Cattle grubs are the immature or larval stages of heel, warble flies or gad flies. Two species of cattle grubs affect cattle: Hypoderma lineatum, the common cattle grub and Hypoderma bovis, the northern cattle grub. Both species occur naturally in Africa, Asia, Europe and North America, but principally from 25 to 60° latitude in the Northern hemisphere. In North America, the common cattle grub ranges from northern Mexico to northern Canada, while the northern cattle grub is found north of a line running west to east from northern California through Kansas to the Carolinas.
Adult cattle grubs are bee-like in appearance and approximately 11 to 18 mm long. The adults don’t feed and need to mate within a lifespan of three to five days. Eggs are attached to lower hairs on the animal’s body, especially the legs. This activity frightens the host animal which responds by running with its tail in the air, bent over the back. This behavior is termed “gadding”. Larvae emerge from the eggs in about four to six days and crawl down the hair and burrow into the skin. Larvae of the common cattle grub migrate through the animal’s body until they reach the gullet, while northern cattle grub larvae move to the region of the spinal canal. Older larvae then spend several months migrating to the back of an infested animal. Once at the back, the larvae cut a breathing hole through the hide and develop a cyst or swelling, called a warble. Warbles are visible and can be detected by touch. After about six weeks full grown larvae exit out of the breathing holes, drop to the ground and burrow into the soil to pupate. Adult flies will emerge in another three to 10 weeks. The entire life cycle takes about a year, with eight to 11 months spent inside an animal.
Cattle fail to graze normally during the warble fly season because of gadding. They seek shade or stand in water to avoid the flies. The failure to graze normally reduces milk production and weight gains. Slaughter losses result when grubby areas must be trimmed from the carcass and from the decreased value of hides containing grub holes.
Cattle grubs are best controlled with endectocides (macrocyclic lactones) applied either as a pour-on or by an injection. Destruction of grubs at the period when they are in vital areas may cause undesirable host-parasite reactions. Consult your veterinarian or state extension service for proper treatment time.