Physocephala tibialis (Thick-headed Fly)
Overview
Physocephala tibialis is a species of thick-headed fly in the family Conopidae. These flies are best known as endoparasitoids of bumble bees (genus Bombus), but they also visit flowers for nectar. Despite their parasitic nature, they are ecologically significant in shaping bee populations and maintaining balances within pollinator communities.
Ecological Role
Parasitoid of bumble bees. Female P. tibialis intercepts a foraging bumble bee mid-flight or on flowers, curls her abdomen under the bee’s body, and forcibly inserts an egg into the bee’s abdomen. The larva develops inside, ultimately killing the host.
Pollinator service. Adults feed on nectar and act as pollinators themselves, often visiting open, nectar-rich flowers such as composites (Asteraceae) and mints (Lamiaceae).
Population regulation. By parasitizing bumble bees, they help regulate bee populations, preventing dominance of a few species and potentially contributing to pollinator community diversity.
Life Cycle
Adult emergence (summer): Adults emerge in warm months (June–September in the Midwest). They are slender, wasp-like, with a distinctive "thick head" and often hover near flowers.
Mating and oviposition: Females seek bumble bees; eggs are injected directly into the abdomen of the host.
Larval development: Inside the bee, the larva consumes hemolymph and tissues. The parasitized bee may continue foraging for a while but eventually dies.
Pupal stage: After the host dies, the larva pupates within the bee’s body or in soil.
Overwintering: Likely occurs in the pupal stage; adults emerge the following summer.
Supporting P. tibialis in Chicago
Because this species depends on both flowers for nectar and bumble bees as hosts, supporting it requires a two-layered approach:
1. Nectar sources for adults
P. tibialis visits many of the same nectar plants as bumble bees. The best support comes from long-blooming, nectar-rich, open flowers:
Mountain mints (Pycnanthemum muticum, P. tenuifolium) – Mid to late summer bloom, swarms with pollinators.
Wild bergamot (Monarda fistulosa) – Lamiaceae; excellent for both bees and flies.
Goldenrods (Solidago speciosa, S. rigida) – Critical late-season nectar.
Asters (Symphyotrichum novae-angliae, S. oblongifolium) – Fall bloom, major nectar draw.
Milkweeds (Asclepias syriaca, A. tuberosa) – Early to mid-summer nectar.
These species are all native to the Chicago region and thrive in prairie-style plantings.
2. Host availability (bumble bees)
To sustain P. tibialis, you must also support healthy populations of bumble bees—their required hosts. This means:
Planting diverse, staggered-bloom natives from spring to fall (e.g., willows in spring, coneflowers mid-summer, asters/goldenrods in fall).
Avoiding pesticides that harm bees or their nests.
Providing nesting habitat (undisturbed ground, tussock grasses, and rodent burrows).
How to Grow These Native Plants in Chicago
Here are practical notes for establishing a supportive habitat:
Mountain Mint (Pycnanthemum muticum): Full sun to part sun, medium soils. Spreads by rhizome—best in dedicated pollinator beds. Plant plugs or sow stratified seed outdoors in fall.
Wild Bergamot (Monarda fistulosa): Full sun, medium–dry soils. Easy from seed (cold-moist stratification) or nursery plugs. Can spread by rhizome.
Goldenrods (Solidago speciosa, S. rigida): Full sun, dry–medium soils. Less aggressive than S. canadensis. Start from seed (cold stratify) or plugs.
Asters (Symphyotrichum spp.): Full sun, medium soils. Sow in fall or plant plugs; pinch back in June for bushier form.
Milkweeds (Asclepias spp.): Full sun, well-drained soils. Seeds need cold stratification; slow to establish but long-lived.
Summary
Physocephala tibialis plays a dual ecological role—as a nectar-feeding pollinator and as a regulator of bumble bee populations through parasitism. To support it in Chicago:
Grow a diverse mix of nectar-rich native plants (mountain mints, bergamot, goldenrods, asters, milkweeds).
Support strong bumble bee populations (its hosts) with season-long bloom and nesting habitat.
Maintain pesticide-free, diverse gardens or prairie patches with bare/undisturbed soil nearby.
