Glyptapanteles sp.

Today we're featuring a guest post by Niamh Dalton - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyer, who has previous written guest posts about salp-riding crustaceans and ladybird STI on this blog. This post was written as an assignment on writing a blog post about a parasite, and has been selected to appear as a guest post for this blog. Anyway, I'll let Niamh take it from here.

Wasps in adult form are terrifying, right? Humans automatically associate the sight of wasps with sudden panic in the fear of getting a minor sting. What do we really have to be afraid of? After briefly studying the life-cycle of a species of wasp, Glyptapanteles, I assure you it’s not adult wasps we should be frantically sprinting away from, it’s their babies.

Glyptapanteles cocoon being watched over by their caterpillar guardian, from Fig. 1 of the paper

Glyptapanteles wasps are parasitoids, a group of parasites that inevitably kill their host.  Adult females, after mating, will inject their eggs into a live caterpillar. The caterpillar will act as a surrogate womb, giving the eggs a chance to develop into mature larvae as they feed of its bodily fluids. The larvae eventually break through the skin of the caterpillar to complete pupation, meanwhile the caterpillar is still living and undergoes mind control by the parasite, becoming a modified bodyguard and surrogate parent until the larvae break out and fly away, leaving the caterpillar to die of starvation.

As spine chilling as this process is, a team of scientists were particularly interested in this survival technique and they constructed an experiment to investigate the behaviour modifications inflicted by the parasite on their host.

It all begins with a female wasp injecting approximately 80 eggs into the body cavity of a caterpillar using an ovipositor or egg layer. Each egg hatches into a larva in the the caterpillar’s body, feeding only off the bodily fluids and being careful not to damage any internal organs in order to keep the host alive and functional. According to the scientists' observations, there is no behavioural modifications of the host during this internal parasitism stage, however, each larva is the size of a rice grain and the density of the larvae in a caterpillar can have morphological alterations. The caterpillar will grow in girth but not in length, looking ready to explode.

It gets worse. Eventually the larvae have to leave the nest, so to speak. To complete the final stage of maturity, all 80 larvae evacuate the host simultaneously by using their newly developed jagged jaws to slice through the caterpillars’ tough skin. Whilst emerging through the tough material, the larvae release a chemical which only paralyses the host, meaning the host is alive throughout this excruciating process. In order for the larvae to keep their host alive, they coincide their last moulting stage with their exit, filling the holes they have excavated with a ‘plug’ made of their sloughed exoskeleton.

Why would the Glyptapanteles larvae have to keep the host alive after emergence? Well, following their exit, the larvae begin to spin silk strings and form cocoons for their last stage of maturity. At this stage, the larvae are vulnerable to predators and other parasitoid wasp species that can inject their eggs into these larvae (ironically). The host develops behavioural modifications during the parasites pupae (cocoon) stage, acting as a bodyguard. As caterpillars are themselves larvae of butterfly and moths, they too construct a cocoon in their life-cycle. As the scientists found, the host caterpillar will use their own silk string to weave a blanket over the Glyptapantele cocoons for further protection.

That’s not all. The host will increase its number of violent head swings in attempt to scare off any form of disturbance. The host is also known to stand on two pairs of back legs in vigilance and spending a substantial amount time bent over the cocoon mound. In the experiments, the research team found an increase in aggression in caterpillars that were infected with the parasitoids compared in caterpillars that were not exposed to parasites.

The main question that remains was: How is there behavioural modifications in the host after the exit of the parasite? After the dissection of previously parasite-stricken caterpillars, there were 1 or 2 active parasitoids found still in the body cavity. The authors of this paper hypothesised that these leftover larvae are responsible for the mind controlling of the host after emergence. In this way, the parasites sacrifice a few individuals for the survival of the majority of the larvae. This is a uniquely evolved survival technique that is obviously very effective and bitter-sweet in a strange way.

Reference
Grosman, A., Janssen, A., de Brito, E., Cordeiro, E., Colares, F., Fonseca, J., Lima, E., Pallini, A. and Sabelis, M. (2008). Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators. PLoS ONE, 3(6), p.e2276.

This post was written by Niamh Dalton