To provide their offspring with plenty of food as they grow, parasitic wasps lay eggs inside other animals. After they hatch, the larvae eat their way out and kill the host in the process, though sometimes other wasps end up attacking them. According to new work published in Proceedings of the Royal Society B, parasitoids are extremely effective in using their food resource: They convert almost all their food (their host) into their own body mass.
Food webs can have several levels. Plants are typically the primary level, the herbivore would be the second level, and a carnivore might finish the chain off at level three. Rarely do food webs have more than five of these so-called trophic levels, and ecologists have long been interested in the factors that limit the length of food chains. One factor that’s thought to be a major limitation is the proportion of biomass that’s transferred from one level to the next level – from resource to consumer. Food chains would stay short if less than half of prey body mass is converted into predator body mass.
Host-parasitoid systems might allow for longer food chains. Parasitoids can parasitize other parasitoids: Hyperparasitoids are those that use other parasitoids as their hosts, and even they might be attacked by other hyperparasitoids higher up the food chain.
"The evolutionary pressure to be efficient should be very strong for parasitoids, where the larvae have just one host individual for development into an adult," Dirk Sanders from the University of Exeter explains to IFLScience. Sanders and colleagues used isotope analyses to measure the efficiency of biomass transfer along a primary–secondary–tertiary parasitoid food chain.
They fed bean plants to Megoura viciae aphids, which were fed upon by primary parasitoids called Aphidius megourae. After killing the aphids by feeding on their vital fluids and tissues, the parasitoid larvae develop into their next stage (called pupae) while inside the mummified skin of the aphid. But before they become adults, there are two different groups of hyperparasitoids that could attack them: One lays eggs in the parasitoid larvae that are inside the still-living aphid, another lays eggs in the parasitoid pupae within the aphid mummy. They’re called endoparasitoids and mummy parasitoids, respectively.
The team released the aphids, parasitoids, and three species of hyperparasitoids into a cage with bean plants in a strict succession over the course of about month. They found that parasitoid wasps are extremely efficient in using their food resource, and they’re even more efficient when they’re higher up the food chain.
From the third trophic level (primary parasitoids) to the fourth (secondary parasitoids), the proportion of host biomass transferred was between 45 and 73 percent, depending on the hyperparasitoid species. And for the two wasp species that can act at the fourth and fifth trophic levels, the team found a 93 percent efficiency. "The nutrient content of the hosts at higher trophic levels more closely match that of the parasitoid," Sanders adds, "so it is easier for them to turn it into biomass."