Cattle Grubs

Ernest W. Laake, Irwin H. Roberts.

Cattle grubs, or heel flies, have been known to man from time immemorial. Few parasitic insects that attack man or domestic animal have received more attention from naturalists than these.

The introduction of the two species of cattle grubs we have in North America undoubtedly dates from the time of the first importation of European cattle. The spread of one, now called the common cattle grub, in the United States advanced with the progress of settlement to all parts of the country. The other, the northern cattle grub, has spread throughout the Eastern, Middle Northern, and Northwestern States. It is moving southward, but we doubt that it will ever invade the States farthest south.

The life history of the two species is similar. The eggs are securely fastened near the base of the hair on the host.

The adult of the northern species generally lays its eggs one at a time on the hind legs above the hocks, on the flanks, and on the sides of the abdomen. It is bold and vicious in its attack.

The adult of the common cattle grub lays its eggs in rows on the heel when the animal is standing or, when it is lying down, on any part of the body that touches the ground. It makes sneaking attacks. Often the animal is unaware of the fly. When the egg is 4 or 5 days old, the eggshell splits at the free end and the first-stage cattle grub emerges, crawls down the hair, and immediately bores into the skin. After migrating through and feeding on the host's tissues for a month or more, the young grub reaches the esophagus or the abdominal viscera, where it moves about for about 6 months to continue its development at the expense of the host animal. It then migrates on through the tissue under the skin to the back of the animal. The young larvae of the northern species do not invade the esophagus and abdominal viscera but migrate instead through the spinal canal on their way to the back of the animal. The grubs of both species usually locate along the median line of the back between the shoulders and hips and then cut holes through the skin in the choicest part of the hide.

After opening the hole through the skin, the grub molts to the second stage and becomes encysted under the hole. At this stage, small swellings on the back of the animal indicate the presence of the second stage of the heel fly grub. Having ample access to both food and air in the second stage, the grub grows rapidly for about 3 weeks and then molts again into the third, or last, stage. Up to this point the grub is opaque. As it goes through its third stage and completes its residence in the back of the animal it becomes dark brown or black in about 20 days, when maturity is reached, and it leaves the host animal through the hole in the skin.

Shortly before the mature grub leaves its host it partially dehydrates itself, shrinks considerably in size, and at the same time greatly enlarges the hole through the skin of the animal. These changes in the grub make possible a rapid and easy escape from the host and the dehydration keeps the grub from freezing after it leaves the body of the animal. The grub is not seriously affected by a sudden drop in temperature from around 103 F., the usual body temperature of cattle, to an outside air temperature of zero or lower. Few other insects can stand such drastic changes in their environments without suffering serious injury or death.

When the grub falls to the ground it crawls under nearby objects for protection and pupates for its transformation to the adult stage. The pupal period may range from 16 to 75 days, depending on prevailing temperatures. The adult fly does not feed. Its sole purpose is to reproduce. Mating is followed immediately by egg laying, which may take place within an hour after the adult emerges from the puparium.

THE ONLY METHOD OF CONTROL which has so far been successful in the United States has been to destroy the grubs by applying a larvicide during the time when the larvae, or grubs, inhabit the backs of animals. This method of treatment has been developed to the point where it is more effective than any other method known and is probably the only one that is practical and economical under our large-herd system of ranching.

Hundreds of materials have been tested, including such chlorinated hydrocarbons as DDT and benzene hexachloride, but rotenone is the only toxicant recommended for cattle grubs. Rotenone occurs in the roots of derris and cube plants; when the roots containing at least 5 percent rotenone are ground to a fineness that permits go percent or more to pass through a 200-mesh screen, the powder can be formulated so that it may be applied to the infested cattle as a dust, wash, spray, or dip.

Power spraying is the most rapid method of applying the powder in suspension to large herds of cattle. The dust or wash application usually is preferred by owners of small herds. The dust application is well adapted for use in very cold climates. If dipping vats are available and large herds are to be treated, the rotenone can be applied as a dip. Because the spray, dust, and wash are applied only to the grub-infested area of the animal, they are cheaper than the dip treatment, even for large herds.

Rotenone should be applied 30 to 45 days after the appearance of the first grubs in the backs of cattle and thereafter every 30 or 40 days during the grub season. Correct timing of the treatment, regardless of the method of application, is essential for satisfactory control.

Individual attempts to reduce cattle grub infestations are usually unsuccessful. If the grub population is to be satisfactorily reduced, control programs must be based on a community or area basis.

Time and again the value of organized community efforts to control cattle grubs has been proved. Grubs were practically eliminated on Clare Island, off the coast of Ireland, in 1920 after a 5-year program that involved the systematic destruction of grubs by all cattle owners working together. In Denmark, legislation in 1922 required that all cattle owners take measures to destroy the heel fly larvae in their herds; at the end of 3 years the percentage of infested hides in Denmark dropped from 20.5 to 2.5. In Prowers County in Colorado, a 6-year program, organized in 1928 and involving 22,500 head of cattle over goo square miles, reduced the average infestations from 35 to 5 per head. Those programs were carried on largely without chemicals--cattlemen then had to squeeze the grubs out of their cysts in the backs of the cattle with the fingers, remove them with small forceps, or in limited trials to destroy them by inserting toxic materials into individual grub sacs.

Since 1932 control projects in the United States and Canada have been made simpler and more effective by the use of derris and cube root powders.

On Calumet Island in the Ottawa River, Canada, a cooperative program reduced the average numbers of cattle grubs in cattle from 16 to 2 between 1933 and 1936.

In Hughes County, S. Dak., a voluntary program effected an 80-percent reduction in grubs between 1947 and 1950; yearling calves harbored only an average of 15.5 larvae in the winter of 1950, while untreated cattle on farms outside the area were infested with an average Of 78 grubs. In this program, as in similar work in New Mexico and Washington, the use of high-pressure spray equipment made it possible to treat large herds of cattle quickly, effectively, and inexpensively.

Common cattle grub.

A COMMUNITY PROGRAM can be organized successfully in almost any locality where cattlemen are willing to pool their efforts in a concerted attack on the grub. Between 1944 and 1949, projects were started in South Dakota, Montana, New Mexico, Oklahoma, Colorado, Washington, and California. Some were sponsored by Federal agencies and some by the State agricultural experiment stations. Several of the programs were assisted by such organizations as the National Live Stock Loss Prevention Board, State livestock sanitary associations, and the county cattlemen's associations.

Any group of stockmen or dairymen interested in a coordinated project against cattle grubs should seek the help offered by the county agent's office. They should first establish that cattle grubs are an economic problem in the locality. In a few sections infestations are sporadic and light and the expense and labor involved in community action would be of doubtful value.