A white maggot emerges from a dead or dying caterpillar or chrysalis. The maggot moves to a dark corner or under whatever is around and changes into a small brown capsule shaped pupa. In a week or two, an adult fly emerges. Yes, that is a Tachinid fly!
Flies around caterpillars can be deadly. Tachinid flies lay eggs on young caterpillars. The eggs hatch and the fly larvae (maggots) begin to drink the hemolymph (blood) of the caterpillar. Just before it pupates, the maggot eats it way out of the caterpillar or pupa. It finds leaf litter, and moves underneath the leaves and twigs. Hidden, it pupates into a small brown pupa.
There are different species of Tachinid flies. Some kill the caterpillars and they fall to the ground. When the butterfly or moth caterpillar is lying on the ground, the fly larvae emerge from the caterpillar and move away to become pupae.
Other species will leave the caterpillar while it is J’ing or after the caterpillar changes into a chrysalis. The fly larvae lower themselves about three or four inches on a mucus string before dropping to the ground.
A caterpillar may have one to over a dozen fly larvae growing inside it.
If you are raising caterpillars indoors, keep them safe from flies. That fly you see may NOT be just a housefly!
We wild collected butterfly and moth caterpillars to raise and photograph their life cycle. We collected some species to raise indoors and re-release outdoors. Every tachinid fly we have seen on our property has come from wild-collected caterpillars because our caterpillars are raised in a closed room in closed rearing containers. Flies cannot fit through the mesh into the rearing containers.
From the wild, we find about a 25 – 50 percent infection rate in some species.
When you find a caterpillar, you can’t always tell that it has a Tachinid fly larva inside its body. You can see eggs if they are freshly laid. Eggs hatch almost immediately after they are laid. When the fly larva hatches, it bends the shell of the egg outward, into the caterpillar.
In some cases, you can see that the caterpillar is infected with Tachinid fly larvae.
Butterfly chrysalises and moth pupae are discolored before the fly larva emerges. You can compare these Tawny Emperor butterfly chrysalis photos. One is healthy with its wings showing through the chrysalis. The other is dark in the abdominal area. A Tachinid fly larva (at least one) is inside the chrysalis and will soon emerge.
Butterfly Fun Facts 