It was around a century ago that etymologists first noticed something peculiar hanging from the catkins of a dwarf chestnut tree in Norfolk, Virginia. They discovered hundreds of dead goldenrod soldier beetles (Chauliognathus pensylvanicus) dangling in groups of 6 or 7, their mandibles clamped firmly to the tree in a “grim death-grip”. Even more bizarrely, the beetles’ wings were splayed wide open. It was as if they had been readying to give a last, final flight before expiring.
Closer examination of the insects yielded a tell-tale clue about what might be happening: the dead bodies had a furry appearance, instantly recognisable as the sprouting bodies of a fungal infection.
Over the past couple of decades, Donald C. Steinkraus of the University of Arkansas and his colleagues have been studying this bizarre phenomenon. Their work has revealed that the fungal parasite Entomophthora lampyridarum lurks in the landscape, waiting to infect soldier beetles. After penetrating the surface of the hapless beetle, the fungus seizes control of the mind of the beetle, which begins marching to the upper leaves of the plant. Here, they clamp their jaws firmly down onto the leaves, before finally succumbing to the infection, causing them to “die in a situation where they will be in contact with healthy beetles that come to the flowers to feed and mate” Steinkraus told me.
Such a mind-bending trick is not unique to the Entomophthora pathogen. An eerily similar feat has been observed in another fungus, where Ophiocordyceps is known to infect carpenter ants on the jungle floor, forcing the victims to climb plant stems, where they also clamp their jaws to the underside of a leaf in an almost identical “death-grip”. Days later, fungal reproductive structures burst through the insect’s corpse and the spores rain down onto the unsuspecting ants on the forest floor, starting a new cycle of infection.
Whilst these two different species of “zombie-funguses” have some eerie similarities, they attack completely different hosts and operate in entirely different ecological niches. Furthermore, the two species are only very distantly related, and according to Steinkraus, it is “a case of convergent evolution” – a process whereby unrelated organisms independently evolve similar traits.
The Entomophthora parasite also appears to have a more complex life-cycle than its distant cousin, employing two alternate phases to subjugate the beetle (Journal of Invertebrate Pathology http://doi.org/b7tx). A further strategy does not involve the “death-grip”, but instead the beetle simply falls to the ground, where it produces a different type of spore, one that is incredibly tough and resilient, designed to survive through the winter and infect beetles into the next season. “Both stages are adaptive for the fungus”, says Steinkraus.
The researchers also discovered a sinister and truly zombie-like final twist to the infective cycle of Entomophthora. The final act of beetles that have been manipulated to climb up and clamp their jaws onto flowers was to spread their wings, which was established to occur nearly a day after death. This was caused by the fungus bursting through the thorax, forcing the wing-cases to open. According to Steinkraus, this process is “as if a human corpse lay all day in the morgue, and late the following evening, it suddenly sat up, spread its arms and legs in a peculiar way.” The researchers speculate that this wing-spreading might stimulate mating-behaviour, tempting uninfected beetles to crawl over the cadavers in a final “fatal attraction” before they themselves succumb to the fungal infection.