Ichneumon Wasps

Wasps may have a bad reputation, but there are beneficial wasps, and ichneumon wasps [pronounced ick-NOO-mon] are one of those Good Guys. Well, mostly.

Ichneumon wasps are parasitic wasps and they have been around for over 15 million years. ​

'Megarhyssa greenei' female (Bruce Marlin) CC BY-SA 2.5

There are somewhere between 60,000 and 100,000 different ichneumon wasp species in the world, with 5,000 species in North America. Clearly, with those numbers, there is significant variety in appearance.

Female 'Xanthopimpla punctata'. Ovipositor longer and more slender than stingers of other wasps (Vengoils) CC BY-SA 3.0

Ichneumon wasp description

Like other wasps, the ichneumons have a narrow body and an even narrower waist. Some females have an especially long ovipositor, which is often mistaken for a stinger. Ranging in length from 1/10” to over 5” long, they can be black, brown, yellow, or some pattern combination of those colors. These wasps have 16 or more segments in their longer than average antennae. Ichneumons are solitary wasps.

​Head of ichneumon wasp showing highly segmented antennae (Martin Cooper) CC BY 2.0

Common ichneumon prey

Adult ichneumon wasps eat little or nothing. Their larvae, on the other hand, are voracious feeders of beetle, butterfly and moth, wasp, ant, fly, and sawfly larvae and pupae or chrysalises. This is what makes ichneumon wasps so helpful in the garden. They also parasitize beet armyworms, some spiders, and wood-boring grubs.

'Echthrus reluctator', female (Charles J. Sharp) CC BY-SA 4.0

Some ichneumon wasps do this by using their long antennae to detect prey, then inserting the ovipositor into the wood, plant, or soil, to strike their prey, piercing its skin and inserting an egg. Other ichneumon wasps crawl down the stems of aquatic plants to inject eggs into water-dwelling insects. Yet another ichneumon is the parasite of a parasite, making it a hyperparasitoid. This ichneumon lays its eggs in moth-eating ant larvae. She emits chemicals that confuse the ants as she does her deed.

'Hercus fontinalis' larvae feeding on caterpillar (Beatriz Moisset) CC BY-SA 4.0

Ichneumon wasp lifecycle

Most parasitic wasps lay their eggs on, in, or near their prey, but ichneumon wasps kill their prey outright and then lay eggs. When the eggs hatch, larvae feed on the host’s body. After hatching and eating their host, ichneumon larvae spin cocoons and pupate in or near the exoskeletons of their first meal. When they emerge from their cocoon, they are adult wasps who go in search of a mate.

​These garden helpers do not sting and they are worth their weight in gold, so check before swatting at something just because it might be a wasp. [Swatting at wasps is usually a bad idea, anyway…}

One of the best ways to attract and provide for beneficial ichneumon wasps is to plant coyote brush (Baccharis pilularis) and to maintain hedgerows.