Egg -- The smooth egg is dull yellow in color. A narrow oval, it is about 0.8 mm long and 0.25 mm wide. At one end it has a short stalk with a clasping device that holds the egg to the host's hair.
Larva -- When first hatched, the larva is creamy white, densely covered with spines and about 0.65 mm long. After the first molt, it is about 13.5 mm long and has a pair of spiracles on the back end. The third stage larva may be as long as 28 mm and nearly 13 mm wide, with the upper side flat and the bottom side rounded. Tapering slightly at both ends, the larva has spines on segments 2 through 10. It turns almost black just before emerging from the back of the host.
Pupa -- The puparium is the hardened, dark, wrinkled skin of the third-stage larva which envelops the pupa.
Hosts -- The cattle grub normally infests cattle. It was once common on American bison and has been found on sheep, goats, and horses. Some cattle grubs have even been removed from man.
Damage -- The adult flies are nuisances, occasionally causing cattle to run wildly about with their tails in the air (gadding) or to stand for long periods of time in deep shade or water. Any of these defensive activities result in reduced milk production and/or subnormal weight gains. After hatching, larvae irritate the host's skin by burrowing into it. Larval migration to the esophagus and other organs, involving abnormal contact with these organs, is injurious, and the cysts on the host's back are swollen, often pus-filled areas which adversely affect the host's health. This is often reflected by loss of weight and a decrease in milk production. In addition, at slaughter some of the damaged meat must be trimmed, often from expensive cuts, and discarded; the hide's value is also greatly reduced by the holes and scar tissue.
Life History -- At least a year is required for the completion of a life cycle. In spring, eggs are deposited on the lower leg hair of hosts, usually cattle. Up to 12 eggs are deposited on each hair, and each female lays a total of about 500 eggs.
Newly hatched larvae immediately burrow into the skin at the base of the hair and migrate through connective tissues to the diaphragm, small intestine, esophagus, or heart. After a few months the larvae migrate via connective tissues to positions just beneath the skin on the back. When they reach the back, the larvae cut minute holes through the host's skin, and cysts form around the grubs. A few days later, the larvae molt for the first time; about 25 days later, they molt again. In December to February, 3rd stage larvae mature, work their way out of the cysts, fall to the ground and seek pupation sites. From 15 to 75 days afterwards, the flies emerge and are ready to fly and mate. The adults do not feed and remain active for only a few days.