Like many other leaf-eating caterpillars, the bagworm falls under the order Lepidoptera and its adult stage is a moth.
Bagworms are very unusual caterpillars in the sense that they construct their own silken bags, which they strengthen and camouflage with small twigs and stalks, creating the worm’s equivalent of a ‘wooden cabin’, and very cleverly attaching it to a branch with a silken strand. When a larva outgrows its bag, a larger one might be constructed and when it is fully grown it will pupate in this bag, very well protected against its enemies.
In the accompanying photo, the larva is still busy constructing and yet it is almost fully grown. It started at the far end of the twig, cutting off well-measured pieces one by one and weaving them together. Males emerging from the pupa inside the ‘nest’ will take off in search of females, which remain in their bags. The female moth is wingless and has to wait patiently for a male to visit. The females are also quite often ‘degenerated’ in the sense that they have reduced mouth parts, antennae or legs; while the males, on the other hand, have well-developed wings and mouthparts. Some species, like the wattle bagworm, is a pest of forest wattles but will also feed on a range of acacia trees. The particular specimen pictured here was found constructing its nest in a cypress tree.
Bagworms are widespread in South Africa, especially in well-vegetated areas including riverine forests, but they are most common in open bushveld areas. Drastic control measures are hardly ever necessary in home gardens but where one needs to reduce their numbers the first course of action would be to physically pick the bags from the trees and kill the life stages inside. If bagworms are present in huge numbers and it appears that they might just cause serious damage to your cypresses you will be able to control them with Bacillus thuringiensis var. kurstaki, which is commercially available. It should be applied to branches as the bagworms start building as teenagers.