Robins reject red glowing grub

Parasitic worms induce a color change in caterpillars that's repulsive to predators

Parasitic worms may be saving their own little hides when they induce the caterpillars they infest to glow a little and blush a furious red.

When infected by a parasite, normally pale caterpillars turn red and actually glow a little (visible to human eyes only in darkness), a color change that may turn away predators. Sarah Clark

As the parasitic nematode Heterorhabditis bacteriophora infects caterpillars of the greater wax moth, the normally pale caterpillars temporarily bioluminesce and also turn persistently pink-red.

In outdoor taste tests with 16 European robins, birds overall preferred uninfected waxmoth caterpillars to ones that had been infected for at least three days. By day seven of infection, odd-colored caterpillars barely even got tentatively picked up by the birds, report Andy Fenton of the University of Liverpool in England and his colleagues in an upcoming paper in Animal Behaviour.

“I think the cool thing is that it’s the first example, to our knowledge, of a parasite manipulating its host to avoid being eaten,” Fenton says. 

It’s to the parasite’s advantage not to be eaten, Fenton explains, because these nematodes don’t infect vertebrates. So if a bird happens to eat a parasitized caterpillar, it’s bye-bye wormy.

Biologists have already uncovered weird examples of the opposite approach, in which other parasites change the appearance or behavior of hosts in ways that attract predators. Ants infected with the parasitic Dicrocoelium dendriticum worm, for example, crawl up grass blades until some grazing cow or sheep inadvertently scoops them up with a mouthful of forage. This worm does infect grazers and thus sacrifices its current host to the next one.

Young nematodes of the species in the new study search through soil for larvae of a range of flies, beetles, butterflies and moths. Slipping into victims through the mouth, breathing holes or anus, the nematodes release live-in luminescent bacteria that reduce the host’s innards to a nutritious broth for both the nematodes and their microbial passengers.

Whether the parasites benefit in some way from their hosts’ reddening and weak, transient glow — which is undetectable to the human eye in daylight but easily visible in a darkened room — has inspired considerable speculation, Fenton says. Researchers have mused about whether the color change may be just a side effect of reducing the buildup of highly reactive forms of oxygen. As far as he knows, Fenton says, his is the first test of the warning-color idea with real birds and caterpillars and parasites.

“This is an old idea but a fun one,” comments Richard ffrench-Constant of the University of Exeter in England, who has studied the nematode but wasn’t involved in the new research.

Studying parasite manipulations does offer the immediate fascination of the “gross and riveting,” Fenton acknowledges. But the parasite takeovers also give evolutionary biologists some extreme examples of evolution acting on the genes of one species, the parasite, as the genes change the body of a completely different species, the host.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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