# Eastern thick-headed fly (Physocephala tibialis) > A wasp-mimicking nectar-feeding fly whose larvae are internal parasitoids of bees (especially bumble bees) in eastern North America. Source: https://insectscanner.com/species/eastern-thick-headed-fly/ (updated 2026-10-10) ## Is it dangerous? **Non-toxic.** Physocephala tibialis does not sting or bite people. Adults may land on skin while nectaring but pose no venomous threat. No special first aid is needed beyond normal wound care if accidentally brushed. **Pets:** Harmless to pets; flies do not parasitize mammals. No known toxin risk to cats or dogs. ## Quick facts - Scientific name: Physocephala tibialis - Also called: Thick-headed fly - Order: Diptera (True flies) - Family: Conopidae (Thick-headed flies) - Size: 8-12 mm body length - Colours: Dark brown, Black, Yellowish-buff, Orange-brown - Habitat: Flower-rich habitats: meadows, field edges, gardens, and woodland clearings where bees forage. - Range: Eastern North America (widespread in the eastern United States and southern Canada). - Active: Late spring through early autumn; diurnal (active in daylight when flowers and bees are active). - Lifespan: Adults live for days to a few weeks during the summer flight season. - Effect on plants: Pollinator - Pest: no ## Overview Physocephala tibialis is a species of conopid fly (family Conopidae) common in eastern North America. Adults visit flowers for nectar and pollen and are often wasp-like in appearance. Female flies are parasitoids that lay eggs into foraging bees; the fly larva then develops inside the bee, eventually killing it and pupating inside the dead host's body. Adults are most often seen during the warm months when their bee hosts are active. ## How to identify it Adults are slender, wasp-mimicking flies with a narrow, waisted base to the abdomen that widens into a curled, club-shaped tip, a large head and a long, slender proboscis. The body is dark reddish-brown to black with a pale yellowish face and pale bands or markings on the abdomen; the front part of each wing is darkened. Antennae are elongate and project forward. Females have a downward-curved abdominal tip adapted for inserting eggs into bees. - Slender wasp-like body with a narrow waist and a club-shaped, curled abdominal tip; only two wings. - Long proboscis and flower-visiting behaviour; frequently seen on composite flowers and other nectar sources. - Dark reddish-brown to black body with a pale face; front edge of the wings darkened. - Females may be observed intercepting bees on flowers or carrying an extended ovipositor. ## Lookalikes - **Thick-headed fly (other Physocephala spp.)** (Physocephala spp.): Other species in the genus are similar in shape and habits; identification often requires close examination of leg and genital features by a specialist. - **Thread-waisted wasps** (Ammophila spp.): True wasps with four wings, long thread-like waists and long antennae, and they can sting; thick-headed flies have only two wings, a shorter waist and short forward-pointing antennae. ## Life cycle - **Egg:** The female inserts a single egg into the body of a host bee while it is foraging; the egg hatches inside the host. - **Larva (internal parasitoid):** The larva develops inside the living bee, consuming internal tissues; infected bees often become lethargic before dying. - **Pupa:** The mature larva pupates inside the abdomen of the dead host bee, which often dies in or falls to the soil; the fly overwinters there as a pupa and emerges the following season. - **Adult (fly):** Adults feed on nectar and visit flowers; females intercept and oviposit into foraging bees or wasps. ## Diet and predators - Young eat: Endoparasitic, feeding on host bee tissues - Adults eat: Nectar and pollen from flowers - Predators of the young: Susceptible to parasitoids, predators that uncover pupae in soil, and micro-organisms - Predators of adults: Birds, predatory insects, and spiders ## Effect on plants and gardens Adults visit a variety of flowering plants for nectar and pollen and can act as incidental pollinators. ## Where and when to find it Look for adults on open, nectar-rich flowers, field edges, meadows, and gardens. Watch flowers where bumble bees and honey bees forage; females may be seen intercepting bees. - Best time: Late spring through early autumn, during warm daylight hours when bees are active. - Tips: Use a camera with a short telephoto or macro lens to photograph without disturbing bees; watch for the rapid interception behavior of females on flower-visiting bees. ## Fun facts - Females actively intercept bees on flowers and can insert an egg into a live bee in a fraction of a second. - Although the larva kills its individual host, conopid flies are part of natural checks on bee population dynamics and are not considered major threats to healthy pollinator communities. ## Frequently asked questions ### Is the Eastern thick-headed fly dangerous? No. It does not sting or transmit disease to people and is not considered dangerous. ### Do Eastern thick-headed flies bite? No. Adults do not bite people; females do insert eggs into bees but that behavior is directed at insect hosts only. ### Why is this fly on flowers in my garden? Adults feed on nectar and pollen; gardens with flowering plants attract them because those flowers also attract their bee hosts. ### Are they harmful to bee populations? They are natural parasitoids of bees and can kill individual bees, but they are a normal part of ecosystems. In most cases they do not cause major declines in healthy bee populations. ### How can I get rid of them? Control is generally unnecessary. Removing flowers would reduce visits but also harm pollinators. If you have concerns about local managed bees, consult a local entomologist or beekeeper for specific advice; widespread eradication is neither practical nor recommended. --- More about this species: [iNaturalist](https://www.inaturalist.org/taxa/133978), [Wikipedia](https://en.wikipedia.org/wiki/Physocephala_tibialis). Identify insects and spiders from a photo with the Insect Scanner iPhone app: https://apps.apple.com/app/id1543946563?pt=126685406&ct=web-markdown&mt=8 Identification from photos is never certain: don't handle insects you can't identify, and get medical advice for bites, stings or allergic reactions.