Tachinid flies
Tachinid flies are a large family of parasitic flies, with over 1,300 species in North America. They look like bristly houseflies, and their larvae develop inside other insects, killing them. The one gardeners meet most is Trichopoda pennipes, the feather-legged fly. It has an orange abdomen and a fringe of black hairs on its hind legs, and it lays its eggs on squash bugs, stink bugs and leaf-footed bugs. Other species lay on caterpillars and beetle larvae. The sign to look for is small white oval eggs glued to a bug or caterpillar. Leave that host alone, because a new fly will come out of it. Tachinids cannot be bought. You keep them by growing nectar plants and not spraying.
At a glance
- Kind
- Wild — attracted, not bought
- Family size
- Over 1,300 species in North America
- Adult
- 2–20 mm; bristly, housefly-like
- Trichopoda pennipes
- 8–12 mm; orange abdomen, fringed hind legs
- Tell-tale sign
- White oval eggs on a bug or caterpillar
- Adults feed on
- Nectar and honeydew
How it works
Tachinid females reach their hosts in different ways. Most glue eggs onto the host's body or insert them into it. Some lay on leaves the host will eat, and a few lay near hosts in the soil. The eggs are oval and clear when laid but soon turn white, so a caterpillar or bug carrying small white dots, each up to 1 mm, has been parasitised. The larva feeds inside the host, which is usually still alive and feeding for a while. When full-grown, the larva leaves the host and pupates, and the host dies. Some species develop singly; in others a dozen or more larvae share one host.
Trichopoda pennipes specialises in true bugs. The female lays one to many small white or grey oval eggs on large nymphs and adults, often on the sides of the body. The hatching larva bores straight through the host's body wall and grows for about two weeks (14–15 days at 78 °F in the lab). It then forces its way out through the rear of the bug, drops to the ground and pupates there. Several eggs may be laid on one bug, but only one larva survives. There are about three generations a year, depending on location, and the fly overwinters as a larva inside its host. Populations seem to specialise on the bugs that are common where they live. Squash bugs and the southern green stink bug are the main hosts, but a California population attacks the bordered plant bug and has never been found on squash bugs.
What it is confused with
A tachinid adult is easy to pass over as a housefly. The eggs are the thing to learn, because they tell you a pest is already taken care of.
- PestHouse flies and blow flies — Similar size and shape, but tachinids have many stiff bristles, often most obvious on the abdomen, and are usually seen resting on foliage or feeding at flowers.
- Also goodBees and wasps — Some tachinids mimic them, but a fly has one pair of wings, where bees and wasps have two.
- PestSquash bug eggs — Laid in clusters on the leaf, pale at first and then darkening to coppery bronze or brown. Tachinid eggs are single white dots glued onto the bug itself.
- Also goodBraconid wasp cocoons — White cocoons on a hornworm's back are wasp cocoons, not tachinid eggs. Also a friend. Leave the caterpillar.
How to keep it around
Adult tachinids feed on nectar and on the honeydew of aphids and scale insects, A flowering milkweed border raised parasitism in one USDA field trial, though a buckwheat border beside squash did not in another. Keep something flowering near squash, tomatoes and beans through summer, and do not use broad-spectrum sprays.
- Buckwheat — T. pennipes adults were seen visiting a flowering buckwheat border beside squash in a USDA trial, though parasitism there was no higher than without it
- Milkweed — a flowering tropical milkweed border raised stink bug and leaf-footed bug parasitism in a USDA trial in Georgia cotton
- A spread of flowering insectary plants for nectar, which UC IPM recommends for adult natural enemies
- Leave squash bugs, stink bugs and caterpillars that carry white eggs where they are
- No broad-spectrum persistent insecticides such as pyrethroids near the crop
Pesticide sensitivity
UC IPM advises avoiding broad-spectrum, persistent insecticides to protect natural enemies, tachinids included. A parasitised bug that is sprayed takes the developing fly with it. Open the compatibility matrix →
When to choose something else
Tachinids are free and common, but they are not a rescue. A parasitised bug goes on feeding and laying eggs for a while, so damage can continue even when many bugs carry eggs. Do something else as well when:
- Squash bugs are on seedlings or at flowering — hand-pick and crush the egg clusters; see squash bug
- The problem is brown marmorated stink bugs — T. pennipes lays on them, but in a 2025 USDA study no flies emerged from them; see brown marmorated stink bug
- Leaf-footed bugs are on the tomatoes now — pick or vacuum them; see leaf-footed bug
- Caterpillars are stripping brassicas — Bt kurstaki works on the feeding larvae
Tachinid flies by target
What it eats and when it turns up differ by pest. Each page below is the picture for one target.