Agrilus

Under-Bark Borers of Kamarere and Talis
(Agrilus viridissimus and Agrilus opulentus)
(Coleoptera: Buprestidae)

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Common pests of the two most widely grown tree species in the lowlands of Papua New Guinea are jewel beetles (Buprestidae). Larvae of Agrilus opulentus affect kamarere (Eucalyptus deglupta) whereas the larvae of Agrilus viridissimus attack swamp talis (Terminalia brassii). Agrilus larvae feed on the cambium of their living host. Adults of both species look very similar, are of about 10 mm length and of shiny green or blue colour with orange or silver spots on the back. The adults can be easily distinguished since A. opulentus has two orange spots on the prothorax. The white larvae of both species are approximately 2.5 to 3 cm long and narrow except for the enlarged thoracic segments. The adult females lay eggs singly into cracks of the bark of their favourite host. When the young grubs hatch, they tunnel into the cambium, where they feed and grow bigger. The zigzag-like tunnels in the cambium are clearly visible on the thin bark of Eucalyptus deglupta (A), but invisible in Terminalia brassii due to its thicker bark. The mature grubs (B) bore radially into the wood and excavate a chamber for pupation (C). After the final emergence the young adults bore their way to the ‘surface’ through the wood and bark, leaving a semicircular hole (D). The adult beetles are associated with the crown of the host tree where they feed on young foliage, before they can become sexually mature and eventually reproduce. They can also be found feeding on regenerating shoots in logged forests. The time span from the egg to the adult depends on the diameter of the host tree and on whether the tree is felled or living. The development from the egg to the adult takes about 6 to 7 weeks in a felled tree of 30 cm diameter. The bigger the diameter, the longer it takes for the beetles to develop. The wild host in the natural forest is lau-lau (Syzygium) which belongs like Eucalyptus to the family Myrtaceae.

Agrilus viridissimus (left) and A. opulentus (right) with two orange spots on the prothorax
(drawings reproduced from Roberts, H., 1987)

Since Eucalyptus deglupta and Terminalia brassii are the major tree crops of lowland plantations in PNG, Agrilus can cause considerable damage worth millions of Kina, particularly on kamarere. A severe infestation by the larvae of either species disrupts the conducting tissues causing the loss of increment and might even result in the death of the host. The zigzag-like feeding in the cambium can be compared with ring-barking. The signs of A. opulentus on kamarere are zigzag-like tunnels visible on the bark (A), that indicate a previous infestation. The cambium-feeding of the larvae results in callus formation after the attack. Semicircular emergence holes (D) of the adults are visible in bark of the trunk. Furthermore, epicormic shoots develop from the lower trunk. An infestation of swamp talis by A. viridissimus is usually less conspicuous than in kamarere and there are usually only semicircular emergence holes visible in the bark of the trunk. Agrilus seems to be specific to a particular host family. An attack of Terminalia by A. opulentus is usually less successful due to the rapid cell division of the host and the copious sap that is produced by the tree killing the larvae.

Life cycle of the under-bark borer Agrilus opulentus on Eucalyptus deglupta
(reproduced from Roberts, H., 1987)

There is a number of factors encouraging the infestation of kamarere and swamp talis, like unsuitable soil conditions ie. badly drained soils for kamarere and soils drying out too rapidly as in the case of swamp talis. Unhealthy young trees are often severely infested and diseased. Old and dying trees are also much more susceptible. In general, any silvicultural treatment that puts the residual trees under stress increases the risk of an Agrilus infestation. Apart from general preventive measures, the following strategies are recommended to minimise and control Agrilus-related problems:

use Agrilus-resistant varieties from Mindanao in the Philippines
select a suitable site for planting
line planting with a spacing of 10 m x 2 m results in fewer attacks than planting the trees with a spacing of 4 m x 4 m
an understorey in the plantation favours predators and other beneficial animals and furthermore makes mate-finding difficult for Agrilus
mixed stands should be preferred and pure stands of kamarere should not exceed 100 ha
remove all small diameter trees that show suppressed growth, preferably by burning or debarking
remove alternative or wild host trees in adjacent areas. This does not seem to be feasible because the local people won’t be very happy if their fruit trees are removed
minimise tending after the first year
after thinning, remove stumps to avoid the regeneration of shoots as an attractive source of food for the adult beetles
carry out pruning and thinning only during the dry season. By doing so the logs dry out immediately and are no longer suitable for the beetles to breed
remove the logs immediately from the plantation after commercial thinning
if possible, debark stems that are left in the plantation, so that the logs are no longer suitable for the beetles to develop
chemical control methods are available but are extremely expensive, time consuming and sophisticated. Systemic insecticides like acephate (Ortheneâ ) can be injected into the trunk. This is usually done only to protect valuable individual seed trees and is not feasible on a large scale in a plantation.

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