Caterpillars on the grass, part four

By the time fungus-infected caterpillars crawl up to the tops of grass stems  (or to other elevated positions), they are almost certainly doomed.  Therefore, it might seem reasonable to assume that this 'height seeking' behavior is instigated by and for the benefit of the fungus.  Yet, there is another possibility.

Though it is too late for the dying caterpillars to save themselves, they may be acting to save their relatives.  Female moths and butterflies sometimes lay their eggs in clumps, with the consequence that sibling caterpillars develop in close proximity.  For these families, it would be advantageous if diseased individuals left the group to avoid infecting the others.  Furthermore, being stuck at the top of a grass stem may have significant drawbacks for the fungus if large herbivores or predators of the caterpillars are around.

Whether climbing up grasses was more beneficial for the caterpillars or the fungus, there were some caterpillars that avoided infection.  About the same time and in the same area that I found the sick, green caterpillar, I caught a much healthier version that had been crawling across the ground.

A healthy moth caterpillar.

I took the healthy caterpillar home to observe whether it too would fall victim to the (at the time) mysterious epidemic.  However, unlike the sick caterpillar, which was rapidly engulfed in fungus, the healthy caterpillar soon metamorphosed into a pupa...

A healthy moth pupa.

... and later emerged as a nondescript, grayish brown moth.

Caterpillars on the grass, part three

I took the sick caterpillars home, sealed within plastic bags to keep whatever might be inside the caterpillars from getting out into my apartment.  Within a few hours, my suspicions had been confirmed: the caterpillars were sprouting fungal fruiting bodies.  By the next morning, the caterpillars were completely covered in shrouds of fungus.

One of the sick caterpillars the next morning.

Once I had solved the central mystery of what was killing the caterpillars, it became possible to find explanations for the strange circumstances surrounding the caterpillars' deaths.

The first thing that had struck me as odd was that I found most of the dead caterpillars on the tops of grass stems.  Yet, the grasses -- unlike many nearby plants -- did not show any signs of being eaten by caterpillars.  Why were the caterpillars on the grasses?  The most likely answer is that the grasses were the tallest plants around.  Insects infected by some types of fungi exhibit "summit disease", in which they climb up to high points shortly before they die.  One hypothesis is that the insect host is being manipulated by the fungus into seeking out a spot that will maximize the transmission of the fungal spores to new hosts.

The second puzzling circumstance was that the caterpillars were firmly attached to the grass stems, despite sometimes being in awkward positions (such as the one pictured below).  Such attachment can be another result of manipulation by the fungus; however, in this case, it was more likely due to the fungus directly.  Fungal hyphae can grow through the insect and into the plant underneath, tightly anchoring the insect into a spot favorable for the transmission of fungal spores.

A dead caterpillar anchored to a plant stem.

The third unusual circumstance was that the only time I found dying caterpillars was in the evening after a rain storm.  Although possibly coincidental, these high humidity conditions are ideal for the production of spores and the infection of new hosts.  Indeed, some fungi have been shown to use the insects' own circadian rhythms to time the insects' deaths favorably (from the point of view of the fungi, of course).

Was the curious behavior of the caterpillars all for the benefit of the fungi?  To be continued...

Explore some more: BIZARRE INTERACTIONS AND ENDGAMES: Entomopathogenic Fungi and Their Arthropod Hosts

Caterpillars on the grass, part two

A few of the dead caterpillars had small amounts of fungus growing on them.  Was the fungus the killer or simply an opportunist benefiting from the work of another?

A dead caterpillar with some light gray fungus visible between it and the grass.

There seemed to be no way to tell.  However, when I returned to the meadow to investigate again a couple of days later, I made an exciting discovery -- in addition to several more dead caterpillars, there were two that weren't dead yet.

The first sick caterpillar, wet from a recent rain shower.

Although they appeared much less dead than the others, these caterpillars were still far from healthy.  When I clipped their grass stems and collected them for observation, the caterpillars did not react and just remained tightly attached to the grass.

Another sick caterpillar clinging onto grass.

There were no signs of external damage, making it unlikely that the caterpillars had been attacked by a predator.  Instead, whatever was ailing them was probably still inside them.

To be continued...