Sirex—the timber bandit

Very elegant members of the insect family are the slender, agile ichneumon wasps, many of which are parasites and can be used in programmes of biological control. One of these however, Sirex noctilio, accidentally introduced into New Zealand from Australia or Europe in a cargo of timber before 1900, was by no means a welcome immigrant: it has the unfortunate habit of laying its eggs in radiata trees.

The female wasp has the remarkable adaptation of being able to bore into hard timber by using her long hair-like ovipositor. When boring she stands on her head, bringing the ovipositor forward between her hind legs, and thrusting the boring end of it rapidly into the tree. As with a dentist's drill, the drill of the Sirex ovipositor must be lubricated, and for this she produces a mucus, which as well as helping the drilling process, has a toxic effect on the tree.

The female is attracted to weak or damaged trees because of the scent they produce. Once she has found such a tree, she drills with her ovipositor to test its suitability. If it seems satisfactory she makes a number of holes, laying eggs in all but the last one. In pockets situated on each side of the end of her abdomen she carries a special fungus, and in the last hole she deposits some of the spores of this. The mucus she has used as a lubricant causes wilting and yellowing of the needles of the tree, so the Sirex infestation can be seen before long, both by these symptoms and by the series of holes in the trunk, which could be dripping with resin. The growing fungus dries out the wood and interferes with the conduction of water in the trunk: the combined effects of fungus and mucus can kill the tree before long, if conditions are right.

The eggs of the Sirex hatch in nine days or so, though they can remain dormant, which probably explains how they managed to live in timber long enough to invade New Zealand. Once hatched, the larvae tunnel through the wood, apparently living on the fungus and gaining all nourishment from this. When the larva is full-sized it will pupate. Before females emerge, some of the fungus will pass up the ovipositor and into the pockets of the abdomen that are prepared ready for the fungus spores.

Thus the adults, varying in size, but often reaching 36 mm in length, emerge from the trunk of the tree. They usually mate, so that the female has a store of sperms; but it is the way she lays her eggs, in the odd manner of all the wasp and bee family, that determines the sex of the offspring. If the egg is unfertilised, it develops into males only; if the egg is fertilised, the progeny will be either male or female. Thus a female Sirex, even if unmated, can still lay eggs in timber and these will grow into grubs.

Mated or unmated, the female will place fungus spores into the wood with her eggs. This is an amazing feat of biological cooperation, but one with undesirable consequences in the timber industry—although trees, to be badly affected by Sirex infestation, have to be unthrifty, crowded or badly damaged in some way—so a number of controls have been introduced to combat the insect. Biological control, with good forest management, is the only answer, as, obviously, spraying with insecticides has no effect on the grubs boring happily away deep inside the trunk of the tree.