Sirex nematode loses potency in culture in Australia
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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.
From Forest Health News No. 43, June 1995.
In 1962 a new species of nematode Beddingia siricidicola, was discovered in the reproductive organs of Sirex noctilio in New Zealand. It was found that the New Zealand strain of this nematode was capable of sterilising over 90% of sirex females and was present throughout North Island pine forests. Beddingia was absent from the South Island sirex population and was deliberately introduced there as a biocontrol agent.
Initially the nematodes live in the tree feeding on the sirex fungus Amylostereum areolatum . When a nematode encounters a sirex larva it turns into an infective form and invades the larva. The infective nematode reproduces and the young nematodes enter and cause degeneration of the reproductive organs of the developing adult insect. Nematode dispersal occurs when infected sirex females lay nematode filled eggs in Beddingia free trees. Since Beddingia can complete its life cycle without infecting an insect it can be maintained continuously in culture. A culturing procedure and tree-injection technique were devised in Australia for introducing the nematode into trees in newly infested areas. In this way massive outbreaks of sirex were rapidly brought under control.
Twenty years ago inoculations resulted in nearly 100% parasitism, however recently levels of parasitism have dropped to less than 25%. According to an Australian paper published in 1993 declining parasitism in inoculated logs is almost certainly a result of genetic change in the cultured nematodes. Over this time the nematode cultures have gone through many hundreds of generations without developing into the infective form and this has resulted in a steadily decreasing ability to cause sterilisation. In contrast New Zealand cultures were started each year from freshly collected nematodes so genetic weakening has been avoided. From now on in Australia cultures will be started each year from fresh material collected from Tasmanian forests where nematode injection last occurred in 1970.
Mike Nuttall, FRI
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