Given that some moths can lay many thousands of eggs, we can be glad that there are many ways by which the lepidopteran population is regulated. Predation by birds and small carnivorous mammals have a major impact on numbers.
In a healthy eucalypt woodland, birds can take about half of the insects produced (some 30 kilograms per hectare per year) and small mammals (bats, sugar gliders, and so on), predatory insects and spiders take a substantial proportion of the rest.Salt, Lindenmayer, & Hobbs - (2004)
Aside from predators, there are also things like fungal and bacterial attack to contend with. One well known bacterium, Bacillus thuringiensis, marketed under names like Dipel, is regularly used by gardeners to control caterpillars.
Anyone who has had a go at rearing caterpillars, will also be well aware of the many parasites of caterpillars such as mites, wasps, and flies. While it can be disappointing for a rearing project to have been parisitised, it is also quite fascinating to get a close up view of these natural population controls in action. Below is a brief look at a few of the parasites I've come across.
Erythraeid mites are the most common ectoparasites of moths and their larvae. (Ectoparasites are those which feed externally on there host.) These do not kill their host. The six legged mite larvae gorge themselves on the blood of their host. When finished feeding they drop off in order to complete their development into the eight legged adult stage.
The photos below shows red Erythraeid mites on Uraba lugens (Gum leaf skeletoniser).
Unlike ordinary parasites, Parasitoids eventually kill their host. They feed on the less vital organs at first. Common parasitoids of Lepidoptera include Ichneumon wasps, Braconid wasps and Tachinid flies.
Ichneumonidae is a very large family of parasitic wasps with around 2000 species in Australia and about 120 of these found in Tasmania. They attack not only several insect orders but also spiders.
Pupation can take place either within the host or it may exit the host first and make it's own cocoon on vegetation or among the leaf litter. The next shot shows an exit hole where dozens of small Ichneumon wasps have emerged from the pupa of Pieris brassicae (Large White Butterfly) These ones have emerged as adults having pupated within the butterfly pupa. (Note: The 4 photos relating to Ichneuman wasps were taken in Scotland)
With larger species of wasp there may be just one egg laid in each caterpillar. For example, here is an arctiid pupa which had a single large Ichneumon wasp emerge from it.
Another major family of parasitic wasps are the Braconidae. They belong to the same superfamily, Ichneumonoidea, as the Ichneumonidae. Australian has about 800 species of Braconid wasp and at least 52 of these are found in Tasmania.
Braconids parisitise insects from many different orders including the Lepidoptera. The female generally lays eggs in the host larvae although sometime they are laid in the host insects eggs. In rare case adult hosts are attacked. Braconids are usually internal parasitoids (Endoparasitic) although some feed externally (Ectoparasitic).
Some Braconids will pupate within their host although more generally the larval grubs will exit their host first. Below is a photo of some Braconid wasp pupae. They had eaten there way out of the caterpillar and immediately made their silk cocoons.
Tachinid FliesThere are over 540 species of Tachinid fly in Australia with around 125 of these found in Tasmania. The larvae of these flies are parasitoids of other insects from several insect orders including Odonata, Hemiptera, Coleoptera and especially the Lepidoptera.
The flies generally lay there eggs on, and sometimes in their host. Others will lay the eggs on plants where the will either be ingested by the host or else hatch first and then find their way onto their host. The photos below show two, of three flies along with an empty puparium. These came from an Arctiid caterpillar. In this case the fly larvae, each only 4mm in length, exited the caterpillar prior to pupating leaving behind the shriveled body of the caterpillar.