Screwworms That Attack Livestock

GAINES W. EDDY AND R. C. BUSHLAND.

THE SCREWWORM, Callitroga hominivorax, is an obligatory parasite of warm-blooded animals. Animals infested with it usually die unless they are treated promptly. It can be controlled by good management and prompt treatment with an insecticide.

The screwworm fly is bluish or bluish green and is about twice the size of the housefly. It lays its eggs on all kinds of wounds but prefers fresh abrasions wire cuts, nail scratches, boils, brand marks, shear cuts, and sores caused by tick bites, by needle grass on the legs and face of sheep and goats, and by prickly pear in the mouths and other parts of the body. The navel of newborn animals is especially vulnerable.

The screwworm fly lays shiny white eggs in masses that contain about 200 eggs. The eggs are glued to each other and are placed on the edge of the wound, usually on a dry part of the skin. The masses of white eggs, arranged in neat rows, are unlike the eggs of other blowflies, which are yellowish and laid at random in uneven masses. Eggs of screwworms hatch in 12 to 24 hours, and the tiny maggots feed as a colony in the living flesh.

They feed with their heads down. The tail, or blunt end, usually is exposed for breathing. During development they have a pinkish color, which is very noticeable by the fourth day. The pigmented air tubes, or tracheal trunks, which open at the blunt end of the body, are also noticeable then.

These show as two almost straight black lines. They and the pinkish color identify them from larvae of other blowflies. Wounds infested with screw-worms smell bad. There is considerable drainage of brownish-red fluid, which stains the hair below the wound.

Larvae complete their growth in 5 to 7 days and drop to the ground, where they burrow into the soil and pupate. The pupal stage lasts 7 to 10 days in warm weather but may last several weeks when the weather is cool. Flies emerge from the puparia in the soil, work their way to the surface, where they crawl up on vegetation, and expand their wings within a few minutes.

For the first few days of adult life, the flies visit animals to feed on wound fluids. They also take water, nectar, and plant juices. The insects mate when they are 2 days old. The females, when they are 5 or 6 days old, seek out wounded animals on which to lay their eggs. The eggs usually are deposited in one mass. The average life span probably is only about 3 weeks but may be as long as 65 days.

The distribution of the screwworm fly in the United States in winter is confined normally to the southern part of California, Arizona, New Mexico, and Texas, and most of Florida. The flies breed the year around in tropical and subtropical North America and South America. The fly occurs only in the Western Hemisphere.

The flies migrate in spring and summer at a rate of about 5 miles a day. The limits of winter survival determine the areas or States infested one year or another. The southeastern areas usually infested by fall include all of Florida, the southern tip of South Carolina, the southern half of Georgia, and the southeastern part of Alabama. Occasionally the flies move into Mississippi and northward into Kentucky and Virginia. Outbreaks have occurred in New Jersey. In the Southwest, the flies migrate out of Texas into Oklahoma, Kansas, Missouri, Arkansas, and occasionally Mississippi.

The infestations in California, Arizona, and New Mexico are usually local and cover only the southern parts. Severe outbreaks have occurred In many of the Northern States; they usually are costly, because many stockmen in those areas are not familial with screwworms and by the time expert advice is available the flies have spread over a large area.

Because many animals, especially cattle, move across the country today by rail and truck, outbreaks are likely unless all animals from infested area, are checked closely. Many of the outbreaks in the Northern States have been traced to the shipment of infested animals.

SCREWWORMS cost the livestock industry an estimated 20 million dollars a year in deaths of cattle, permanent injury, and poor weight gains.

Another loss lies in the constant watch necessary to find and treat infested animals. Animals with screw-worms tend to stray from the herd or flock and hide in underbrush to escape the flies. Sheep and goats hide under old buildings or in caves. Stockmen may spend the equivalent of several months each year looking for screw-worm cases and treating them.

In some years the loss of game animals, especially deer in Florida and Texas, is great. When the fly season comes early and the fawn season is late, a heavy toll of the fawns and their mothers can be expected. Deer are one of the favorite hosts of certain ticks in Florida and Texas, and the sores the ticks cause are also favorite spots for screwworms to lay their eggs.

CONTROL OF SCREWWORMS consists of good ranch practices and the use of an approved remedy.

Corrals, fences, and gates should be kept in good repair so that livestock do not suffer snags and cuts from them.

Dogs used in working stock should not be allowed to bite animals. Necessary surgery, branding, earmarking, and birth of young should be scheduled for the winter months, when screwworm activity is at its lowest.

Livestock need special watching during the fly season. Because animals are killed so quickly by screwworms, every animal should be seen at least once a week. In brushy country, cowboys need to ride the pastures early in the morning and late in the afternoon to find wormy animals grazing and watering when flies are least annoying.

In places where sheep and goats are pastured, brush should be controlled so that infested animals can be found easily.