Science - February 17, 2005

Butterfly gives lift to killer of its babies

Parasitic wasps track down fertilised female Cabbage White butterflies because they can smell an anti-sex scent, according to a publication in Nature by entomologists in Wageningen and Berlin. The parasitic wasps also make use of the butterfly to hitch a ride to the Brussels sprout plants where they parasitize the freshly laid butterfly eggs.

The results of a joint research project of entomologists at Wageningen University and the Free University of Berlin are published on 17 February in Nature. ‘Parasitic wasps often hitch a lift with butterflies, but in this case we discovered that the parasitic wasps make use of an anti-sex scent emitted by the butterfly. It is an ingenious form of scent espionage that enables the parasitic wasp to ignore the sex female butterflies that are still virgins, and therefore will not lay eggs,’ explains Nina Fatouros of the Freie Universität Berlin. She worked together with her colleagues in Wageningen on a series of smell experiments with the large Cabbage White and the parasitic wasp Trichogramma to unravel this unusual form of scent espionage.
During mating, Cabbage White males transfer a special scent to their partners, benzyl cyanide. This scent repels male competitors and works therefore as a sex repellent, the opposite to a sex attractant. The miniscule parasitic wasp Trichogramma, a natural enemy of the Cabbage White, uses this scent to recognise female butterflies that have mated and therefore are going lay eggs. ‘If we treat virgin female butterflies with the scent they suddenly also become attractive to the parasitic wasps,’ says Fatouros.
Thanks to the scent spy work, the parasitic wasps can track down the fertilised females when they deposit their eggs on the Brussels sprout plants. Fatouros and her colleagues discovered that in almost half of the cases the parasitic wasps hitch a ride with the butterfly. Once the butterfly has landed on a sprout plant the wasps climb down from the butterfly and lay their own eggs in the eggs that the butterfly has laid. ‘It’s a clever strategy as Trichogrammas are not known for their good flying,’ according to Fatouros.
The entomologists expect that this understanding of how the parasitic wasps track down the eggs of the Cabbage White butterflies will help to make biological control of caterpillars more efficient. / GvM

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