Anthela acuta caterpillar with red parasites.
Female moths lay between a hundred and several thousand eggs, depending on species. If all these matured and laid more eggs, then after several years the world would be covered in caterpillars.
Their parasites, parasitoids, and predators stop caterpillars in nature from taking over the world. Note that 'parasitoids' are parasites that actually kill their host, whereas 'parasites' just feed off it but leave it alive.
The main parasites of caterpillars are particular species of viruses, bacteria, fungi, mites, ticks, wasps and flies.
Caterpillars are just as susceptible to disease as we are, and get the equivalents of flu and measles etc. The viruses in the Cypovirus genus, and the bacterium Bacillus thuringiensis are especially fond of attacking caterpillars, and are used as a biological control agents against pest caterpillars.
The exact relationship of the various species of Lepidoptera with the various parasitic species of mites (Acarina), flies (Diptera), and wasps (Hymenoptera) is often very specific, and is also of great importance in pest control. If you should get some flies and/or wasps emerge from your caterpillar or pupa, you might consider donating them to your local natural history museum or university entomology department, who may have study programs on these parasites.
The flies attacking caterpillars come mainly from the family TACHINIDAE,
although Tritaxys flies lay eggs on the caterpillar foodplant that are swallowed but not digested by the caterpillar. In due course: the eggs hatch inside the caterpillar, and eat it from inside out.
Wasps that attack caterpillars come from a variety of families, including:
When a caterpillar is infected with a parasite, its behaviour changes, just as ours does when we get a cold or a fever. So though normally caterpillars will keep hidden, caterpillars when infected sometimes start wandering aimlessly about. So if you find a caterpillar wandering about, it is probably ill already.
Predators also control the caterpillar populations,
such as spiders:
and adult wasps which catch caterpillars, paralyse them, and lay eggs on them, so that when the wasp eggs hatch: the wasp larvae have a ready food supply.
and various Bugs attack caterpillars,
and of course Praying Mantids will happily feed on any life stage of any Lepidoptera.
Even plants attack Lepidoptera:
(updated 3 February 2013, 28 May 2017)