Intrigued to see a large, shiny black wasp that lays eggs in trees in your nearby park or garden? This is probably the giant ichneumon wasp, and it is perhaps one of the most fascinating creatures among us. Let’s learn more about this gentle giant.
The giant ichneumon wasps are strikingly large insects found across the wooden areas of North America.
These parasitic wasps lay eggs in the larvae of a pigeon trimix horntail, which grows in dead tree trunks.
Intrigued to learn more about these wasps and how they lay eggs in trees? Continue reading this article.
What Are They?
The long-tailed giant ichneumon, scientifically known as Megarhyssa macrurus, belongs to the wasp species of the insect order group Hymenoptera.
A typical giant ichneumon is 2 inches long (about the size of your index finger) with a thin body and long abdomen. It has a reddish-brown body with black and yellow-orange stripes and transparent wings.
The female giant ichneumon is distinctly known for its 5-inch long ovipositor, which it uses to insert eggs in the potential host.
The male does not have an ovipositor since it’s the female who lays eggs on the larva. So, the female wasp has a longer body as compared to her male counterpart.
Why Do They Lay Eggs In Trees?
The giant ichneumon wasp is a parasitoid in nature; it needs a host to lay eggs in order to complete its life cycle.
The larvae of a giant ichneumon feed on the larvae of a pigeon horntail wasp, who develops in a rotting tree trunk.
Thus, you need a dead tree with a pigeon horntail larva to attract a female giant ichneumon wasp to lay eggs.
How Do They Know Where To Lay Their Eggs?
There are two explanations for how the female giant ichneumon wasp locates the pigeon horntail larva. We have discussed them both below.
The first explanation is that the giant ichneumon female probably uses her sense of smell to locate the host.
Once she is assured about the presence of the pigeon horntail larva in the tree, she uses her long ovipositor to drill the bark and insert her eggs next to the larva.
The other understanding is that the female wasp possibly uses her antennae to pick up the vibration of the larva’s location in the tree.
She then drills through the tree bark to find and paralyze the larva and lay her first egg on its body. The minute the egg hatches, the ichneumon larva starts feeding on the immobilized wood wasp larva.
How Do They Lay Their Eggs?
Since we have learned how the female wasp knows where to lay the eggs, we must discuss how they do it.
The female ichneumon is a natural predator and finds the host through her sense of smell or picks up its vibration using her antennae.
Once she locates the host, her 5-inch long ovipositor starts drilling through the tree bark until it reaches the pigeon horntail larva.
It takes about 3 to 4 hours for her to get to the larva and lay a long, skinny egg on its body.
Once she inserts the egg, the mother wasp’s job is finished. The minute her egg hatches, the larva starts eating its host’s body.
It takes several weeks for the larva to finish its meal. When it finally reaches the pupal stage, the larva consumes the remaining part of the host’s body and starts chewing its way out of the tree.
What Kind of Trees Do They Prefer?
Horntail wasps generally attack trees that are dead or in the process of dying. Ichneumon wasps lay their eggs on these larvae, so where they go, the wasps go too.
If the tree has a possibility of reversing its decline, these wood-boring insects will leave it alone.
Thus, if a tree near you has become a shelter for horntail wasps, it is a sign that it is dead.
Frequently Asked Questions
Do ichneumon wasps sting humans?
Ichneumon wasps are not interested in interacting with human beings. They don’t even have a stinger, so it is simply not possible to sting us.
The female has a large ovipositor which can’t sting us, and the males don’t even have that.
They share a beneficial relationship with us since they naturally help in pest control. They are wasp pollinators and can travel up to 2000ft to disperse pollen and find horntail nests.
Are parasitoid wasps harmful to humans?
Parasitoid wasps are not harmful to humans. In fact, they are quite beneficial to human beings.
They act as a biological control agent and help in eliminating pest populations, especially in agricultural areas.
Many of these wasps control crickets, grasshoppers, caterpillars, spiders, aphids, thrips and other such insects that can ruin your precious plants.
What does an ichneumon look like?
A typical ichneumon wasp is about 2 inches long and has a reddish-brown body with multi-colored stripes.
The female has about 4 to 5-inch long ovipositor and, thus, is bigger than the male, who does not have the appendage.
Both are bigger than other commonly observed wasps but are extremely thin.
Can parasitoid wasps lay eggs in humans?
