|December 2002. Niue asked about borers in mahogany. The Forestry Division of Niue is thinning mahogany and has left some of the cut branches on the ground. They have noticed that wood borers are eating the cut branches. There is concern that these insects will eventually move onto the live trees. Therefore, they are very interested to know how they should monitor the damage that might be done. Also, could anyone suggest how to trap the insects adn what insecticides could be used for monitoring purposes.
PestNet members explained that the insect borers of mahogany are Ambrosia beetles (Scolytidae and Platypodidae) and are quite common in Fiji’s mahogany plantations. These beetles attack disturbed/stressed plantations and wherever mahogany debris is left after logging or the trees are weakened after fungal infection. The best method of collecting the beetles is by light trapping at night as these beetles commonly fly at dusk or between 6 pm and 9 pm.
Drawings of the beetles were sent (above). Some general information was also provided on the problem in Fiji. Ambrosia beetles rarely attack old mahogany plantations, they are more common in those up to 12 years. Five species of Platypodidae (3 genera) and 103 species of Scolytidae (21 genera) have been recorded in Fiji (revised list 21/05/97). The main cause of infestation is reported to be associated with interference or actual destruction of the plantation’s vegetation matrix. The intensity of attack is usually dependent on the extent of disturbance. A few species are more common after such disturbance and occur in large numbers. Ambrosia infestations have also been recorded from nurseries containing mahogany seedlings (1-2 years’ old) which were water stressed.
Light trapping: The method currently used in Fiji consists of a white sheet of 1.65 m by 0.65 m that is tied vertically between a wooden frame. The lamp used to illuminate the surface of the sheet is a 125 Watt high pressure mercury vapour bulb with internal phosphor coating. This is placed about 1/3 distance from the top of the cloth. The lamp is powered by a petrol driven generator and the circuit has a control gear (Watt Saver 125 M). The most abundant species coming into light traps is Xyleborus perforans (Scolytidae) which varies between 1180 to 20,059 within a year per site. Other common ones attracted in large numbers to light are Xyleborus volvulus and Hadodemus fijianus. Overall, 18 to 25 different species were collected in light traps in mahogany plantations (just two sites and once per week) within a year. The disturbed site reported much higher incidence of Ambrosia compared to the undisturbed site.
The biology of these beetles can be assessed using infested billets collected from the plantation and reared in cages in the lab. These billets may be kept moist but not wet by spraying with water every 3-4 days.
An unpublished report by Dr. M. K. Kamath, the previous Fiji’s forest entomologist mentions that gummation in mahogany is responsible for naturally controlling these beetles. Apart from these, a wood swallow, skink and gecko were seen feeding on these beetles as well as insects from Cleridae, Brentidae and Colydiidae. As for biocontrol, very little info is available save the mention of a few, for example, a Hymenopterous parasite of the genus Perilampid and a coleopteran predator of the Colydiid genus Sosylus.