Hypoderma spp: Cattle grub fly; warble fly

General Description: The adult flies are hairy and bee-like, about 15 mm. long. The mature larvae, or grubs, are up to 3cm. long, and resemble bot larvae.

Life Cycle: The adult flies deposit eggs in summer on the hair of the legs and lower body of cows and horses. The eggs hatch in 2 to 7 days, and the larvae burrow into the subcutaneous connective tissue, where they wander for several months. Eventually, the larvae appear under the skin of the back, where they bore breathing holes and develop for up to 3 months. In early summer the grub emerges through the breathing hole, falls to the ground, and pupates. The adults emerge in 1 to 3 months, depending on weather, and live for one week.

Location: Subcutaneous tissue, especially under the skin of the back.

Geographical Distribution: Many countries in the northern hemisphere.

Significance: Hypoderma is of minor significance in horses, except where horses are in constant contact with infested cattle. Range horses may be unrideable if they develop reactions to larvae which are under skin in the saddle area.

Effect on Host: The adult flies annoy and frighten horses, especially in midsummer. Subcutaneous larvae damage the skin and underlying tissues. Allergic reactions to these grubs can occur. The backs of afflicted horses become irritated and disfigured. Breathing holes are visible in the skin.

Diagnostic Information: Finding mature larvae under the skin of the back.

Control: Since the flies like bright light, providing shady shelter in midsummer can help. Larvae should not be squeezed out, as this can precipitate an allergic reaction. Horses may be treated with insecticides. This practice is advisable for range horses in contact with infested cattle.