Ames Free Library

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A Glimpse of Nature - A Tale of Two Wasps and a Fungus

See what’s happening on the grounds of the Ames Free Library or nearby areas with “A Glimpse of Nature.”  Offered by Lorraine Rubinacci, the library's resident naturalist, this weekly photo blog is a gentle reminder to enjoy the wonders that surround us.


A Glimpse of Nature - "A Tale of Two Wasps and a Fungus"


Two weeks ago, on October 7, I was examining this small fungus on a maple tree behind Queset Garden’s bocce court. 

There weren’t a lot of fungi on the property, and this species, though tiny, was distinctive and charming.  I was having a tough time getting good images and was very absorbed in the effort.  In fact, I was so focused that I didn’t realize I had “company.”  Just a few inches away, an extraordinary creature was laying its eggs. 


Initially, I was so surprised that I didn’t comprehend what was happening.  I had never seen this animal before, at least not a female . . . and she was large.  Her body was almost two inches long, and she had an exceptionally long ovipositor, the organ used to lay eggs.  Here’s another individual with its ovipositor extended.


I snapped out of my reverie and shot some more photos . . . the research would come later.  

At home, I learned that this amazing animal is a long-tailed giant ichneumon wasp.  Each of the several individuals that I observed on the tree were females who were ready to lay their eggs. 

These wasps are parasites who lay their eggs on insect larvae that live under a tree’s bark.  Their host species is also new to me:  the pigeon horntail or wood wasp.  It, too, is substantial and seemingly scary.

Horntail Wasp

Billmcmillan, CC BY-SA 4.0

In case you are wondering, neither species is dangerous to humans.  They do not sting.  Indeed, the ichneumons ignored me and went about their business, even though I was close by. 

Here’s a summary of the relationship between the two species.  First, the female pigeon horntail bores tunnels into dying or recently dead hardwood trees where it lays its eggs.  At the same time, it introduces a white rot fungus that will be helpful to the larvae’s development.  As the University of Wisconsin’s BugLady explains, “The fungus starts to grow, rotting the surrounding wood. When the eggs hatch, the “pre-treated” wood is softened so they can eat and tunnel more easily.”  The horntail spends most of its life -- as an egg, a larva, and a pupa -- inside the tree.  Hence, the nickname “wood wasp.”  Upon reaching adulthood, the horntail cuts a circular exit hole in the bark. 

This arrangement would seem to offer a safe haven to the young insect, but the giant ichneumon has just the anatomy and behavior to capitalize on the horntail larvae’s vulnerability.  First, it can detect the presence of horntail larvae under a tree’s bark.  According to most accounts (including The Audubon Society Field Guide to North American Insects and Spiders), the insect uses her antennae to detect the vibrations of horntail larvae in the wood. Then she uses her lengthy ovipositor to drill through the bark to the larva’s tunnel.  Upon locating the hapless host, the ichneumon paralyzes the horntail larva and deposits her egg, which is bad news for the horntail who will be consumed by the ichneumon larva.  The young ichneumon will grow rapidly, pupate, and wait until the following summer to emerge as an adult. 

I did not witness the entire cycle, but I was fortunate to see a fascinating segment.  Let’s examine the photographs more closely.  The first thing to notice is that not all wasps look alike.  These two species are both dark with yellow markings, but the horntail’s body is more stout and cylindrical -- there is no waist between its thorax and abdomen.  Indeed, if I had seen a male giant ichneumon, it would have presented another body shape, one more akin to a damselfly’s, with a long, narrow abdomen.

Long-tailed Giant Ichneumonid Wasp (male)

© Joe Bartok

When I first saw the adjacent wasps shown below, I thought they were mating.  [My vision leaves much to be desired.]  Look again.  Both of these are females who have inserted their ovipositors into the tree’s bark.

Two females laying eggs


A closer look also reveals the giant ichneumon’s remarkable anatomy.

Positioning the ovipositor

At first glance, the ovipositor of this species seems like a simple, needle-like projection.  A description by the Colorado State University Extension reveals its complexity:  “Although appearing as a single filament, it is actually made of two parts, that interlock, slide against each other, and are tipped with the cutting edge.”  This structure is sheathed by two protective filaments which bow out as the female deposits her eggs.  The image above also demonstrates how the ovipositor loops over the female’s body before penetrating the bark. 

The exit holes of the pigeon horntails are large, circular, and hard to miss.  This makes me wonder how the giant ichneumon wasps find the right trees.  Do they return to the trees where they were born?  Do they see the holes?  Or, as BugLady suggests, perhaps their antennae can smell the horntail’s frass or the fungus.

Exit holes of pigeon horntails (with giant ichneumon)

Now it’s time to put the pieces together.  This 2 ½ minute video shows two females:  one who is already laying eggs, and the other who is using her antennae to pick the best spot.  Towards the end, she adjusts her position to drill into the bark, a movement that is truly amazing.


This phenomenon didn’t last long.  When I returned a few days later, the only sign was a dead ichneumon which was still attached to the tree.  Next year, we can look for the pigeon horntail wasps and the male long-tailed giant ichneumon wasps.  If you and I are really lucky, we’ll witness the female ichneumons depositing the next generation of eggs.  At least we’ll know where and when to look.

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