The Tachinid fly looks like a house fly except that it has red eyes and a hairy back. An adult will generally live for about 30 days. Tachinid flies go through a complete four-stage life cycle (egg, larva, pupa, and adult). In its larval stage it is commonly referred to as a maggot.
While several Tachinid fly species are known to parasitize Lepidoptera (butterflies and moths), the species that is most likely to parasitize the monarch is the Lespesia archippivora. The female Tachinid will lay one of more of her eggs, generally up to five eggs, on a monarch caterpillar. When the eggs hatch, the larvae (maggots) burrow inside the host caterpillar and feed on it until they are fully grown.
Either when the monarch caterpillar is in its 5th instar or shortly after it pupates, the fully grown fly maggots exit their host. If the host caterpillar is already attached for pupation, the fly maggots will drop from the host (caterpillar or pupa) via a white, silk-like thread. In nature the fly maggots will generally pupate just below the soil level. Ten to 14 days later, adult Tachinid flies will emerge from the pupae.
If the Tachinid fly larvae exit butterfly larvae that are being raised in captivity, the cream-colored maggots might be seen crawling on the floor of the rearing cage. Less than an hour after emerging, the maggots will pupate. Newly formed Tachinid pupae are light brown and hardened ones are reddish- brown. Each pupa is an elongated oval and approximately 1/4” long.
Tachinid flies are parasitoids, meaning that the host organism, the monarch in this case, dies as a direct result of the Tachinid maggots feeding on the insides of the butterfly larvae.
Should you find white strings hanging from monarch caterpillars that have “J”ed or from pupae, the infected stock should be immediately discarded. They are likely already dead (or in the process of dying) and they cannot be saved. To keep them in the cage is asking for trouble. Dead and dying monarchs will attract bacterial and fungal microorganisms that could infect otherwise healthy monarch caterpillars. Also discard whatever fly maggots or pupae you find on the floor of the cage.
Tachinid flies, in nature, seem to be more abundant in August and September than they are in spring and early summer. This is likely because they, like other predators and disease, have had the summer to multiply.