Plants that are under attack from caterpillars call on parasitic wasps for help. They do this using volatile scents, which they produce in response to the damage caused by being eaten. The parasitic wasps then help to free the plant of its hungry attackers. However, the scents also attract other insects. A new study published by a Wageningen research team in the open access journal PLoS Biology on 27 November shows how other hyper-parasitic wasps pick up these plant scents to track down their hosts, the larvae of parasitic wasps.
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Photo: Lysibia nana infests pupae of the Cotesia glomerata, by Nina Fatouros.
A team of Dutch researchers led by Erik Poelman from Wageningen University, part of Wageningen UR, studied the behaviour of what are known as secondary parasitic wasps (or ‘hyperparasitoids’), which attack other species of parasitic wasps while they attack caterpillars. The research has shown that these hyperparasitoids can smell whether plants are being attacked by healthy caterpillars or caterpillars that have been parasitized by parasitic wasps. Experiments carried out both in the laboratory and the field have shown that hyperparasitoids prefer scents emitted by plants being eaten by parasitized caterpillars to the scent of plants under attack from healthy caterpillars. The results provide evidence that these enemy’s enemy’s enemies use a complex network of interactions between the plant, the caterpillars and the parasitic wasps to locate their host, larvae of the parasitic wasp.
To show how this complex network of interactions provides reliable information about the presence of parasitic wasps, the researchers collected saliva from the infested and the healthy caterpillars. Substances in the caterpillars’ saliva play a major role in the change in the scent that plants emit when under attack by caterpillars. Dr Poelman's team discovered that adding saliva from parasitized caterpillars to a plant caused the plant to produce a different scent profile than when saliva from a healthy caterpillar was introduced. Furthermore, the scents that a plant produces in response to saliva from parasitized caterpillars attract hyperparasitoids, which then attack the larvae of the parasitic wasps.
“You have to think of plants producing scents in reaction to plant eaters within the context of the food web as a whole. This includes the enemies of the parasitic wasps”, says researcher Erik Poelman, who headed the research funded by a Veni grant from the Netherlands Organisation for Scientific Research (NWO). “This is the only way to understand the ecological functions of plant scents." In addition to the ecological aspects of their work, the authors are also keen to stress the importance of their findings in terms of developing strategies for comprehensive pest control whereby parasitic wasps are used to regulate insect plagues. “Although parasitic wasps are an effective biological control agent, it would now seem that optimising biological pest control by means of plant scents can also have side-effects. And this could reduce the efficiency of the pest control”, according to Poelman.