Tachinid flies are a large and valuable group of parasitic insects. There are more than 1,000 species in North America. They are endoparasitoids, which means they develop inside their hosts. They tend to have a wider range of hosts than parasitic wasps. These flies can be very important natural enemies of leafrollers, cutworms and other Lepidoptera that feed exposed on foliage in the Northwest.
Tachinid flies attack the larvae of moths, beetles, sawflies, stink bugs and other insects and are effective biological control agents because they can increase their populations rapidly. Tachinids from five different genera have been reared from leafroller larvae.
The egg is white, oval and flat. It is not unusual to see a caterpillar with several tachinid eggs attached to its body or a stink bug nymph or adult with eggs on its shoulders.
The maggot develops inside the host body. In some species, the adult keeps the larvae within its abdomen and deposits them directly on or near the host.
The pupa is dark reddish brown and oval, like a large grain of wheat.
The size and appearance of the adult varies depending on the species. It is usually hairy or bristly and dark gray or black. Many are about the size and color of a housefly.
The life history of different species can vary considerably.Some tachinid flies complete only one generation a year, spending much of the season in the pupal stage. Others have several generations and complete the life cycle in 3 to 4 weeks.
The life cycle of some tachinid flies corresponds closely with that of the host. If the victim—usually a caterpillar—overwinters as a larva, the tachinid larva also spends the winter at rest inside the host. Some species of tachinid flies overwinter as pupae in the soil or in leaf litter.
Adults emerge in the spring and feed on insect honeydew and flower nectar. After mating, the female begins searching for hosts. Many tachinid fly species that lay eggs deposit them directly on or in the body of their host. However, some simply deposit their eggs on the host’s food plant and leave it up to the larvae to find suitable victims. Many tachinid flies do not lay eggs. Instead, they deposit young larvae on, in or near their hosts. The young larvae feed their way into their hosts, where they chew on the gut wall. Usually, a single larva develops inside an individual host insect. Many tachinid larvae almost totally consume the host insect before they bore out of the host to pupate and complete the generation. Usually the tachinid larva will leave the host pupa, forming its own pupa close by.
Adult flies can often be seen visiting flowers in the orchard or on plants near the orchard. They can be recognized by the thick bristles on the abdomen. Often the large white eggs of tachinid fly species that deposits eggs on the host can be seen on the head or thorax region of caterpillars that have been attacked. Usually, the presence of tachinid flies will become evident during sampling for pests. The parasitoid’s pupa, or the empty pupal case, often remains beside the dead body of the host.
The impact of tachinid flies on pests in the Northwest is not well understood. Broad-spectrum insecticides are probably highly toxic to these natural enemies.