Pigeon Horntail Wasp Ovipositing

I first wrote about the Pigeon Tremex horntail wasp, Tremex columba, back in September, 2012. At that time I had not seen a female of that species here in Colorado. That all changed on the afternoon of August 22 when I witnessed two females of this species laying eggs (ovipositing) in an elm tree at my workplace, a plant nursery here in Colorado Springs.

Horntail wasps belong to the family Siricidae. They are mostly large, cylindrical wasps, the females of which have intimidating spear-like organs protruding from the end of the abdomen. They use this "ovipositor" and its associated appendages to bore into trees and lay their eggs. The larvae that hatch tunnel through the wood, aided by a wood-softening fungus that is deposited by their mother at the time she lays her eggs.

Close-up of ovipositor apparatus

Horntails get their name not from the ovipositor, but from a knob or spur at the tip of the abdomen above the ovipositor in females, but present in both genders. This “cornus” is a feature of the larva, too, aiding the grub in locomotion. The function of the cornus in adult wasps is apparently unknown.

The video below shows the female wasp already engaged in oviposition. Her ovipositor is the black rod perpendicular to her body under her hind leg. The rhythmic movements of her abdomen probably help drive the organ deeper into the wood. The pair of pale appendages extending from the tip of her abdomen function as a sheath that protects the ovipositor when it is not in use.

One reason the mother horntail attempts to lay her eggs deep inside the tree is to keep her offspring out of reach of giant ichneumon wasps and ibaliid wasps that lay their eggs on horntail grubs.

Female giant ichneumon wasp, Megarhyssa sp., from Colorado

Most horntail species are not considered pests because they will lay their eggs in dead, dying, or weakened trees. A handful of species have made their way to the United States from Asia and elsewhere, and those are invasive species that have no known natural enemies here in North America.

The favorite hosts for the Pigeon Tremex are maple and beech, but obviously elm is also acceptable, along with apple, poplar, oak, and other hardwoods. It takes one to two years for a horntail wasp to complete metamorphosis from egg to adult.

Sometimes, the female wasp dies in the act of ovipositing, or is consumed by a predator during this lengthy period of vulnerability. Indeed, on September 4 I found a deceased wasp still embedded in the tree where she was ovipositing; and nearby were the remains of another, just the needle-like ovipositor protruding from the tree trunk.

Deceased female horntail stuck in tree

This is the time of year to be watching for the Pigeon Tremex in your neck of the woods. Watch for their giant ichneumon parasites, too. You might have to spend a lot of time looking up and down dead trees, though.

Source: Eaton, Eric R. 2014. "The Wasp and the Fungus," Northern Woodlands Educators.

Wasp Wednesday: Pigeon Tremex Horntail

People who have minimal knowledge of wasps can’t really be faulted for having a startled or panicked reaction when confronted with one of these creatures, especially if it is a large insect. Among the more intimidating of wasps are the horntails in the family Siricidae. They do not sting, but they look like they can. Here in the Front Range of the Rockies, the most common species of Siricidae is the “Pigeon Tremex,” Tremex Columba.

So far, I have encountered only male specimens, resting quietly on foliage at about eye-level. They can be approached easily and closely if you are careful. This is a native species ranging widely from Nova Scotia, New Brunswick and Quebec, south to Florida, and west to Mexico, Arizona, Utah, and Colorado. There are even a few records from southern California.

Siricids in general are more closely related to sawflies than any other wasps. The abdomen is joined broadly to the thorax, giving the entire creature a cigar-shaped appearance. They are about a third the size of a stogie, too. Adult females range from 37-50 millimeters (1.5-2.0 inches). Males average smaller, from 18-37 millimeters. Note that western specimens of Tremex Columba are usually much paler in coloration than eastern specimens.

© Andrew Williams

The females are equipped with what looks like two stingers, one short one protruding from the top of the tip of the abdomen, and another longer, needle-like rod at the bottom of the abdomen. The wasps are, in fact, non-venomous. The top “horn” is called the “cornus,” and both genders possess this spur, which gives the family its common name of “horntails.” I have been unable to find an explanation of the function of the cornus, if there is one. The longer appendage is an egg-laying organ called the “ovipositor.” The female uses this complex part of her anatomy to insert eggs into dead, dying, or weakened hardwood trees. Maple, beech, elm, apple, pear, poplar, oak, hickory, sycamore, and hackberry are all known hosts of this wasp, but other deciduous trees in poor condition are probably utilized as well.

The ovipositor acts like a drill and hypodermic needle. It is housed in a two-part sheath that helps to brace the sawtoothed egg-laying organ as it works its way into bark and wood. Lumberjacks have a vulgar nickname for these wasps, owing to what the wasps appears to be doing when they oviposit into a bole or stump. I’ll leave that epithet to your imagination.

© Andrew Williams

Not only does the wasp lay her eggs in the tree, she also delivers a wood-rotting fungus, Cerrina unicolor to the site of each egg insertion. She stores the fungus in special abdominal glands called mycangia, until she is ready to deploy it. The fungus breaks down cellulose, and both fungus and decayed wood are then consumed by the larval wasp.

Larval horntails are grub-like, and easily mistaken for beetle grubs save for the cornus on the very tip of their worm-like bodies. It usually takes more than one year for the horntail to complete metamorphosis.

© Project Gutenberg (Google)

You would think that a larva tunneling inside a tree or log would be safe from its enemies, but such is not the case. Huge wasps called giant ichneumons can drill down and reach the horntail grubs. One of my most popular blog entries chronicles the life cycle of these Megarhyssa wasps. Yet another kind of wasp attacks younger larval stages of horntails. Wasps in the family Ibaliidae (and genus Ibalia) drill down to reach horntail larvae that are at a more shallow depth.

Female Ibalia sp. © Mark MacMillan/Colorado State University

Pigeon Tremex horntails that survive the perils of youth eventually pupate, emerging as adults most often in late summer or fall. Right now is the ideal time to find them, but you have to look at a lot of dead, standing trees to find them. Horntail species that use coniferous trees as hosts are often attracted to wildfires, since fire often weakens trees. Many wood-boring beetles are also active in the wake of conflagrations.

Please help spread the word that with few exceptions (introduced species like Sirex noctilio for example), horntails are not pests, but merely exploit trees that are already damaged by environmental stress, and/or diseases or other destructive insects. They are important to forest ecology and the average homeowner has no need to control them.

Sources: Cranshaw, Whitney and Boris Kondratieff. 1995. Bagging Big Bugs: How to identify, collect, and display the largest and most colorful insects of the Rocky Mountain region. Golden, CO: Fulcrum Publishing. 324 pp.
Drees, Bastiaan M. and John Jackmann. 1999. A Field Guide to Common Texas Insects. Houston: Gulf Publishing Co. 359 pp.
Evans, Arthur V. 2008. National Wildlife Federation Field Guide to Insects and Spiders of North America. New York: Andrew Stewart Publishing, Inc. 497 pp.
Schiff, Nathan M., Steven A. Valley, James R. LaBonte, and David R. Smith. 2006. Guide to the Siricid Woodwasps of North America Morgantown, WV: USDA Forest Service Forest Health Technology Enterprise Team (FHTET-2006-25). 102 pp.