Parasite turns host caterpillars red to warn predators

By Emma Brennand
Earth News reporter

The infected larvae (right) gradually change colour

A parasitic worm that infects insect larvae turns them red to discourage predators, say scientists.

The nematode, Heterorhabditis bacteriophora, is a body-snatcher that attacks developing larvae buried in soil - liquefying their insides.

Once infected the larvae change colour.

Their study, published in the journal Animal Behaviour, reveals that the nematode has developed this tactic to warn birds to keep away from its host and therefore avoid being eaten.

It is the first reported case of a parasite changing its hosts' colouration to avoid predation.

Red is well known as a warning colour Dr Andy Fenton University of Liverpool Parasitic facts, pictures & stunning videos: BBC Wildlife Finder

The parasite is specific to insects and is so effective that it is commercially available as a pesticide.

But despite its widespread use in the control of unwanted insects, until now scientists did not understand why Heterorhabditis bacteriophora changes the colour of its host so dramatically.

Dr Andy Fenton and his research team from the University of Liverpool, UK have been studying the actions of this parasite for 12 years.

"The nematodes die if their insect host is eaten," explained Dr Fenton.

"So it seemed reasonable that such a vivid colour change may be a warning signal to predators - red is well known as a warning colour."

With help from colleagues from Glasgow University, UK, Dr Fenton's team developed an experiment to test the responses of wild robins to infected and uninfected larvae of the greater waxmoth.

They found that the robins avoided eating the infected caterpillars - preferring the uninfected larvae.

The parasite liquefies and feeds on the infect larva's insides

During several trials, the researchers watched the birds approach, and in some cases handle, infected larvae before rejecting them and opting for an uninfected alternative.

And as the red colour developed over time, the birds handled the red caterpillars less and less.

"We were surprised the results came out as clear as they did - particularly when we compared infected insects with ones that had been dead a while," remarked Dr Fenton.

"We thought the birds wouldn't be interested in old dead caterpillars, but they seemed to prefer them to the infected ones."

In addition to this visual "warning sign", the team think the infected caterpillars may have a particular odour or taste that puts the birds off.

Taking over

The nematode enters its victim via the mouth or anus, and quickly releases a lethal bacterium that kills the host.

It then digests its tissues into a nutrient-rich broth that it feeds on.

This nematode-bacterium "tag team" works quickly to take control of their new host, allowing the nematode time to reproduce and pass through several generations within a single dead insect.

When the nutrients run out the host splits apart releasing an army of nematodes into the environment.

This study is the first to show a parasite changing its host's colour to deter a predator, and the knock-on effect that has on the predators prey choices and diet.