There are over 1300 species of tachinid fly in North America, all belonging to the family Tachinidae from which they get their common name. Most trachinid larvae are non-selective parasites of many different kinds of insects including armyworms, cabbage looper, Colorado potato beetle, squash bugs, cucumber beetles, gypsy moths, and tent caterpillars. Most are parasites of the caterpillars of moths or butterflies or the larvae sawflies but a few parasitize true bugs, beetles, grasshoppers or centipedes. Tachinids are a major natural factor in controlling these and other insect pests but unfortunately, also they parasitize caterpillars of desirable butterflies such as the monarch.
The adult flies are about 1/2 inch long and resemble houseflies. They are gray, brown, or black often with colorful markings, are hairy, and have bristles on the end of their abdomen. They feed on nectar and honeydew. Adult tachinids vary in their egg laying practices but in all cases their larva end up inside the body of the host insect and consume the internal organs. A few flies lay the eggs directly inside the host. Others lay eggs on the host and enter the host after hatching. Still other flies lay eggs on foliage; these eggs are either ingested by the host or hatch to release larvae that penetrate a host that passes by. Many species are able to have multiple generations per year.
There are three ways to encourage populations of tachinid flies. The first is to grow flowers that attract the flies for the nectar they provide including dill, parsley, Queen’s Anne’s lace, buckwheat, goldenrod, pigweed, and sweet clover. Second, avoid broad-spectrum insecticides, and third, allow parasitize caterpillars and leaves carrying eggs in the garden so the larvae can complete their life cycle and lay more eggs.