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Ichneumonid wasp

Quick facts 

This insect is a type of ichneumonid (ich-new-MON-id) wasp known as Megarhyssa (meg-a-RISS-uh).

  • It is a large wasp that has been associated with declining or recently dead hardwood trees between May and July.
  • The wasp parasitizes horntails that are present in declining trees, but does not injure trees.
  • The wasp is harmless to people and ignores people when possible.
  • If mishandled, it may use its ovipositor to protect itself.

How to identify ichneumonid wasps

Megarhyssa macrurus is a reddish brown and yellow insect with black coloring throughout. Another species, M. atrata is black with an orangish head.

A reddish-brown and yellow wasp with black patterns
Ichneumonid wasp drilling in a tree. Note the long ovipositor
  • one and half inches long
  • ovipositor is another 2-3 inches long
  • With the ovipositor, the wasp has a total length of about 4 inches.
  • The wasp has two sheaths protecting it which sometimes gives the appearance that it has three 'tails'.

Sometimes this wasp is mistaken for other large sized insects, like mayflies or dragonflies, so look closely to be sure it is correctly identified.

Damage caused by ichneumonid wasp

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.
  • Megarhyssa can drill 1/2 inch or more into the wood with its ovipositor to deliver an egg into the horntail larva.
  • Once the egg hatches, the Megarhyssa larva slowly feeds on the horntail, eventually killing it.
  • After maturing, the adult wasp emerges from the tree.
  • It might seem like the wasp is attacking the tree, but the wasp only feeds on the horntail.

How to get rid of ichneumonid wasps

Although ichneumonid wasps appear dangerous due to their large size and threatening ovipositor, they are not harmful to people.

  • It is possible that if someone handled a Megarhyssa, it may jab with its ovipositor in self-defense. This will generally result only in a minor wound.
  • Ignore these wasps, if you see them.
  • They will go away on their own in a short time.
  • It is not necessary to treat them with a pesticide.

Jeffrey Hahn, Extension entomologist 

Reviewed in 2018

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