Zoological investigations

Ah, time to go back to school. If you’re anything like me you’ve probably bought a bunch of new pens and stationary you don’t really need and have resolved to read your textbooks and class notes every day, maybe even get up extra early before classes to do something physical. Soon enough, though, as the fall semester kicks into high gear, most of us are bound to feel as exhausted as though the life were being sucked right out of us. That’s why I have chosen to begin the first fall issue of the Manitoban by telling you about the parasitic wasp Dinocampus coccinellae.

D. coccinellae is found throughout Eurasia and North America and parasitizes numerous species of lady beetles (your friendly, aphid-eating, garden variety “lady bug”). The wasps are primarily female and reproduce by parthenogenesis — laying unfertilized eggs, thereby creating clones of themselves. Males are very rare, although they are produced every once in a while so they can disperse, mate and thereby preserve the genetic diversity of the species via sexual reproduction.

The wasps prefer to parasitize female lady beetles and do so by injecting an egg directly into the beetle’s body cavity. Once the egg hatches, the larva eats the lady beetle’s own eggs to nourish itself and eliminate competition for nutrients. Once the eggs have all been eaten the larva begins to eat the surrounding body tissue, including the reproductive structures.

About 20 days after hatching the larva is ready to emerge. To do so, the baby wasp chews a hole through the lady beetle’s body. It is then ready to spin a cocoon, inside which it will metamorphose into an adult wasp. The larva spins its cocoon within the hind limbs of the lady beetle, which is still alive, paralyzed and very miserable.

Currently unidentified venoms secreted by the wasp larva are believed to be responsible for the next stage in this ghastly business. The lady beetle, paralyzed with a wasp cocoon under her body, is now also a victim of behaviour modification courtesy of the wasp — the beetle begins to twitch and grasp erratically. This movement, along with the familiar red and black colouration of the lady beetle, serves to ward off predators, thus protecting the developing larva.

Many species of parasitic wasps lay eggs in a variety of host species, but most of these lead to the death of the host. Not so with D. coccinellae, although I bet the lady beetles wish they were dead. After the adult wasp emerges from its cocoon, the effects of paralysis wear off, as does the behavioural control exerted by the larva. Approximately 25 per cent of parasitized lady beetles survive this process and presumably amble off to continue their regular lady beetle duties.

Yet leaving the host alive is not without costs of its own. After all, nothing in life is truly free, right? Researchers at Laboratoire Maladies Infectieuses et Vecteurs: Ecologie, Génétique, Evolution et Contrôle (CNRS/IRD/Université Montpellier 1) and the Université de Montréal were able to show that while wasp larvae guarded by lady beetles are less likely to experience predation, they also lay fewer eggs (Maure et al. 2011). The developing larvae lose resources by keeping the lady beetle alive, resulting in the observed reduction in fecundity of the adult wasps.
So when the term hits full swing, just remember — you don’t have it half as bad as the spotted lady beetle does!

Comments are closed.