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BBC NEWS | Science/Nature | Grubs fight parasites with food
http://news.bbc.co.uk/2/hi/science/nature/4722233.stm
        
Last Updated: Monday, 1 August 2005, 09:55 GMT 10:55 UK
Grubs fight parasites with food
Caterpillar, Michael Singer
The caterpillars develop a fondness for certain plants to fight a 
parasite infection
Tiger moth caterpillars have been seen medicating themselves to treat a 
nasty influx of parasites.

Scientists found the caterpillars' sense of taste actually changed when 
they became infected with parasites.

Instead of avoiding certain alkaloid plants, the caterpillars actually 
developed a fondness for them.

This change in diet helps to beat the creatures' parasite infection, the 
researchers report in Nature.

The finding is slightly unusual because often when animals change their 
behaviour following a parasitic infection, it is to the invaders' benefit.

"It is a new and surprising kind of interaction between organisms," said 
Elizabeth Bernays, of the University of Arizona, US.

"When parasites change the behaviour of their hosts, it's usually to 
their own advantage."

Race to live

Caterpillars of the tiger moths Grammia geneura and Estigmene acrea , 
which live in the grasslands of southern Arizona, US, are susceptible to 
a range of insect parasites.

The parasitic flies lay eggs on the surface of the caterpillars' skin 
which, on hatching, bore into the larvae's flesh.

        
They can survive because they find the protective plants more tasty
Elizabeth Bernays, University of Arizona
It is here that the struggle for life between host and parasite really 
begins.

The parasite's objective is to feast on the generous supply of live 
tissue, before it pupates and bursts out of the dead caterpillar.

If the caterpillar is to survive such an onslaught, it must develop an 
effective defence strategy.

Instead of eating a wide range of plants, which the tiger moth larva 
usually does, the caterpillar becomes much more specific: it homes in on 
plants that are particularly toxic to its parasites.

Chemical war

These plants, such as woolly plantain, produce chemicals known as 
pyrrolizidine alkaloids and iridoid glycosides.

Not only do these chemicals wash through the caterpillar's body, making 
it an unattractive meal, they also collect in its skin, deterring future 
invaders.

Dr Bernays and her colleagues found that, when a caterpillar acquires a 
parasite infection, its taste cells react differently to the chemicals 
in food.

These cells become more responsive to the protective chemicals and less 
sensitive to other chemicals, which are usually distasteful to the 
caterpillars.

Just how this change occurs in the caterpillars is not known yet.

"It's still a mystery how they do it," Dr Bernays said. "But the result 
for the caterpillars is the same: they can survive because they find the 
protective plants more tasty."