Parasitoid wasp may be the most diverse animal group

On Costa Rica, an orb weaver spider is doomed to die. She doesn’t know it yet but there is a white grub stuck to her abdomen and it is growing bigger, feeding on her while the spider shrinks.

One day, the large grub will suck out the spider’s remaining life juices and throw out the skinny remains. It will spin a cocoon for itself and a few weeks later, a wasp will emerge from the cocoon, all grown up. 

It may seem like a raw deal for the spider. But wasps that lay their eggs inside other insects, which their larvae will eat from within, are all too common. In fact scientists think that

“There are probably more species of them than any kind of animal on Earth”

In many cases, the host insect is still alive while their insides are being eaten by the wasp’s larvae. When the larvae are ready to pupate, they normally kill their host.

This lifestyle has an unusual name. The wasps are not strictly parasites like fleas, which do not kill their hosts. But wasps also do not kill their hosts as quickly as a predator like a lion would. They are somewhere in between – hence the term “parasitoids”. 

Parasitoid wasps infest a host of bugs, from ants and bees to flies and caterpillars. Each species of wasp specialises in attacking only one species of host insect, leading to a huge diversity in the types, or species, of wasps.

One wasp species lays its eggs into a moth larva when it is already in its cocoon. Another species risks its life wrestling with the dangerous jaws of an antlion larva to lay eggs in its throat. 

A different wasp species will dive underwater to lay an egg in the caddisfly larva while another uses a caterpillar both as food and as a bodyguard for her young. This wasp lays up to 80 eggs inside a caterpillar, which hatch into larvae that eat through the caterpillar. The larvae then spin cocoons near the still-alive caterpillar, which uses its body as a tent to protect the pupae. 

You may feel that parasitoid wasps are simply horrible. But in the grand scheme of ecology, they are rather useful, as each parasitoid is so specific in its choice of hosts. For example, many parasitoids are used to control crop destroying pests, without the need for damaging insecticide. To date, there have been more than 3600 parasitoids introduced against more than 500 pests. 

As unfortunate as it may be for the host insect, even parasitoid wasp larvae have a purpose in this world.

Featured imgae of a female parasitoid wasp (credit: Wikimedia Commons under Creative Commons licence)