population control is unknown.^[7] Scientists have tested strains of B. bassiana, B. brongniartii and Metarhizium anisopliae which are found in southern beech forests or in places where no Platypus are found.^[4] They were tested on both the adult and larvae stages of the insect, a total of 10 isolates where tested and scientists found that all killed the adult stage of the insect.^[4] This provides a natural and common control method that could be used in the future to help control the population of the Platypus apicali.^[4]

Other information

Platypus apicalis has both biodiversity and economic impacts.^[1] From a biodiversity perspective, the beetles are a threat to beech trees. While some trees have been known to survive, in most cases the combination of the tunnels through the xylem and phloem and the subsequent fungal infection prove lethal. This and the fact that it has no known predators and few parasites makes it a potential threat to beech tree population numbers.^[1] There is also the threat of the species escaping New Zealand via exports of lumber. Beech species exist throughout much of the world and it is conceivable that the insect could survive quite well elsewhere.^[1] From an economic standpoint, the tree reduces the amount of native timber available.^[1] Even if a tree is not killed outright, the invasion often causes defects such as concealed rot pockets and irregular wood cores.^[1] Damaged trees are also easy targets for other species of fungi. This not only affects logging companies, but also associated industries.

References