Wood Wasps Identification Guide

Wood Wasp Identification Guide

Physical Characteristics

Wood wasps, also called horntails, are among the largest insects homeowners may find. Adults measure 20 to 50 mm (about 3/4 to 2 inches) long, not counting the female’s egg-laying tube. Their most obvious feature is a thick, tube-shaped body. Unlike true wasps, they lack the pinched “wasp waist.”

The name “horntail” comes from a short, pointed spine at the tip of the abdomen. Both males and females have this spine. Females also have a long, needle-like tube called an ovipositor. This organ drills into wood to deposit eggs. It is not a stinger and cannot hurt you.

Colors vary by species. Most are black, blue, brown, yellow, orange, or rust. Many have banded patterns on their abdomen. They have two pairs of clear or smoky wings and short antennae. Their broad heads have small, widely spaced eyes.

Common Species

• Pigeon Tremex (Tremex columba) is the most common species in eastern North America. Adults are reddish-brown to black with yellow bands. Females can reach 5 cm (2 inches) long. They grow in hardwoods like maple, elm, oak, and beech.

• Blue Horntail (Sirex cyaneus) is found across North America. Adults have shiny blue-black bodies with smoky wings. Larvae grow in conifers like pine, spruce, and fir.

• Western Horntail (Sirex californicus) is common in western states. It looks like the blue horntail with a dark shiny body. It infests pines and other conifers.

• European Woodwasp (Sirex noctilio) is an invasive species first found in North America in 2004. Unlike native species, this horntail can attack and kill healthy pine trees. It is dark blue to black with orange legs.

Wood Wasp Behavior and Biology

Wood wasps go through four life stages. They start as eggs, then become larvae, then pupae, and finally adults. Females use their long ovipositor to drill up to 3/4 inch into wood to lay eggs. Each female can lay 300 to 400 eggs in her lifetime. She places them one at a time inside the wood.

When laying eggs, females also inject a fungus from glands in their abdomen. This fungus (in the genus Amylostereum) causes white rot decay in the wood. The fungus breaks down tough wood fibers. This makes the wood softer and easier for larvae to eat.

Eggs hatch after three to four weeks. Larvae then start tunneling through the wood. They are legless white grubs with a spine on their rear end that helps push them through tunnels. They pack their tunnels with fine, powdery waste called frass. Larval growth takes one to five years, depending on the species, wood moisture, and temperature.

When ready to become pupae, larvae move close to the wood surface. Adults come out in summer, usually from late June through September. They chew round exit holes about 1/4 to 1/2 inch wide. Adults live only a few weeks. During this time they mate and females look for trees to lay eggs in.

Wood wasps only attack weak, dying, stressed, or freshly cut trees. They find these trees by smelling chemical signals and fungus from damaged wood. Males sometimes gather on high ground, a behavior called hilltopping. There they wait for females to arrive for mating.

Why Wood Wasps Appear in Homes

When wood wasps appear inside a home, they almost always emerged from lumber used during construction. Here is what happens:

1. A tree becomes stressed or dies and wood wasps lay eggs in it
2. The tree is harvested and milled into lumber while larvae are still developing inside
3. The lumber is used in construction before larvae mature
4. One to five years after construction, adult wood wasps emerge inside the home

This can be alarming because the insects are large and appear suddenly. They may chew exit holes through drywall, plaster, hardwood floors, or other surfaces. But wood wasps will not reinfest the wood in your home. They need fresh, moist wood with certain fungal conditions. Seasoned lumber in homes does not have these conditions.

The damage from wood wasp emergence is purely cosmetic. Exit holes can be patched and repaired. Since they cannot reinfest the wood, no treatment of the structure is necessary.

Treatment Methods for Wood Wasps

Because wood wasps cannot reinfest seasoned lumber, structural treatment is generally unnecessary. Here are the recommended approaches:

Wait and Repair

The most practical approach for wood wasps emerging in a home is patience. Adults that emerge indoors are trying to find their way outside. They may appear over several months as different larvae reach maturity. Once emergence stops, simply repair any exit holes with wood putty or patching compound.

Preventive Measures for Timber

For sawmills and lumber operations, kiln drying wood to reduce moisture content below 20% kills developing larvae. Proper seasoning of lumber before use in construction also prevents issues. Avoid using freshly cut or unseasoned wood in construction projects.

Managing Trees on Your Property

Keeping trees healthy through proper watering, mulching, and pruning reduces their appeal to wood wasps. Promptly remove dead or dying trees before wood wasps can infest them. If you store firewood, use it within one year and keep it away from your home.

Natural Controls

Several natural enemies help control wood wasp numbers. Large ichneumon wasps (Megarhyssa species) lay eggs on horntail larvae inside the wood. Woodpeckers also eat the larvae. A tiny worm called Deladenus siricidicola infects larvae and stops adult females from reproducing.

When to Call a Professional

While wood wasps themselves rarely require treatment, you should contact a pest professional if:

• You cannot tell if the insects are wood wasps or a wood-destroying pest
• You see many large exit holes and want to confirm the cause
• The insects look like powder post beetles or other wood-boring beetles that can reinfest wood
• You see sawdust or frass appearing over time, which may mean an active pest problem

A professional inspection can confirm whether you have harmless wood wasps or a pest that requires treatment.

References

• Wood Wasps and Horntails - UC IPM Pest Notes
• Horntails - Colorado State University Extension
• Siricidae of the Western Hemisphere - Canadian Journal of Arthropod Identification
• Horntail - Wikipedia
• Sirex noctilio Woodwasp - LSU AgCenter