Tiny Insect Develops Long-term Memory

Research in Wageningen has shown that a parasitic wasp can learn to recognize a scent secreted by mated (or inseminated) females of the large cabbage white butterfly Pieris brassicae. This takes place while they hitch-hike with the cabbage white butterfly to a host plant where they parasitize the freshly laid butterfly eggs. A day later, the parasitic wasp is able to travel the same route. Parasitic wasps that did not have this ‘pleasant’ experience do not distinguish between mated cabbage butterflies and ‘uninteresting’ virgin ones. The researchers demonstrated in experiments that long-term memory is developed to remember the scent.

During mating, male large cabbage white butterflies transmit a special scent, benzyl cyanide, to their partners. This scent repels male rivals. The Minute (barely 0.5 mm long) parasitic wasps of the species Trichogramma brassicae take advantage of the anti-sex scent of the cabbage butterfly. They essentially engage in chemical espionage. Inexperienced parasitic wasps detect the anti-sex scent and use it to recognize mated female butterflies. The parasitic wasps hitch-hike with these mated butterflies. When the butterflies lay their eggs on a cabbage plant, the parasitic wasp climbs off the butterfly to lay its own eggs in the freshly laid butterfly eggs. In this way it parasitizes and kills the offspring of the butterfly.

For Trichogramma brassicae wasps this is an instinctive, innate behaviour. But this does not hold true for the closely related parasitic wasp Trichogramma evanescens. This wasp parasitizes the eggs of many species of moths and butterflies, including the large cabbage white butterfly. Inexperienced wasps of this species climb onto cabbage white butterflies just like Trichogramma brassicae, but show no preference for female butterflies that have mated. The researchers therefore concluded that they do not respond instinctively to benzyl cyanide.

A research team led by Dr Ties Huigens of the Laboratory of Entomology at Wageningen University wondered if the parasitic wasps could learn this behaviour. That appeared to be the case: Trichogramma evanescens do react to the anti-sex scent benzyl cyanide if they have previously had a positive experience with mated butterflies. After hitch-hiking once with such females and parasitizing freshly laid butterfly eggs, they thereafter climb specifically onto mated butterflies and hitch-hike with them. Either only hitch-hiking or only parasitizing the eggs were not sufficient by themselves. The researchers therefore concluded that the parasitic wasps learn to associate the anti-sex scent of the female butterflies with the reward of parasitizing the butterfly eggs.

The parasitic wasps continued to exhibit this learned behaviour even a day after the initial experience. This is an indication of long-term memory. In order to demonstrate this, researchers provided inhibitors for long-term memory. The parasitic wasps that ate these inhibitors (protein synthesis inhibitors) did not respond to the anti-sex scent a day after they had hitch-hiked with mated butterflies and parasitized freshly laid butterfly eggs. Apparently, the proteins needed for the development of long-term memory could not be produced. Obviously, these insects, with their extremely small ‘brains’ (estimated to be less than 10 nanolitres in volume) are able to develop a long-term memory, even though the formation of such memory requires a great deal of energy. Scientists know this, for example, because insects are less resistant to stress situations such as dessication after they have formed long-term memory.

The possession of long-term memory is very important for these short-lived parasitic wasps because they will probably only have a few opportunities to hitch-hike with mated female butterflies. If they were to immediately forget a positive experience, they could miss these few opportunities. The entomologists from Wageningen expect that learning the espionage-and-ride strategy is a widespread phenomenon because it is adaptive for many species of parasitic wasps killing insect eggs.