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.
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