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Several thousand species of insects are parasitic in some way, many of them killing the victims on which they feed internally or externally.

The most efficient parasites are those that live at the expense of their hosts but do not kill them. However, most parasitic insects, with the exception of fleas, lice, and a few others, attack only other insects and do kill them. These parasitic insects are known as internal carnivores and are called parasitoids.

If you want to know more about the parasitic way of life in insects, please read on.

Photo of horsefly

How Do Parasites Keep Insect Numbers In Check?

Parasitoids are of immense importance in keeping the number of insects down. After climate change and disease, parasitoids do more to keep the population down than any other factor.

One study showed that out of every 10,000 caterpillars of the large cabbage white butterfly, 6000 die from the disease, 3500 are killed by parasitoids, and less than 500 by birds. This leaves just 30 to emerge as butterflies.

Caterpillars are an easy target for a host, but parasitoids attack a wide variety of insects, and very few species seem to be free from them. Parasitoids will attack their host at all stages of their life cycle. No matter whether in the egg stage, pupae stage, or as an adult, all are vulnerable to attack by different parasitoids.

Different parasitoids may lay a single egg in or on a host, but sometimes many are inserted, usually through the ovipostor, a tubular organ.

The larvae may feed internally or externally, depending on the attacking species. The larvae of many live inside the body of the host, which is generally a caterpillar, although bugs, earwigs, and woodlice are open to attack.

Ichneumon wasps and chalcid larvae include internal and external feeders, the latter attacking mostly hidden prey such as tunneling larvae. When the parasitoid larvae are young, they feed on the body fluids and less important tissues so that the host remains alive until the later stages of larval development, when all its tissues are attacked.

If you want to know more about parasitic wasps, I have written this article here.

Aphid Hosts

Aphids are a common host for braconid wasps of the genera Aphidius or Praon. The small females lay their eggs directly into the bodies of the aphids, one each, where they quickly hatch and grow.

When attacked by the tiny wasp, the aphid struggles and tries to fight off the braconid. Ants also help in the protection of the aphid colonies.

When the parasitic larva is full grown after a few days, the aphid dies, and its skin hardens. The Aphidius larva cuts a slit in the lower side and glues the aphid to the plant with saliva before pupating in a cocoon spun inside the hardened body. After a few more days, the adult emerges through a hole cut in the top.

These parasitoids though are sometimes attacked. There are other small wasps, related to gall wasps, that lay their eggs within the parasitized aphids, and their larvae feed on the Aphidius larvae within. This is called hyperparasitism.

Are wasps social or solitary insects?  The answer may surprise you.

Ladybirds As Hosts

Ladybirds are subject to attack by the common braconid Dinocampus coccinelli. The female lays an egg inside the adult ladybird, attacking from below and inserting her ovipositor between the body plates.

The ladybird doesn’t die until the parasite emerges from the pupal sac.

Spider Hosts

Externally feeding ichneumon wasp larvae often attack spiders. The female manages to evade the dangerous fangs of the host spider to lay an egg on its body.

The larva clings tightly to the spider’s abdomen, absorbing the spider’s body juices through a small hole made with its mouthparts. As the wasp larva grows, the spider slowly dies, and the larva spins a cocoon which they use to pupate.

Some other species of ichneumon wasps lay their eggs in spider egg cocoons. When the larvae hatch out first they feed on the contents of the spider eggs.

Photo of ichneumon wasp

Parasitic Wasps

Chalcid wasps across different families form a vast group of very small parasitoids, mostly 1-3mm long and often with brilliant greenish metallic coloration. They attack all stages of the host, although eggs and larvae are preferred, and many of them are hyperparasites.

Members of the genus Torymus are green chalcids that attack the larvae of gall wasps, reaching them with their long ovipositors.

Torymus bedeguaris attacks the larvae of the gall wasp Diplolepsis rosae, while Torymus cyanimus, a very similar species, attacks the larvae of the fly that causes galls in thistles.

Trichogramma species are a group of tiny chalcids that attack insects’ eggs. Before laying, the female walks over the selected egg, leaving a scent trail that informs other females that the egg is already parasitized.

Thickheaded Flies

Another interesting family of parasitoid flies is thick-headed flies Conopidae. They often mimic bees and wasps in coloration and in their flight movements around flowers and bushes.

Conopidae specialize in attacking insects that they mimic. They grab their prey in a quick strike, sometimes falling to the ground with them in a violent struggle.

The fly lays one egg inside the abdomen of the bumble bee or wasp, and the larva grows inside while the host continues working. When the host’s abdomen is emptied, the larva reaches into the thorax, and the host quickly dies.

Pupation takes place within the dead shell of the host, and winter is passed in this stage, with the adult coming out the following year.

Do you know how insects avoid predators?  Find out in this article I wrote