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How to identify it
- Adult heel flies are large, hairy, bee-like flies about 11 to 18 mm long (roughly 1/2 to 3/4 inch); the northern cattle grub fly is the larger and more bee-like of the two, with black and yellow striped hair on the abdomen.[1][2]
- The eggs are about 1 mm long and dull yellow, glued to hairs on the legs and belly: the common cattle grub lays a row of 5 to 15 on one hair, the northern cattle grub one egg per hair.[1][2]
- Later in the season, run a hand down the backline and feel for cyst-like lumps; parting the hair may show the grub's breathing hole. Mature grubs are black and about 30 mm (just over an inch) long.[1][2]
- Cattle bothered by the flies panic, kicking at their legs and running across the pasture, which is where the names heel fly and gad fly come from.[1]
Look-alikes
Where it lives
Two species occur in the United States. The common cattle grub ranges through the Americas from northern Mexico to northern Canada; the northern cattle grub lives mainly north of a line running from northern California through Kansas to the Carolinas.[1]
Biology and life cycle
Adults live only 3 to 5 days and do not feed; they mate within a day of emerging, and in Florida egg laying runs from February to May.[1]
The common cattle grub fly walks onto a standing or resting animal and lays its eggs with little alarm, around 500 on one animal; the northern cattle grub fly chases fleeing cattle and lays eggs while they run.[1]
Eggs hatch in about 3 to 7 days, and the larvae crawl to the base of the hair and bore through the skin.[1][2]
The larvae then migrate through the body for months: the common cattle grub rests in the lining of the esophagus and the northern cattle grub in the spinal canal before both move to the back.[1][2][3]
On the back each grub cuts a breathing hole, and the animal forms a warble around it. After 1 to 2 months in the warble the grub drops out through the hole and pupates on the ground, and the adult emerges 1 to 3 months later, so the whole cycle takes about a year.[1][2]
Damage and signs
- Panicked, gadding cattle spend less time feeding and gain less weight, can injure themselves on fences, and pregnant cows may abort; in first-year heifers it can spread out breeding and calving.[1]
- Grubs riddle the hide with breathing holes and leave green or yellow jelly-like meat around the warbles that must be trimmed; UF/IFAS also cites milk yields 10 to 30 percent lower in infested cows.[1]
- Killing grubs at the wrong time is dangerous. Common cattle grubs dying in the esophagus can make it swell shut so the animal bloats, suffocates and dies, and northern cattle grubs dying in the spinal canal can paralyze the hind legs, though animals often recover.[1][2][3]
- Infested cattle also have lower resistance to disease and a higher chance of secondary infections.[1]
Health risk
People and horses can become accidental hosts. In people the larvae wander under the skin, causing itchy red tracks (cutaneous larval migrans), and the northern cattle grub can do more harm if it goes deeper; in horses the grubs reach the back and cause abscesses that make riding impossible.[1]
Control
Prevention
Watch for gadding, which signals heel fly activity, and check the legs and belly of cattle for eggs, focusing on animals under five years old, since older cattle develop immunity and carry fewer grubs.[1]
Traps and physical control
Mature grubs found in warbles can be squeezed out through the breathing hole. Avoid rupturing them, since the grub's body fluids can cause inflammation, abscesses or severe allergic reactions in animals exposed before.[1]
Pesticides
Systemic insecticides are the main tool, and timing is everything: treat after the heel fly season ends and before the migrating grubs reach the esophagus or spinal canal. The cutoff depends on region: August 31 in Florida and no later than November 30 in the Pacific Northwest, while Nebraska advises against systemic products from November 1 to February 1 for herds that did not get one at fall weaning.[1][2][3]
Pour-on and spot-on products have given the best results, and a well-timed treatment can also control horn flies, ticks and lice. Observe slaughter withholding periods; UF/IFAS noted that only one systemic insecticide was permitted for lactating dairy cows, and the PNW handbook lists eprinomectin pour-on for dairy cattle.[1][2]
Common questions
- What are the lumps along a cow's back?
- They may be warbles, cyst-like swellings around cattle grubs that have reached the back. Part the hair over a lump and you may see the grub's breathing hole.[1][2]
- Why do cattle suddenly stampede across the pasture in spring?
- Heel fly activity is one cause: the buzzing, bee-like flies panic cattle into running and kicking, which is called gadding.[1]
- Why is there a cutoff date for grub treatment?
- Grubs killed while they sit in the esophagus or spinal canal can cause bloat, suffocation or paralysis, so systemic treatments must go on before they reach those sites. The safe cutoff depends on your region.[1][2][3]
Sources
- UF/IFAS Extension EDIS, “Cattle Grub Management, ENY-290 (P. E. Kaufman and E. N. I. Weeks, March 2013; archival copy).” journals.flvc.org/edis/article/download/120986/119573/181085 (accessed 2026-11-08)
- Pacific Northwest Pest Management Handbooks (Oregon State University, Washington State University and University of Idaho Extension), “Dairy cattle: Cattle grub (Pacific Northwest Insect Management Handbook).” pnwhandbooks.org/insect/livestock/dairy/dairy-cattle-cattle-grub (accessed 2026-11-08)
- University of Nebraska-Lincoln Extension, UNL Beef, “Cattle Lice (D. Boxler, February 2, 2020).” beef.unl.edu/cattle-lice (accessed 2026-11-08)
- iNaturalist, “Hypoderma lineatum taxon page (order and family).” www.inaturalist.org/taxa/626994 (accessed 2026-11-08)