Pinhole borer found in oak tree
A DUNEDIN arborist called at the museum recently to report that the trunk of a mature healthy oak tree on a Mornington property was riddled with 2mmwide holes. At the foot of the trunk were conspicuous 20cmdeep piles of whitish powdery sawdust.
When I visited the tree, adult males and females of the native pinhole borer Platypus apicalis
(subfamily: Platypodinae; family: Curculionidae) were walking on the trunk near the holes. This beetle is an elongated cylindrical highly specialised weevil, just under 2mm wide and 67mm long, and glossy reddish brown to dark brown. It is presocial: adults and larvae both communicate, the latter with clicks.
Both adults and larvae make complex systems of branched cylindrical tunnels, termed nests, which are 2mm wide throughout. These tunnel systems are made in both healthy and weakened living trees, in stumps, and in freshly felled logs. The beetles do not eat wood — this being pushed out of the burrows as sawdust. Instead, both adults and larvae feed on yeasts (singlecelled fungi) that completely coat the walls of fresh tunnels. A beetle that feeds in this way is termed an ‘‘ambrosia feeder’’ and carries internally the yeasts with which it infects its burrow, and thereby supplies its own food.
Platypus apicalis nests in all species of native beech and kamahi, and introduced trees such as Spanish chestnut and coast redwood. Trees may die as a fungus pathogenic to sapwood spreads from its burrows.
Nests are begun by male beetles attracted to volatile substances released by trees under stress. As it begins its nest, the male beetle produces pheromones (a chemical substance produced by an animal that influences the behaviour of another) that attract both other males and females, which results in a concentrated attack on the selected tree.
The male bores a radial entrance hole. He is joined by a female that mates with him and takes over excavation, while the male ejects the sawdust from the tunnel. The tunnel extends to the dry inner sapwood, then turns at right angles and follows the boundary between sapwood and heartwood. Eggs are laid at the end of the tunnel, after which a further tunnel is made in the opposite direction, and more eggs are laid. The larvae eat yeast on the walls and further extend the burrows, or start new burrows inside the tree. When ready to pupate, the larvae construct short tunnels parallel to the woodgrain. The life cycle takes one and ahalf to two years. Adult beetles live for up to six years.