Bug of the Week

Tachinid Fly (Family Tachinidae)


Howdy, BugFans,

When you are scrutinizing the prairie flowers in late summer and you spy a “plus-sized” fly with a teeny tiny tutu, it’s probably a tachinid fly. The adults are there to drink nectar, though they have an ulterior motive for visiting the flowers.

Tachinid flies have bristly abdomens.
Tachinid flies have bristly abdomens.

Tachinids are, in a sense, the cuckoos or cowbirds of the insect world. But, instead of laying their eggs in another insect’s nest, they lay one to two eggs in an unsuspecting caterpillar’s “hard-to-reach spots” (along with Lepidopterans – butterflies and moths – they also target members of the True Bug family, Hemiptera). Or, they inject eggs or live maggots under their quarry’s skin; mom hatches these internally, on-the-spot, when she finds a likely victim. Or, they just deposit eggs on leaves so their hosts will graze on and ingest them. Or, creepiest of all, the eggs of a few species hatch on the leaf and the maggots stalk their own potential hosts (permission to shudder). No, the Bug Lady could not possibly be making this up, although it does sound like a 1950’s Japanese horror film. The prairie hosts many small and violent stories.

The maggots live as internal parasites, consuming their hosts’ less important tissues first and not finishing off the vital organs until they are ready to pupate. The larvae leave the host and pupate on the ground. It’s been said that “a good parasite doesn’t kill its host” and one author differentiates these maggots as “parasitoids.” Because of this lifestyle and because of their preferred hosts, tachinids act as controls on a variety of pest insects, and some alien tachinid flies have been imported as biological controls. Alas, the one introduced to control the gypsy moth never met a caterpillar it didn’t love, and it especially loves the caterpillars of Saturnid Moths - the giant silkworms, the Luna and Cecropia moths of previous Bug-o-the-week fame – whose numbers decline dramatically when tachinids come to town.

Parasitoids 1; Biologists 0.

The BugLady