Critters: Giant ichneumon wasps help the environment, are harmless to humans |

Critters: Giant ichneumon wasps help the environment, are harmless to humans

Giant ichneumon wasps can be found on Grand Mesa around dead and dying hardwood trees.
Submitted photo |

Editor’s note: “Critters” features insects, spiders, snakes and other freaky/fun creatures unique to the Grand Valley. Have a critter you want discussed? Email

Giant ichneumon wasps — spindly, brown creepy crawlers, the stuff of children’s nightmares — may look scary at first glance, but they won’t sting people.

According to Bob Hammon, Colorado State University Extension’s western Colorado entomology expert, “Giant ichneumons are found in association with wood wasp larvae [pigeon tremex horntail],” which occur in forested areas with dead and dying trees.

To spot one in Mesa County, head to Grand Mesa and look for infected hardwood trees (like silver maple, ash, cottonwood, and elm). They’re hard to miss — ichneumons grow up to 5 inches long, with 2-inch-long tails extending from the body. Coloring is brown with yellow and orange marks.

“The giant ichneumon wasp is the most common natural enemy of the pigeon tremex,” W. Cranshaw wrote in Colorado State University Extension Fact Sheet No. 5.604. “ … The pigeon tremex develops as a wood borer and the larvae are cylindrical-bodied, cream-colored grubs that live within the [infected] wood.

“Developing horntail larvae can be detected under the bark by the female [giant ichneumon wasp] and she subsequently drills into the wood to the tunnel of the horntail larva. During egg laying (oviposition) the host larva is paralyzed with a sting after which the egg is laid.”

Giant ichneumon larva then feed off horntail larva for a few weeks while developing and remain dormant until emerging as adult wasps the next summer season.

“In the big natural history picture, ichneumons are beneficial insects that help regulate population levels of other insects,” Hammon explained. “They are an amazing example of how specialized some insects can be. They can locate their host under bark and through wood, which is amazing …”

Male ichneumons are smaller than females, and lack the egg-laying drill mechanism called the ovipositor.

“We get a few calls a year about giant ichneumons, usually curiosity,” Hammon said.

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