Cattle Grubs

IRWIN H. ROBERTS AND ARTHUR W. LINDQUIST.

NEARLY all cattlemen know the conspicuous swellings that appear in the backs of cattle in winter. The swellings contain grubs, which are the maggot stage of heelflies.

Of all the insect pests that prey on livestock, heelflies are among the hardest to control. They and the cattle grubs (which are known also as warbles and wolves) may be to blame for losses to producers, feeders, dairymen, packers, and tanners. The losses may nearly equal those caused by all other insect pests of cattle combined.

Two species of heelflies, or warble-flies, parasitize cattle in North America. The common heelfly, Hypoderma lineatum, and its larva, the common grub, exist in all parts of the United States and Canada. The northern species, Hypoderma bovis, is found in Canada and the northern half of the United States.

Heelflies are true flies of the order Diptera. They are closely related to other flies commonly encountered on farms, but look like small bumblebees hairy, black, and striped with yellow. The common heelfly is nearly three times larger than the housefly.

The northern heelfly is much larger and stouter.

From the eggs that the flies lay on cattle emerge tiny, white larvae. The flies, their eggs, and the young maggots are seldom seen by casual observers. The young maggots penetrate the skin of the host animals, move through the body tissues, and eventually emerge as the familiar cattle grub in the backs. Mature grubs are black and may be an inch long. Heelflies develop from grubs that have dropped to the ground from the backs of cattle.

Common heelflies appear during the first warm days of spring. They have no mouth parts and do not bite or Sting. They live only a few days, rarely as much as 2 weeks, but in that time they can deposit hundreds of eggs. The heelflies are active about 2 months. They lay their eggs in neat rows on hairs, usually low on the legs on and above the fetlock hence the name "heelfly." If the animal is lying down, the eggs may be laid on the hairs closest to the ground.

The eggs hatch in 3 or 4 days. The minute, spiny maggots crawl down the hair and begin to burrow through the skin. Then they begin an amazing journey through the tissues of the body. They appear to limit their travels to the soft connective tissues between the muscles. Most of the larvae of the common heelfly congregate in the tissues of the gullet after about 5 months.

The larvae spend 3 months or so in the gullet. After that they begin their final migration, again through the connective tissues, to the region beneath the skin of the back. Almost immediately they make breathing holes in the skin.

In a very few days a pocket, or cyst, developed from the tissues of the host, surrounds each larva, and in this the grub remains for about 6 weeks. At all times during its stay in this swelling, the breathing hole is kept open. The tail end of the grub, with its two prominent breathing pores, lies just under this opening.

The grub molts twice in the cyst. When it is mature, it works out of the now enlarged hole in the skin. Once free of the animal, the grub seeks the protection of soil or trash. Its outer cuticle becomes hard and black. Within that protective or pupal case, in a month or so, the change from grub to fly takes place.

The fly emerges from its pupal case through a hinged cap at the upper forward end. It may crawl from the puparium in less than a minute, dry its wings, and be in the air within half an hour. Shortly thereafter, the flies mate, and if the weather is favorable, egglaying may begin on the same day the adults emerge.

NORTHERN HEELFLIES appear later in the season, after the egg-laying activities of the common species are well underway. The activities of the two flies overlap for a short time, and then, after the common fly has disappeared, the northern fly continues to plague cattle well into the summer months. The northern heelfly deposits its eggs at about the height of the hock, striking the animal a number of times in rapid succession and cementing an egg on a hair each time. Its eggs are deposited singly, but it can lay several hundred in its lifetime.

The young larvae penetrate the skin and travel through the soft tissues of the body (as do those of the common heelfly), but they congregate in the spinal canal instead of in the gullet on their way to the skin of the back. When fully developed, the grubs of the northern fly are larger than those of the common heelfly, but their appearance otherwise is much the same. By the time northern grubs reach the backs of cattle, the common grubs normally have been present for about 2 months, and most of them are ready to emerge from their cysts. The northern grubs are most abundant in the backs of cattle after the larvae of the common fly have practically disappeared. It is not unusual in Northern States to find a few of these grubs emerging from the backs of cattle as late as July.

IN ORDER TO CONTROL cattle grubs effectively, it is particularly important to know when they are likely to be found in the backs of cattle, so that they may be attacked when they are most vulnerable.

Common grubs are said to appear as early as the first week in September in southern Texas. They appear progressively later as one goes northward. Common grubs may not appear until February or March in Montana, and the northern grub generally appears a month later.

