Cattle grubs are the larval stage of heel flies. Two species of these flies occur in the Northeast: the common cattle grub (Hypoderma lineatum) and the northern cattle grub (Hypoderma bovis). Both have similar life cycles. Adult flies emerge during the spring and summer. They are large, hairy flies that resemble bees. After mating, the females locate cattle on which to lay their eggs. Egg laying occurs between late May and August. Cattle often panic in the presence of the fastmoving flies and may run wildly with their tails high in the air in an effort to escape. In spite of this gadding response by cattle, the flies neither bite nor sting the animals. In fact, the adults do not feed at all and survive only 3 to 8 days.
Female flies attach their eggs to the hairs of the cow’s legs and lower body regions (hence the term "heel fly"). Each can lay up to 600 eggs, which hatch in 4 to 7 days. Newly hatched larvae burrow into the skin, causing the animal considerable irritation. The young larvae then migrate through the animal’s connective tissue. By November 1 most larvae of the common cattle grub have migrated to the submucosa of the esophagus, whereas those of the northern cattle grub migrate to the epidural tissues of the spinal canal.
During the winter months, the larvae of both species migrate again, this time into the animal’s back. By February most larvae have reached the back and have cut a breathing hole through the hide. There the larva forms a warble (swelling) between the layers of the hide. Within the warbles, the grubs grow rapidly for about two months, reaching a final size of about an inch in length.
Young animals are more heavily infested with grubs than mature milking cows are, because older animals develop a degree of immunity to the grub larvae. When mature the grubs emerge through the breathing holes, drop to the ground, and pupate in pasture litter and soil. During this stage the grub’s skin hardens and turns black. The metamorphosis from grub to adult fly takes from 2 to 8 weeks. Adult heel flies emerge from the pupae and are active from late May through August. Most activity occurs during June and July.
Economic losses to cattle grubs take several forms. First, gadding behavior in response to adult fly activity decreases the animal’s ability to graze efficiently. Gadding also makes cattle difficult to handle and increases the risk of self-inflicted injuries. Second, tunneling by cattle grub larvae through the animal’s tissues causes great damage. Heavy infestations in replacement animals can result in poor weight gain, delayed time to first lactation, and long-term production losses.
Third, the breathing holes cut by the grubs damage the most valuable portion of the hide, substantially decreasing its value at slaughter. Moreover, the meat surrounding the warbles is discolored and must be trimmed at the slaughter house, further reducing the carcass’s value.
Backs of cattle should be examined during March and April for the presence of warbles. Warbles are detected by rubbing the cow’s backline and feeling for the cystlike bumps. When the hair around a warble is parted, the breathing hole may be visible. Because animals develop some immunity to infestation by grubs, the most important animals to examine are those under 5 years of age. Calves born after the fly season and animals kept indoors during the summer will not have cattle grubs and need not be monitored.
Gadding behavior during late spring and summer indicates that female heel flies are laying eggs. Pastured animals may also be examined for the presence of eggs on the hairs of the animal’s legs, udder, escutcheon, thighs, and rump.
Cattle confined in barns from May to August are protected from cattle grubs, because heel flies do not enter barns to lay their eggs. But individual production and management practices often rule out this method of cultural control. The most effective method of actually reducing fly populations is to organize a community-based, area-wide program for treating all nonlactating cattle with systemically active insecticides. Such an area-wide treatment can substantially reduce heel fly activity the following year.
In the absence of regional control programs, individual producers may minimize damage to their own animals by using systemically active insecticides on their young, nonlactating heifers. Several systemic insecticides are available as pour-ons, spot-ons, and injectables. It is essential, however, that systemic insecticides not be used on lactating animals because of the danger that insecticide residues will appear in the milk. At present no cattle grub treatment is available for lactating animals.
Proper timing is critical for the safe, effective use of systemic insecticides. Treatment must be made after adult heel fly activity ceases, but before the migrating grub larvae reach the esophagus or spinal cord. This means that systemics should be used in September, and never after November 1. Treatments made after November 1 may cause severe allergic reactions in the animals, resulting in bloat, paralysis, and death. A list of systemic insecticides for grub control is presented in Table 3.