There is one parasitoid wasp that lays eggs in humans – the German wasp! The larvae feed on human skin, which can result in a painful rash or swelling.
Apart from this one wasp, most other parasitoid wasps only focus only on insects and small invertebrates.
We hope this article helped you learn more about this beautiful wasp that lays eggs in trees.
When Charles Darwin first encountered this species, he remarked that he could not believe in a God who would design creatures with the intent that they would feed on live caterpillars.
While the description is apt, ichneumon wasps are much more than just parasitoids – they are our friends, gentle pollinators, and also devoted parents to their young.
If you liked what you read, please do give us a shoutout!
Ichneumons look very intimidating, and their actions (such as laying eggs on trees) are even more so.
Many of our readers have sent us emails over the years asking us about these wasps. Please go through these letters.
Letter 1 – Giant Ichneumon Ovipositing
Huge Hornet or Wasp?
Hi I found this on one of our trees in the front yard. I wanted to know if you could tell me what kind of bug it is and what it’s doing with it’s extremely long tail?
Hi there Scott,
This is one of the Giant Ichneumons in the genus Megarhyssa, probably Megarhyssa macrurus. Giant Ichneumons are non-stinging relatives of wasps. This female is ovipositing. The long stingerlike “tail” is her ovipositor and it enables her to lay eggs deep in borer infested wood. The food for the young Megarhyssa is the larval form of wood boring insects like Horntails.
Letter 2 – Giant Ichneumons Ovipositing
I believe that these are Ichneumon wasps. We watched them for quite some time as they took turns laying eggs. The “fan” at the tail was iridescent and pulsating. Thanks!
Congratulations on your stunningly beautiful image of two female Giant Ichneumons, Megarhyssa atrata, ovipositing. We especially love the tiny centipede visible under the bark. We do not fully understand the mechanics governing the ovipositor, but this membrane is visible on numerous photos in our archives and elsewhere.
Letter 3 – Giant Ichneumon Ovipositing
What’s this bug
I spotted this bug soon after I cut a limb of a large maple tree. I’ve never seen anything like it before. What is it? Thanks,
This is a female Giant Ichneumon, Megarhyssa atrata. She is ovipositing or laying eggs. She locates the tunnel of a wood boring insect and lays the egg where her larva can find the wood boring grub and begin to feed on it. Your photos are excellent. The portrait is a nice angle.
Letter 4 – Two species of Giant Ichneumon Ovipositing (photo by Allen’s brother Jon)
two sp. of ichneumon ovipositing together
Thanks for your helpful comments and photos of ichneumons. My brother took this picture next to his house in Bloomington MN that might be nice for your collection. We identified the two ichneumons using your page; if I read correctly, it’s a Megarhyssa macrurus and a Megarhyssa atrata laying eggs side by side.
Thank you for sending us your brother’s fabulous photograph. It is one of the best images we have received in a long time. Your identification is correct, with Megarhyssa atrata on the right. Out of curiosity: Does your brother have a name? or, Is he just your brother?
My brother’s name is Jon, and he deserves all the credit! All the best,
Letter 5 – Giant Ichneumon Ovipositing
Sabre wasp laying eggs
I love your site and use it all the time; I thought I’d share with you this photo I took of a female Sabre wasp laying her eggs in a tree stump in my back yard. Enjoy!
We were not familiar with the common name Sabre Wasp with regards to the Giant Ichneumon, Megarhyssa atrata. Thanks for enlightening us about this fitting name.
Letter 6 – Two Giant Ichneumons Ovipositing
more Giant Ichneumon
Here are a few nice pics of (what I assume are) Giant Ichneumon from trees in our yard. Take care,
Thanks for sending us your images of two female Megarhyssa atrata laying eggs.
Letter 7 – Giant Ichneumons Ovipositing
Sat, Dec 20, 2008 at 9:11 AM
This was taken at night in Ontario Canada while camping at a provincial park.
We have a sneaking suspicion that this observation did not happen while camping this week. These are Giant Ichneumons in the genus Megarhyssa, most likely Megarhyssa macrurus. Giant Ichneumons are related to Wasps since they are in the same insect order, but they are in a different family. These are females and they are ovipositing. Giant Ichneumons are parasitic on the larva of wood boring insects, especially the Pigeon Horntail. The female Giant Ichneumon locates the tunnel of one of the wood boring larvae and uses her nearly five inch long ovipositor to deposit an egg deep inside the infested tree.
