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