Jeffrey Hahn, Asst. Extension Entomologist
A large wasp has been noticed in the landscape recently. This insect is a type of ichneumonid (ich-new-MON-id) wasp known as Megarhyssa (meg-a-RISS-uh). Megarhyssa is a reddish brown and yellow insect (another species is black with an orangish head) with a body length of about one and half inches long. That does not include it ovipositor which is another two to three inches long (that's up to over four inches total length!). In addition to the ovipositor, you will find two sheaths protecting it which sometimes gives the appearance that it has three 'tails'. Sometimes Megarhyssa is confused for other large sized insects, e.g. mayflies or dragonflies, so look closely to be sure it correctly identified.
Ichneumonid wasps are parasitic upon other insects. Megarhyssa is a parasite of horntails. Horntails attack dying or recently dead hardwoods, such as oak, maple, birch, and elm. With her long ovipositor, Megarhyssa can drill 1/2 inch or more into the wood to deliver an egg into the horntail larva. When people see this ichneumonid wasp ovipositing into a tree, they believe that it is attacking it. Of course, this is not true. Once the egg hatches, the Megarhyssa larva slowly feeds on the horntail, eventually killing it. After it matures into an adult wasp, it emerges from the tree.
Despite her large size and her menacing ovipositor, ichneumonid wasps, like Megarhyssa, pose no threat to people. They are not aggressive and avoid us when possible. It is possible that if someone handled a Megarhyssa, she could try to jab you with her ovipositor if she felt threatened but she could only inflict a minor wound at best.
If you see Megarhyssa, just ignore her. Any that you find will go away on their own in a short time. It is never necessary to treat them with an insecticide.