Letter 8 – Giant Ichneumon Ovipositing
Possible Giant Ichneumon – Megarhyssa atrata
May 31, 2009
We found a number of these bugs on the side of a tree while walking through a forest preserve in Northern Illinois (Rockford). They didn’t seem skittish and allowed us to take quite a few hi-res photos of them. There were probably eight or so of them on a single tree. We never saw them anywhere else in the park, only on one tree.
We are trying to devote a bit of time to addressing unanswered mail, and your photo of a female Megarhyssa atrata in the act of ovipositing is beautiful.
Letter 9 – Giant Ichneumon ovipositing
What kind of bug is this?
What kind of bug is this?
Location: Warwick, Rhode Island
August 29, 2010 4:17 pm
These bugs just showed up and crawl all over the dead tree on the side of my house the long antenna looking thing seems to maybe suck something out of the tree! I have never seen these before and they look like they could be 2inches long some shorter some longer! Its the end of August so we are slowly going into fall.
Thanks, Dawn Bergeron
This insect is actually depositing eggs into the tree, not sucking something out as you thought. Several days ago we decided to make the Giant Ichneumon our Bug of the Month for September 2010 so we included information on how the female oviposits her eggs in the wood of dead and dying trees that contain the burrowing larvae of the Pigeon Horntail. The Giant Ichneumon is a parasitoid whose larvae feed solely on the larvae of the Pigeon Horntail. Coincidentally, yesterday we posted an image of a female Pigeon Horntail, another impressive non-stinging member of the order that includes wasps, in the act of oviposition. Though we wrote a lengthy response, we were not able to include images of the actual egg laying or oviposition process. Your photos clearly illustrate the process of a female Giant Ichneumon laying eggs, though your species is different from the species in the Bug of the Month posting, which is Megarhyssa atrata. We suspect your Giant Ichneumon is Megarhyssa macrurus. We are going to combine your letter and images with the previously selected letter to be a joint Bug of the Month posting for September 2010.
Letter 10 – Giant Ichneumon Ovipositing
Location: Mansfield, Ohio USA
September 19, 2010 9:48 pm
Found this ”big lady” laying eggs I believe in a log in my backyard (Mansfield, Ohio) Any idea what type of wasp, I assume wasp. Pictures I know aren’t fabulous, though I do have great video. The appendage on the backend she used to deposit her eggs was 6 inches long when fully put out. Was very cool to watch.
Theoretically, a Queen Wasp is a reproductive female that creates a nest or colony that consists of workers. Solitary Wasps are not considered queens. Your individual is a Giant Ichneumon, a Parasitoid species that deposits her eggs in wood that has been infested with wood boring larvae a Wood Wasp known as the Pigeon Horntail. Your Giant Ichneumon does not form a nest or colony, so she is not a queen. There are several species known as Giant Ichneumons, and it appears that your individual may be Megarhyssa macrurus. You can see other examples in our archives as well as on BugGuide. Though it might seem that the Giant Ichneumon could sting a person, her ovipositor is not multi-purpose, and she is incapable of stinging.
Letter 11 – Giant Ichneumon Ovipositing
on hickory tree
Location: northern mid TN
September 26, 2010 7:06 pm
We were cutting up a downed Hickory here in mid Tn and these bugs were a little territorial of their section of wood. They seemed to be male and female sticking the long tail end into holes in the wood. They were overall around 5-7 inches long.
It is the female Giant Ichneumon, Megarhyssa atrata, that sticks her ovipositor into diseased wood because her larvae parasitize the larvae of the Pigeon Horntail that are eating the wood. The female in your photo appears to be depositing her egg into a egress tunnel bored by either the emerging Pigeon Horntail Wood Wasp or an adult Giant Ichneumon. We suspect it is the tunnel of the Pigeon Horntail.
Letter 12 – Giant Ichneumon Oviposits
I haven’t the slightest…
Location: Rochester, Minnesota
December 30, 2010 7:07 pm
Hi, I found this bug in my very own backyard, it was sitting on a tree, and appeared to be pumping the tree with… something with it’s 3 inch sting, and I really have no idea what it could be.
Signature: Gannon S.
