Dutch researchers have discovered that cabbage volatile odour released in response to white butterfly caterpillar attack can attract multiple parasites making the food chain unbeneficial to the plant.  The study, published in journal PLOS Biology Tuesday, highlights the possible side effects of parasitoid use for pest control.
A second parasite, called hyperparasitoid, is attracted by the cabbage odour only after a first parasite infects the caterpillar. The researchers performed both laboratory and field experiments to verify the data.
As described by the study, the food chain starts when a white butterfly caterpillar, Pieris rapae, attacks a cabbage leaf.  In response, the cabbage releases volatile odour to attract the caterpillar parasites. The odour attracts parasitical wasps, such as Cotesia glomerata, that infect the caterpillar with their eggs. Such behaviour is called parasitoid, when larvae lead a parasitical lifestyle and the adult individuals don't. Such reaction favours the plant.
However the odour also attracts a hyperparasitoid wasp, Lysibia nana, which infects the Cotesia glomerata's larvae and takes over the food chain. Field study confirmed the hyperparasitoid's behaviour is well tuned to the environment: it does not follow cabbages nearly as much until they are infected by the first parasite, too.
Dr Erik Poelman, the research leader, commented about the specifically well adapted behaviour of the parasitoids, "In controlled laboratory assays as well as under field conditions, hyperparasitoids were offered plant odours coming from two types of plant: ones damaged by healthy caterpillars, and ones damaged by parasitoid-infected caterpillars. We found that they preferentially detected odours of plants damaged by infected caterpillars. We were excited by these results as they indicate that hyperparasitoids rely on a network of interactions among plant, herbivore and parasitoids to locate their host".
The scientists hypothesised the saliva of infected caterpillars is different and affects the cabbage odour. The study involved collecting air around healthy, infected, and twice infected cabbage plants. Then the researchers checked the hyperparasitoid reaction to the collected odours. The hyperparasitoid attraction to the twice infected cabbage odour was nearly five times more compared to the odour of a cabbage infected only with caterpillars.
The researchers also carried out tests of hyperparasitoid attraction to the healthy and infected caterpillar saliva alone, without the cabbage. There was no such attraction, confirming the cabbage odours are affected by the saliva.
The scientists say, "Our results show that hyperparasitoids may parasitize up to 55% of the parasitoid offspring, therefore potentially playing a major role in parasitoid population dynamics. ... Overexpression of herbivore-induced plant volatiles [HIPVs] in crops or field application of synthetic parasitoid attractants may not benefit pest control in conditions where the responses of hyperparasitoids to HIPVs cause major mortality to parasitoids."

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Of cabbages and things: Dutch researchers study wasp hyperparasitoid