"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

June 26, 2015

Lysibia nana

Lysibia nana photo by Nina Fatouros
Used with permission
In order to live, a parasite must find its host. Whereas some parasites take a passive approach and simply wait for a chance encounter, many species are more proactive. In the case of parasitoid insects that have free-flying adults, they have various adaptations for tracking down their hosts. But what about the hyperparasites - parasites that infect other parasites? How do they find their host, which themselves are hidden within the body of a host animal? It seems as if they would need to have X-ray vision in order to complete their life cycle.

The parasite we are featuring today is Lysibia nana, a hyperparasitoid that infects Cotesia glomerata - the parasitoid wasp which lays its eggs inside caterpillars. It turns out that L. nana does not rely on superpowers like X-ray vision, but a far more parsimonious ability. To find out how L. nana finds a host, first of all, we have to ask; how does C. glomerata itself find its hosts? A few months ago, we featured a parasitoid fly that uses sound to track down its prey, but most parasitoid wasps use scent to sniff our their hosts. But this scent does not come directly from the host itself, but rather, the host's food.

When a plant comes under attack by herbivores like caterpillars, they emit volatile chemical signals call kairomones that acts like a dinner bell for parasitoid wasps, which have evolved to use those chemicals to guide them to their prey. Feeding by different species of caterpillars elicit different chemical emissions from the plant, which provides a signature of their presence and attract different species of parasitoids.

Parasitoid wasps are master body-snatchers, they don't just consume their hosts from within; while they are in residence they also change the caterpillar's physiology, altering its growth pattern and behaviour - so much so that on some levels the parasitised caterpillar can be considered as almost a different animal. But they have their own enemies in the form of hyperparasitoids like L. nana.

A research group in the Netherlands conducted a series of experiments to figure out how this hyperparasitoid tracks down its hidden prey. They first tested how wild cabbage plants responded when they come under attack by two different species of caterpillar - Pieris brassicae and P. rapae.
Dead caterpillar with Cotesia glomerata cocoons
Photo by Hectonichus
They found that two caterpillars induce very different blends of chemical volatiles from the plant. But it is a different story when those caterpillars are parasitised by C. glomerata. The physiological alteration that the parasitoid imposed on their host was reflected in how the caterpillar's food plant responded. Cotesia glomerata manipulated their hosts to such a degree that once parastisied, both P. brassicae and P. rapae elicited a far more similar blends of chemical emissions from the plant.

This is where the hyperparasitoid L. nana comes in. The researchers put some female hyperparasitoids in a Y-maze and exposed them to combinations of different volatile chemical released by; caterpillar-free plants, plants which had been chewed on by caterpillars, or plants which have been chewed on by parasitised caterpillars. They noticed that given the choice between the chemicals of plants damaged by parasitoid-free and parasitised caterpillars, the hyperparasitoids preferred overwhelming to go in the direction of the latter - regardless of what species the host caterpillar might be. To L. nana, whether those caterpillars had parasitoid babies onboard is far more important than their species identity, and they showed no clear preference for either caterpillar species as long as they were parasitised by C. glomerata.

The researchers also conducted a field-based study that corroborated the results from the behavioural experiment. They did so by attaching C. glomerata cocoons to some wild cabbage plants that they have grown in an experimental plot. Some of the plants had previously been munched on by parasitoid-free caterpillars, others by parasitised caterpillars. After 5 days, they checked the parasitoid wasp cocoons for signs of L. nana and found that cocoons on plants which have been chewed on by parasitised caterpillar attracted far more L. nana than those munched on by parasitoid-free-caterpillars

So while parasitoid wasps like C. glomerata may have masterful control over their host body's physiology, this also leaves a calling card to their own hyperparasitoids. For the hyperparasitoids, it's what's inside that counts.

Zhu, F., Broekgaarden, C., Weldegergis, B. T., Harvey, J. A., Vosman, B., Dicke, M., & Poelman, E. H. (2015). Parasitism overrides herbivore identity allowing hyperparasitoids to locate their parasitoid host using herbivore‐induced plant volatiles. Molecular Ecology 24: 2886–2899.

P.S. I will be attending the New Zealand Society for Parasitology and Australian Society for Parasitology joint conference in Auckland, New Zealand. So watch for tweets with highlights from conference at my Twitter @The_Episiarch! All tweets related to that conference will have the #NZASP15 hashtag.

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