Insect guide > Flies > Horn Tails

Horn Tails

(Super-family Siricoidea.)
These insects form an old series of Hymenoptera, known as the wood-eaters Xylophaga. They are distinguished from the true saw-flies by the fact that the foreshanks have only one spur at the tip instead of two. They have the same broad abdomen and broad head and thorax.

The group includes the families Oryssidae, Siricidae, Xiphydriidae and Cephidae. The larvae of all of these insects are wood­borers, living in the stems of plants, and even in the trunks of trees. The adult flies are called horn tails, because the end of the body usually bears a spine or horn. The ovipositor is fitted for boring instead of sawing, and with it the female bores into woody tissue and lays her eggs.

The group is not a very large or a very important one, although it contains many common species. A noted example is the European Cephus pygmaeus, which bores into the stems of wheat. This species was accidentally introduced into this country some years ago, and is now found in portions of Canada and New York State. Its damage, however, has not attracted the attention of farmers of late. The large pigeon Tremex (Tremex columba) is a not uncommon enemy to shade trees in certain of the northern states. It attacks the elm, oak, sycamore, and several varieties of maple. The holes of this borer may be recognized by their regular, evenly-cut shape, about the diameter of a lead pencil. Isolated shade trees along roads and in streets are favorite habitats. The writer, as a boy, saw them in great numbers in the maple trees on the grounds of the old Ithaca Academy, at Ithaca, New York. In midsummer a large number of females would be seen boring into the trunk of a single tree, laying their eggs. The female plunges her borer perpendicularly into the trunks, holding it at right angles to the abdomen. The insertion requires evidently great muscular effort, and the egg is deposited at the bottom.

It was a common sight to see females which, after laying, had been unable to withdraw the ovipositor, so that they had been held to the trunk until they died. The eggs are oblong-oval and pointed at each end.

The common parasites of the larvae of these horn tails are the very large and extremely long tailed Ichneumon flies known as Thalessa lunator and Thalessa atrala. These Ichneumon flies lay their eggs in the burrows of the Tremex, and their larva; feed upon Tremex larvae.

The exact facts concerning the life of the Tremex larvae in trunk of the tree have not been studied, and the species is such a common one that it will be an interesting matter for some observer to work out the life history in detail.

Life History of a Horn-Tail
(Phylloecus integer, Norton.)
This insect, which is known as the willow-shoot horn tail, is found throughout the eastern part of the United States. The female, after boring a hole some inches below the tip of a willow twig, pushes her ovipositor in an oblique direction into the pith of the twig, inserting the eggs at the bottom of the puncture.

She then girdles the twig below the eggs to prevent it from growing any farther, obviously to prevent the egg from being crushed by the rapid growth of the plant. After a week the eggs hatch, the young larvae bore their way down through the pith to a distance sometimes more than two feet, filling the channel behind them with their excrement as they proceed. The eggs having been laid in the spring, the larvae feed all through the summer and become full-grown in late fall, filling the lower end of the burrow for half an inch with frass. They then eat a passage through the side of the twig about a quartet of an inch above the prospective cocoon, but without cutting through the bark. Then the cocoon is spun in the burrow and the larva remains within it all through the winter, changing to pupa early in the spring. In young willow groves, the shoots of which are intended for basket-making, the withered tips should be pruned off as soon as noticed in the spring.