We are guessing that you did not recently take this photograph of a female Giant Ichneumon, Megarhyssa atrata, in the act of ovipositing recently, but rather, you are trying to identify an insect you witnessed during the summer. Though she is laying eggs, this Giant Ichneumon will not harm the tree. Her brood will feed on the wood boring larvae of the Pigeon Horntail, an insect that is found in dead and dying wood as well as the wood of trees that are already in decline.
Letter 13 – Giant Ichneumon Ovipositing
Flying Sap Eating bug?
Location: Long Island, NY
July 26, 2011 6:38 pm
This bug looks like some sort of dragon fly. If you look closely you will see 2 loops coming over the tail and then come together between the hind legs. This stick looking appendage? was then inserted into a round hole in the tree which looked like a hole made from a and or termite. The holes were as much as 2 inches deep. At the end of the tail where the 2 ”loops start looked like some sort of organ moving/pulsing. I think either the bug was eating the sap of this maple tree which was exposed by these holes or maybe laying eggs. What is it?
This is a Giant Ichneumon in the genus Megarhyssa, probably Megarhyssa macrurus. As trees age, portions of them begin to die and they become susceptible to disease and infestation. One of the insects that targets old and weak trees is the Pigeon Horntail, a wood wasp whose larva bore in the wood. Your creature, a Giant Ichneumon, is a parasite whose larvae feed on the larvae of the Pigeon Horntail. This female Giant Ichneumon is ovipositing. She will lay her eggs on or near the wood boring larva of the Pigeon Horntail, which she senses beneath the bark. Giant Ichneumons will not harm your tree, but you should know that your tree has already been compromised and is in a state of decline, however, it may still live for many years.
Letter 14 – Two species of Giant Ichneumons Oviposit on same tree in Canada
Subject: Unknown bug
Location: NorthWest Ontario
June 27, 2016 8:14 pm
This tree fell down in a recent storm and is known as a Manitoba Maple. This photo is taken of two of the several black yellow winged dragonfly scorpion like taled flyer. Behaviour of which is interesting and peculiar caught the eyes of many… They appeared to be mating or birthing or fertilizing… Hard to say if the tentacles at the end of their tales were their own or coming out…or if the green thick fluid like stuff was a placenta or mating fluids or ? Very bizarre as they seemed to be labouring if not pleasuring. One I thought was birthing actually burrowed it’s once widened tail tip into a hole in the bark made by termites….nestled it there and rested. A couple of these bugs looked rather fresh, young, and of the brighter yellow and reddish brown, one somewhat curled up and drying off …I wondered if was born recently. A couple of the bugs were blacker than others, almost all black and their tails so black you would hardly notice them in the shadows of the tree pieces or against the earth.
I live in northwestern Ontario bordering with Manitoba. We just had a lot of rain, 44mm this past weekend.
What is this bug
Signature: Curious C
Dear Curious C,
In your image, you have captured two different species of female Giant Ichneumons in the genus Megarhyssa, sometimes called Stump Stabbers, and each is in the process of laying eggs or ovipositing. Giant Ichneumons are parasitoid wasps and their host insects are the wood boring larvae of Wood Wasps in the subfamily Tremecinae, including the Pigeon Horntail, that feed on the wood of deciduous trees. The smaller brown and yellow individual is Megarhyssa macrurus, and the larger, black and yellow individual is Megarhyssa atrata.
Wow! Very informative and fast ID’ing!! Thankyou so much!!:)
Curious C 😉
Letter 15 – Giant Ichneumon Ovipositing in Canada
Subject: What is this
Geographic location of the bug: Montreal , Quebec
Time: 07:06 PM EDT
Your letter to the bugman: We found this on a diseased Maple tree in our backyard. Can you identify what this is
How you want your letter signed: Alan Kelly
This is a female Giant Ichneumon in the genus Megarhyssa, probably Megarhyssa macrurus which is pictured on BugGuide, and she is ovipositing or laying eggs. Since you have indicated that your maple tree is diseased, it is most likely infested by various wood boring insects, including the larvae of the Pigeon Horntail, the prey of the Giant Ichneumon. The female Giant Ichneumon senses the presence of the larval Pigeon Horntail burrowing in the wood, and she inserts her long ovipositor so she can lay an egg on or near the larval Horntail. When the egg hatches, the larval Giant Ichneumon will parasitize the Pigeon Horntail larva. The Giant Ichneumon will not harm your tree.