Female moths lay between a hundred and several thousand eggs, depending on species. If all these matured into adult moths, and these laid more eggs, etc, then after about four years, all the world land masses would be about a metre deep in caterpillars.

In nature: the diseases, parasites, parasitoids, and predators of caterpillars stop them from taking over the world.

PARASITES

Agrius convolvuli caterpillar with a parasite:
(possibly a Sand Fly CERATOPOGONIDAE)
(Photo: courtesy of Diana Davey, Woolgoolga, New South Wales)

Parasites usually live on the external body of the caterpillar, sucking juices from the caterpillar through a tube-like tongue pked through the caterpillar's skin.

Cryptoptila immersana caterpillar carrying green parasites
(Photo: Don Herbison-Evans, Sydney, New South Wales))

Parasites seldom kill their hosts.

Hyposidra talaca caterpillar with a parasite
(Photo: courtesy of Harold McQueen)

DISEASES

caterpillar with a bad fungal disease
(Photo: courtesy of Marita Macrae, Avalon Beach, New South Wales)

Caterpillars are just as susceptible to disease as we are, and get the equivalents of flu and measles etc. Caterpillars suffer from diseases caused by various specialised species of viruses, bacteria, and fungi that specialise in infecting and often killing caterpillars.

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.

fruiting body of the fungus Ophiocordyceps sinensis growing out of tail of a Hawkmoth caterpillar
(Photo: courtesy of Nicolas Merky)

PARASITOIDS

Note that 'parasitoids' are parasitic and actually kill their host, whereas 'parasites' just feed off their host but leave it alive.

The main parasitoids of caterpillars are particular species of viruses, bacteria, fungi, mites, ticks, wasps and flies.

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.

Theretra latreillii caterpillar with cocoons of a number of wasp parasitoids
(Photo: courtesy of Jan MacDonald, Queensland)

If 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. In due course, there will be study programs on what particular parasites attack what particular Lepidoptera species, to understand our ecological systems.

two Tachinid fly larvae recently emerged from a moth chrysalis,
one grub having already pupated.
(Photo: courtesy of Peter Street, Gosford, New South Wales)

The flies attacking caterpillars come mainly from the family TACHINIDAE.

dying Brunia replana, caterpillar with cocoons of a number of wasp parasites.

Wasps that attack caterpillars come from a variety of families, including:

wasp checking out a caterpillar
(Photo: courtesy of Michael Watt, Cairns, Queensland)

When a caterpillar is infected with a parasite or parasitoid, 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.

caterpillar vs Red-back Spider (Latrodectus hasselti)
(Photo: courtesy of Robin Sharp, Korong Vale, Victoria)

PREDATORS

Predators also control the caterpillar populations,

spiders love HESPERIIDAE caterpillars
(Photo: courtesy of Kath Vail, Inner Pocket, New South Wales)

such as spiders:

Dasypodia selenophora caterpillar vs Victorian Huntsman Spider (Isopedella victorialis, SPARASSIDAE)
(Photo: courtesy of Wendy Moore, Melbourne)

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 fresh food supply.

wasps love caterpillars
(Photo: courtesy of Trevor Jinks, North Burnett, Queensland)

and various Bugs attack caterpillars, like Shield Bugs (PENTATOMIDAE)

Doratifera casta caterpillar attacked by the Shield Bug Oechalia schellenbergii
(Photo: courtesy of Jenny Holmes, Great Western, Victoria)

and Ants attack caterpillars and pupae,

ants love pupae, in this case a Graphium agamemnon pupa
(Photo: courtesy of Michael Watt, Cairns, Queensland)

and Assassin Bugs (REDUVIIDAE)

a caterpillar stabbed in the head by young Pristhesancus plagipennis,
(Photo: courtesy of Harold McQueen, Goodna, Queensland)

and Scorpion Flies (MECOPTERA : BITTACIDAE) :

caterpillars versus Harpobittacus australis,
(Photo: courtesy of Ken Thomas, Coree, Australian Capital Territory)

and of course Praying Mantids (MANTODEA) will happily feed on any life stage of any Lepidoptera:

Mycalesis perseus butterfly vs Praying Mantis (MANTODEA)
(Photo: courtesy of Jeevan Jose)

and so will birds:

caterpillar vs Australian magpie (Gymnorhina tibicen)
(Photo: courtesy of Harold McQueen, Goodna, Queensland)

who also like adult moths.

moth vs Willie Wagtail (Rhipidura leucophrys)
(Photo: courtesy of Deb & Rod Ralph, Fyshwick, Australian Capital Territory)

Even plants attack Lepidoptera:

Acraea terpsicore, caught by a Sundew plant (DROSERACEAE)
(Photo: courtesy of Alexis & Geoff Ward, near Mt Garnet, Queensland)

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Australian Lepidoptera

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(updated 3 February 2013, 28 February 2023)