Sirex
A genus of Sawflies, the larvae of which feed in the wood of Conifers, in which they bore tunnels, often about 1/3in. wide. They thus inujure the trees, and ruin the wood for carpentry. Only two species are known as British; these are S. juvencus (the Steel-blue Sirex) and S. gigas (the Giant Sirex). They agree in general form. The body is nearly cylindrical. In the female, the last ring of the abdomen bears a strong spine, directed backwards; and from the lower surface of the abdomen arises a strong ovipositor, also pointed backwards, in which lies the "saw," used for boring into tree-trunks. The ovipositor is about half as long as the body. The four wings are large, powerful, and transparent. The legs and antennae are also well developed. The length, without the ovipositor, varies from about 1/2in. to 1 1/2in., but is usually over 1in. The mode of life is as follows: The female bores with her saw into the bark of trees, preferring sickly ones if they are to be found, and deposits in each hole an egg, from which, after a time, a white, soft, cylindrical larva emerges. The larva tunnels in the wood of the tree. The duration of the larval stage is uncertain. Some believe that it is passed through in a few weeks; but the perfect insects have emerged at intervals for many years from wood to which the larvae could not have got access after the trees had been felled and sawn into planks. The larvae have strong jaws, six very small feet near the head, and a bluntly-pointed tail. They change in their tunnels into pupae. From these the perfect insects emerge, from July to September.
S. juvencus, or the "Steel-blue Sirex," is, as the name denotes, usually of a dark steel-blue colour, with redbrown feet; in the males, several segments of the abdomen are rusty-red. The wings, in both sexes, are yellowish, with smoky hind margin. The females are usually 1 1/4in. long, the males rather below 1in. This insect is not rare. It prefers Scotch Firs, though it also feeds in other Conifers.
S. gigas, or the Giant Sirex, is rather larger than the other species, from which it also differs in colour. It is ringed with black and yellow; the latter colour is duller in the males. This species is less common than the former. The larvae are said not to attack Scotch Firs; but feed in Spruce, Silver Firs, and occasionally in Larch.
Remedies. It is not possible to destroy the larvae in infested trees. To prevent the injury from spreading, sickly trees, and all fallen branches and trunks, should be cut up and removed; and this should be done also with all trees that show traces of serious injury, in the form of holes through which the insects have escaped. The timber of such trees is of little value, save as firewood, because of the injury done to it by the larvae.