Unicorns Are Real

I guess there are some unicorns out there somewhere.

~Tedy Bruschi

Indeed there are, and I have several in my back yard right now. In my last post, I mentioned I periodically check the leaves on a young wild cherry tree in the yard to see what might be using it for food or shelter. A few days ago, I discovered an early instar of a unicorn caterpillar, Schizura unicornis. They are named for the prominent horn-like protuberance on their first abdominal segment.

unicorn caterpillar second instar 1

Early instar of a unicorn caterpillar feeding on wild cherry (click photos to enlarge)

This is the first early instar of this species I have seen (I am guessing it may be a second instar – an instar is a development stage between molts).

unicorn caterpillar second instar 1 close up

A closer look shows what look like droplets on the caterpillar spines

When I looked at my image I saw what looked like tiny droplets of liquid on most of the small spines covering the caterpillar’s body. So, I did an online search and found some surprising results. Turns out this species has some interesting defenses that include the ability to spray an attacker with a chemical concoction made of formic acid, acetic acid and other compounds. Perhaps these droplets can also be secreted from the small projections on the larva’s body, although I can’t find any confirmation of that.

Unicorn caterpillar compared to dry edge of leaf

Unicorn caterpillars resemble the curled dead portions of leaves

Another defense is their unusual shape and coloration. The wild cherry has a lot of dried leaf edges this time of year and the larvae tend to feed along the margins of these. They blend in remarkably well, even in their posture, as seen above.

Unicorn caterpillar late instar horizontal

Late instar unicorn larva (photographed a few years ago on wild cherry)

A close look at a later stage of the larva shows how well their shape, and even the smallest details of the patterns and lines on their body, help them to resemble dried portions of a leaf.


Unicorn caterpillar on cherry leaf

Early instar larvae have a white slash mark toward their rear

Unicorn caterpillar on small cherry leaf

The small bit of green on the thoracic segments add to the illusion

Most of the ones I found on this sapling are small, and are feeding on the undersides of the leaves, making it even more difficult to spot them (I turned them up for the photos). But, one caterpillar was a bit easier to see since it was sitting atop a white blob of silk.

Unicorn caterpillar with braconid eggs

Their defenses are not always fool-proof

The tuft of silk turned out to be a cluster of cocoons of a species of a parasitoid, some species of braconid wasp. Braconid wasps are minute parasites (most are about the size of a large gnat) that lay eggs in a variety of other insects. There are over 17,000 recognized species and they are considered important biological controls of many other insect groups, especially the larvae of flies, beetles, and moths and butterflies. Upon hatching, the wasp larvae feed on the host, consuming non-vital tissue so that the host continues to live. Then, one day, the larvae emerge and pupate, and the host eventually dies.

Hog sphinx with braconid wasp cocoons

Hog sphinx larva with braconid wasp cocoons (ironically, I had just commented to someone that I usually find the green form of this caterpillar, and, this week, I discovered this brown one)

Many of us are familiar with the external cotton swab style cocoons found on many species of caterpillars (especially tobacco hornworms on your tomatoes). In the photo above, you can see the wasps have already emerged since the caps of the cocoons are open. But there is another group of braconids that have a different strategy.

Unicorn caterpillar with braconid eggs 1

Unicorn larva guarding the pupating wasps

They may even alter the behavior of the host caterpillar, creating a so-called zombie caterpillar (what’s with all the zombie things in my yard?). When the wasp larvae emerge, they form an array of cocoons that the caterpillar sits atop and guards (as best it can) until the wasps emerge.

Unicorn caterpillar with braconid eggs in hand

The unicorn caterpillar and its parasitoid pillow

I normally don’t interfere much with nature but I decided to remove the wasp cocoons and the doomed caterpillar so I could see what, and how many, parasitoids emerged. I’ll try to get some photo of the tiny masterminds whenever they complete their life cycle. Here’s hoping the other 5 unicorn caterpillars I found on the wild cherry sapling have a better fate.


Seeing the Wild in Wild Cherry

The most beautiful gift of Nature is that it gives one pleasure to look around and try to comprehend what we see.

~Albert Einstein

There is a wild cherry (Prunus serotina) sapling just outside our screen porch that is a favorite spot for all sorts of natural events. Wild cherry is a great host plant for a variety of moths and butterflies so I let this young tree grow in a spot too close to the house to ever reach any height just so I can keep track of the comings and goings of its tenants. It has been a busy place these past few days.

red-spotted purple early instar

Red-spotted purple early instar larva (click on photos to enlarge)

Throughout the year, I can always count on seeing some sign of one of the primary occupants of this species of tree, the red-spotted purple butterfly, Limenitis arthemis. They lay their eggs at the tip of cherry leaves, and the larvae feed on the leaves through their entire caterpillar and chrysalis stage, appearing as bird poop mimics. And they even overwinter on the plant, with the third instar larvae of the fall generation making tiny sleeping bags, or hibernacula, by cutting away much of a leaf and rolling the base into a hollow tube where they spend the winter. Next spring, when the cherry leaves first sprout, the tiny larvae will emerge form their tube, begin feeding on the fresh leaves, and begin the whole cycle again. In the photo above, the larvae has already attached the leaf to the twig with silk (so the leaf fragment remains on the tree all winter) and is just beginning to curl the base of the leaf with even more silk (silk strands shrink as they dry, pulling the leaf together).

red-spotted purple hibernaculum 1

One day later, a hibernaculum!

By the next day, the larvae had finished constructing its hibernaculum and was resting inside. I’m a bit surprised it has constructed this so soon as there is still plenty of time for it to grow, pupate, and start another generation before cold weather. But, there are not many leaves left on this tree at this point, so maybe caterpillars can take a cue from food availability and go ahead and go into a resting phase for the winter.

red-spotted purple last instar

Last instar red-spotted purple caterpillar on a different sapling

On a nearby cherry sapling, I found a much larger red-spotted purple larva which will soon, no doubt, form a chrysalis.

white-marked tussock early instar

White-marked tussock moth larva, early instar

Back at the original tree, there were a couple of other caterpillars to observe. One of my favorite finds this time of year is the white-marked tussock moth caterpillar, Orgyia leucostigma . They remind me of a combination caterpillar and toothbrush, due to the four prominent tufts protruding near the head, plus the two black-colored tufts of setae out front that resemble some fancy flossing tool.

white-marked tussock just after molt

White-marked tussock moth larva and shed skin

Nearby was another one that had just molted. This species is a generalist feeder, so I find it on a variety of plant species around the yard.

Unicorn caterpillar second instar

An early instar unicorn caterpillar

Nearby was another of my favorites, an early instar of the unicorn caterpillar, Schizura unicornis. These guys do an amazing job of blending in with the edges of the leaves of whatever they are feeding on. As I looked around, I found a few more…and that will be some fodder for my next post.