(Vanapa oberthuri Pouillaude)
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|The Hoop pine weevil Vanapa
launched a serious attack in the plantations around
Bulolo between 1962 and 1969. At one stage 1,500 trees
had to be removed and burnt or debarked. The weevils
started their attack after a pruning operation. Another
outbreak occurred after thinning operations, so that
these had to be halted. A study revealed that Hoop pine
is hardly attacked in its natural stands. In plantations
the damage was lowest in trees aged four to seven years
and highest in trees between nine and 22 years. The
damage usually occurs clumped in groups of up to ten
trees. Secondary Curculionidae, Cerambycidae,
Platypodidae and Scolytidae pests are
quickly attracted to trees that are infested by Vanapa.
The attack can result in the quick death of the host
after five months, but vigorous trees survive the
infestation by drowning the larvae in a copious flow of
resin. According to the literature, Hoop pine seems to be
the only host of Vanapa, however, after the fire
in McAdam National Park in 1997, many of the gigantic
Klinkii were infested by Vanapa.
The black adult weevils are conspicuous due to their variable body length of up to 7 cm and the size of their curved long proboscis. The females lay their eggs preferably on the margin of resin exuding from pruning nodes or other injuries. If no resin is present the eggs are either laid under loose bark or on to the outer bark of the stem and then covered with a cemented layer of masticated bark. The oviposition sites are prepared by the help of the strong rostrum. Ten days after oviposition, the newly hatched larvae excavate a tunnel into the outer bark. After about six weeks, when the larvae have reached the third instar, they bore deeper into the fibrous bark and the wood. Initially the tunnel is dug horizontally, then curves upwards and finally turns outwards. The J-shaped tunnel reaches to a depth of about 4 cm into the wood. If many larvae infest the same stem, a crushing noise produced by the larvae is audible. After approximately five months the larvae pupate in a pupal chamber that is plugged with slivers of wood. The pupal stage takes about one month. The newly hatched adults stay for another two to four weeks in the pupal chamber before they chew their way outside through an emergence hole with a diameter of 0.8 to 1.3 cm. The whole life cycle takes about six months but can last eight months if the conditions of the hosts bark are less favourable. The life span of an adult is between six and eight weeks. The larvae feed only on wood whereas the adults eat the green bark of the pine twigs.
Life cycle of the Hoop pine weevil Vanapa oberthuri (reproduced from Gray, B., 1968)
Successful control requires regular monitoring of the Hoop pine weevil populations. This is unfortunately difficult and expensive. Deposits of clean wood debris on the bark and the occurrence of dead branches are typical signs of a Vanapa attack. For the control of Vanapa the felling of the infested trees is suggested. These trees have to be removed by either burning or debarking. The debarked tree should be laid on the open ground so that all eggs and larvae are killed by the heat of the sun and by desiccation. The application of various insecticides dissolved in creosote on the pruning scars showed little effect in preventing Vanapa attack, probably due to the washing off of the insecticides by rain. However, pure creosote, a repellent against Vanapa, was used for a long time to treat pruning scars and injuries inflicted during thinning. Biocontrol agents are not available, even though predacious click beetle larvae (Elateridae) and a virus are known natural enemies of Vanapa oberthuri. The most feasible remedies see to preventive measures.
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© Michael F. Schneider, 1999