Horntails are wasplike insects with a taillike spine that projects from the tip of the abdomen. They have cylindrical bodies and lack the narrow waist so common in wasps. They are usually black or brown, sometimes with rust, orange, or yellow markings. Some species are fairly large. Males and females both have a hornlike spine at the abdomen tip; females have an additional projection below, which is the ovipositor, used for laying eggs.
Horntails do not sting or bite.
Larvae are pale yellow or white segmented grubs with 6 tiny legs. They are rarely seen because they bore in the wood of trees.
A number of species might possibly be found in Missouri.
- The pigeon tremex, or pigeon horntail (Tremex columba) is variable in coloration, ranging from dark to light brown, often with yellow bands on the abdomen and dark or amber-colored wings. Antennae are brown and relatively short and blunt. This horntail can be 2 inches long. Because it is one of the few horntails that prefers deciduous trees, and not conifers, and because Missouri's forests are mostly of deciduous trees, this is the horntail most commonly encountered in our state. The larvae can damage wood of living trees, but this species usually selects declining trees and not healthy ones. Their numbers are usually low enough that they are not a serious pest.
- Urocerus taxodii has yellow-tipped antennae and a pale spot on each side of the head. Females are black or dark-bodied with dark wings and a yellowish horntail. Males are orange- or yellowish-bodied with paler to clear wings. This species lives on bald cypress trees, which is common in southeast Missouri and planted statewide.
- Urocerus albicornis is a black horntail with 2 white or yellowish bands on each leg, white or yellowish antennae with black tips and bases, a white or yellow spot on either side of the head, and brown wings. It sticks to areas with conifers (pines, cedars, and so on).
- The sirex woodwasp or European woodwasp (Sirex noctilio) is an invasive nonnative species that has appeared in the United States. This species is known to cause the death of up to 80 percent of the pine trees in an area, and Americans should be aware of it. Adults have metallic bluish-black bodies with reddish-yellow legs, black feet, and black antennae. Males have an orange band round the abdomen and black hind legs.
- The Asian horntail (Eriotremex formosanus) is native to southeast Asia and has been accidentally introduced to North America in shipping crates or other imported wood products. On our continent, it mainly is known from US southeastern coastal areas ranging from Texas to Florida to Virginia, though it has been found in Utah and Arizona. It could potentially occur anyplace that receives shipments from overseas. Fortunately, its larvae mainly eat dead wood or wood from dying trees, so it is not a serious pest or threat to our forests. It is a dark brown and yellow-striped wasp with short, black, blunt antennae and long yellow hairs on the top side of the abdomen. Like the sirex woodwasp, it could potentially appear in our state.
Length: ½ to 2 inches (varies with species).
Habitat and Conservation
Horntails may be found nearly anyplace with trees but are rarely numerous. Some species lay eggs in hardwood and others in coniferous trees. Historically, Missouri had large tracts of native shortleaf pine forest, but those are mostly gone today. Because most North American species of horntails require pines and other conifer trees, most North American horntail species occur in regions to our north, northeast, southeast, and west, where pine forests are common.
Missouri's most common species, the pigeon tremex, prefers maples, oaks, elms, sycamore, hickories, apple, pear, and hackberry. When laying eggs, horntails tend to select trees that are weakened or dying. The tunneling of the larvae can weaken wood, making it prone to further disease or damage. Sometimes they lay eggs on recently cut wood.
The young of horntails are wood borers, tunneling into the sapwood and heartwood of trees as they eat. Like several other insects that eat wood, larval horntails possess a symbiotic fungus that helps them digest the tough materials wood is made of.
Horntails cannot sting people, even though the horntail looks pretty fierce. The appendage (ovipositor) at the female's abdomen tip is used to insert eggs into trees.
After finding a host tree, the mother horntail uses her ovipositor to bore through the bark and inject eggs, one by one, as much as ¾ inch into a tree. After about a month, the eggs hatch into grublike larvae that eat wood, tunneling into the heartwood. They can grow for 1 to 3 years before pupating. Prior to pupation, they tunnel to just beneath the bark. When pupation is complete in late summer, they emerge as winged adults, chewing their way out of the bark, and repeat the cycle.
Horntails usually do not occur in large enough numbers to be serious pests, though the boring of the larvae does damage trees. Typically, they select declining trees and not healthy ones.
The species name of the pigeon tremex, columba, means pigeon or dove. It's not clear how it came to be applied to this species. With their large, heavy bodies, pigeon horntails are not the most graceful of insects.
Like many other organisms that feed on dying trees, horntails help begin the process the recycles wood back into soil. Larval horntails are not safe in their tunnels; they are eaten by larval ichneumons, whose mothers also possess ovipositors for boring into wood.
If you think the horn tail of horntails is freaky, you should check out the ovipositor of the ichneumon wasps that prey on horntails. Female giant ichneumons (genus Megarhyssa) somehow are able to detect the larval horntails within a tree; they insert their super long, needle-like ovipositors into the tree and stick their eggs onto or into the horntail larvae within. Their larvae parasitize and kill the horntail larvae.