Along the Gulf of Mexico and in southern Arizona and California, all grubs are likely to have matured and dropped from their cysts by the first days of March, at a time when the arrival of the grubs in North Dakota, Minnesota, and northern New York has scarcely begun.

In each section of the country, the appearance of grubs can be anticipated annually with some accuracy, but an allowance must be made for seasonal variations of approximately a month. In an average locality, grubs may show up in January one year, December the next, and in February the third year. Greater year-to-year variations than those have been known to occur, but are uncommon.

HEELFLIES AND CATTLE GRUBS injure cattle in various ways.

One is the annoyance to cattle when the flies are laying eggs. The northern fly excites stock most. Heelflies cause no pain while depositing their eggs, but the reaction of cattle to them may be one of great fear. Animals attempting to escape them run with their tails held high in a characteristic manner. Occasionally they injure themselves in their wild flight from the flies. More often, however, they seek protection from the flies in shade or in water, and fail to graze for hours at a time. The fact that heelflies attack range cattle in early spring, when cows are poor and weak, with calves at their sides, intensifies the losses. Milk production of dairy cattle may drop 10 to 25 percent when heelflies are active.

The grubs migrating through the body tissues and the larvae boring into the backs of cattle cause injuries that one can appreciate but cannot assess easily. Losses after slaughter, however,are obvious. When hides are removed from grub-infested cattle, a mass of yellowish, gelatinous meat may be seen around the grub holes. It must be trimmed out as inedible. Fourteen million pounds of choice meat were wasted because of such trimming in 1948. Carcasses so trimmed are lowered a full grade, or loins may be so badly damaged that they must be boned.

The damage grubs do to hides is the easiest of all losses to evaluate. A third of all cattle hides produced in the United States in 1948 contained five or more grub holes and were sold at a discount of a cent a pound. Many grub holes impair the quality of the skin for leather. Occasionally 100 to 200 grub holes are found in a hide. Such hides are not worth tanning and are sold for use as byproducts.

Losses due to cattle grubs and heel-flies in the United States have been set at loo million to 300 million dollars a year.

AN ACCEPTABLE METHOD of controlling heelflies and cattle grubs has been the object of a diligent search by many investigators since 1890. Attempts to destroy or repel the adult flies have not been successful. Efforts to destroy the eggs after they have been deposited on the hair coats of cattle or to keep the larvae from penetrating also have resulted in failure.

Research was begun in 1947 to find a drug which, given internally, would destroy the grubs after they reached the backs of cattle and would kill all the young larvae as they wandered through the tissues of the body. By 1955, several insecticides showed promise against grubs when injected into cattle or when administered in the feed or mineral-salt supplements. Among the most successful were lindane, aldrin, dieldrin, as well as some of the organic phosphorus compounds. Work on the internal administration of those and related insecticides had not advanced beyond the experimental stage in 1956. Additional research is underway to determine their effectiveness and the hazards involved in their use.

A satisfactory method of attack is to destroy the grub after it has formed an opening in the skin of the backs of cattle. Killing the larvae at that stage interrupts the life cycle before a fly can be produced. Cattle thereby are spared the annoyance of attack by the heelfly and the hundreds of larvae that could hatch from the eggs produced by each female fly.

The larvae within the cysts of the skin can be killed in several ways. An early and simple method was the removal of the grub by pressing firmly with the fingers. That is a slow and laborious method, but many dairymen have used it.

Research workers the world over have experimented with hundreds of materials in the search for insecticides for the mass destruction of grubs, but few have proved effective.

The chlorinated hydrocarbon insecticides, among them DDT, lindane, chlordane, methoxychlor, and toxaphene, which are used against other insect pests, have little effect against cattle grubs when applied externally over the cyst openings.

A few of the organic phosphorus compounds, a group of insecticides developed during the Second World War, are highly lethal to grubs when applied in that way. They have been used experimentally but are not recommended for general use because they possibly may have toxic effects on the animals.

Benzol and carbon tetrachloride, injected into grub cyst openings with an oil can, will destroy grubs, but this method is not a great improvement over manual removal.

Rotenone remains the only effective toxicant that can be safely recommended for use by cattle owners. Rotenone occurs in tropical shrubs, especially Derris eliptica and Lonchocarpus nicou. The finely ground roots of the plants are known as derris and cube powders. The powders, adjusted to contain about 5 percent of rotenone, can be applied to the backs of cattle in ointments, dusts, washes, dips, and sprays.