Caterpillar attacked and killed by predatory fungus

Here's a bizarre and terrible fate. A caterpillar is encased in Metarhizium fungus, which ultimately will kill its host. This cat was still mobile but is not long for this world. The fungus will soon overwhelm it, leaving a hollowed-out husk. This caterpillar was on the abundant native Horseweed (Conyza canadensis), and I was searching it for a beautiful caterpillar known as Speyer's Cucullia, which I've never seen. This cuculiid feeds primarily, if not exclusively, on this plant. The fungal-infected cat is about the right size and shape for Speyer's Cucullia, but the field marks are obliterated by the fungus. The fungus even turns the head capsule and feet a sickly shade of tan. Pickaway County, September 9, 2022.
 

Black Cohosh, caterpillars, and ants

A can't miss plant of late June and early July, the black cohosh, Actaea racemosa. This robust member of the buttercup family (Ranunculaceae) grows in the shady understory of rich woods, and were it a lesser plant it'd be easy to miss. However, black cohosh towers up to six feet, or even taller, and the luminescent spikes of white flowers make it stick out like a sore thumb.

I ran across a vigorous colony yesterday on a steep slope in a Hocking County woods, which instantly caused a lepidopteran red flag to fly. There is a very cool - amazing, really - relationship between this plant, a little butterfly known as the Appalachian azure, Celastrina neglectamajor, and ants. The only host plant for this azure is black cohosh, and with a bit of knowledge one can often find the caterpillars.

The search window is narrow, as the cats primarily eat only the flower buds and then the flowers. The two little racemes to the left in the photo above, and the upper half of the larger raceme, are still in the bud stage. They'll quickly develop into flowers, at which stage the caterpillars will be pretty well grown. So, the azure caterpillar hunter will do best to look for black cohosh still in bud, and then watch for ants swarming a localized area of the budded out flower raceme.

And presto! A caterpillar! It didn't take too long to drum up a few, but out of perhaps two dozen plants checked, only three produced caterpillars. That seems about par for the course. The caterpillars' coloration mirrors that of the buds and stem, and were it not for the ants they'd be incredibly difficult to locate. Not only do these caterpillars prefer cohosh flower buds, they are even finickier. They're only interested in the innards of the bud, and the cat in the photo has its head (to the right) buried inside a flower bud. The flower bud immediately above it is cored out, leaving the tell-tale feeding hole of these larvae. Later, as the buds evolve to flowers the caterpillars will switch to those, and apparently if hard pressed will also consume foliage. I think this caterpillar is in one of its last instars - near fully mature - and if you scroll back to the first photo and look at the overall plant, you'll get an idea as to how small they are.

So, the million dollar question: Why the ants?

Their presence is part of a fantastic relationship between caterpillars (and other organisms such as aphids) and ants known as myrmecophily. This is especially common with gossamer-winged butterflies or Lycaenids, a very large family with some 6,000 species currently known. Basically, the ants involved in myrmecophic relationships "farm" their subjects, and in return are rewarded with highly desirable "honeydew".

The ants (I think a number of species may be involved and I don't know the identity of those in the photo) tend the azure caterpillar closely. They are essentially the cat's private security force, warding off attacks by parasitoid wasps, flies, and other would-be predators. In return, the caterpillar secretes nutritious "honeydew", a liquid excreted by the larvae. This stuff is clearly quite valuable to the ants, as they not only do not let the cat out of their sight, they often remain atop it. In my photo, seven ants are in the photo. No bad guys are breaking through these bodyguards.

So, if you run across black cohosh in flower, take a closer look for a squadron of ants swarming around the buds and flowers. Closer inspection may reveal one of these interesting caterpillars.

Nature: Wasp turns caterpillars into zombified protectors

A Glyptapanteles wasp cocoon guarded by a parasitized saddled prominent caterpillar

Columbus Dispatch

October 1, 2017

NATURE
Jim McCormac

George Romero, who died in July, made some of the greatest horror flicks ever. He is especially known for his films about zombie apocalypses, such as “Night of the Living Dead.”

I don’t know if Romero knew anything about the subject of this column, but I’m sure he would have been intrigued.

During a nocturnal field trip in southern Ohio’s Scioto County in August, my group noticed an odd thing. Plastered to a tree branch was a strange, fluffy cocoonlike object. Perched atop was a saddled prominent caterpillar, a type of moth larva.

Upon closer inspection, I realized what we were seeing: a zombie caterpillar, rewired and forced to act as guardian of its killers.

If you were a being that reincarnates, choosing a caterpillar for your rebirth would be a bad idea. You could take myriad forms, as there are more than 2,000 species of moths and a bit more than 100 species of butterflies in Ohio, all of which are caterpillars for part of their life cycle. But the problem is they have a mortality rate near 99 percent.

These tubular bags of goo are preyed on by all manner of beasts. Songbirds snap them up, mice relish them, and scores of predatory insects feast on them.

To survive this predatory gauntlet, many moths and butterflies practice carpet-bombing reproduction. A female might lay hundreds or thousands of eggs to ensure that a few of the offspring survive to the adult reproductive stage. The rest will fuel the food chain.

Perhaps no caterpillar predator is stranger or more horrifying than the tiny wasps in the genus Glyptapanteles. These wasps are parasitoids, and it’s worth noting their difference from parasites.

Lice, ticks and chiggers are all parasites. They’re annoying and might even carry diseases, but they do not, typically, directly kill their hosts. Not so the parasitoid. The ultimate fate for their hosts is usually death, and sometimes in grislier ways than those concocted by Romero’s fertile imagination.

A female Glyptapanteles wasp seeks out appropriate caterpillar hosts, and when she finds one, the attack commences. She alights on the victim and uses a needlelike ovipositor at the end of her abdomen to inject several dozen eggs.

The eggs quickly hatch, and the wasp grubs feast on the caterpillar’s nonvital tissues and fluids. Eventually, the mature larvae exit en masse by boring holes through the caterpillar’s skin. The shed skins of the grubs’ final molt apparently plug their exit holes.

Upon emergence, the grubs begin spinning silken cocoons under the caterpillar, which, incredibly, begins helping them by enshrouding all of the wasp larvae in a thick, protective bag of its own silk.

Once all of the wasp grubs are safely ensconced in the silken shelter, the caterpillar stands guard atop the structure, thrashing its body at any potential predator that dares to encroach. The mechanisms that trigger the “zombification” of the caterpillar are imperfectly known, but it’s probably due to some chemical brew injected by the wasp.

By the time the grubs transform and emerge as wasps, the caterpillar will be dead or nearly so, having given its life to help ensure the survival of its nemesis.

In spite of all the apparent protections, some predators slip through and attack the grubs. Note the tiny wasp in the photo. It might be a “hyperparasitoid,” a predator of the predator. The wasp appears to be injecting eggs into the grubs within the cocoon.

Nature is seldom Disneyesque.

Naturalist Jim McCormac writes a column for The Dispatch on the first, third and fifth Sundays of the month. He also writes about nature at www.jimmccormac.blogspot.com.