Screw-worms

W. G. Bruce.

Screw-worms, if they are not controlled, can wipe out entire herds of cattle, hogs, sheep, and goats. Good livestock management and prompt treatment of all infestations with an approved remedy are the ways to combat the parasites.

Screw-worms have been known in Texas since about 1842. Frequently in summer they have spread to adjoining States. Localized outbreaks have occurred occasionally in the Central and North Central States mainly because of the shipment of infested livestock into the areas. Screw-worms were unknown in the Southeastern States until 1933, when the first infestation was reported near Boston, Ga. By the end of 1933, infestations were reported in 30 counties in southern Georgia and in 18 or 20 counties in northern Florida. Infestations spread rapidly in Florida. By early 1935 screw-worms were found in every county of that State, and heavy losses of livestock were reported by hundreds of stockmen.

An extensive program was started in all Southern States in May 1935 by the Department of Agriculture and State agencies. The aim was to disseminate information on the control of screw-worms and to demonstrate the proper methods and materials for treatment. The program was discontinued in 1937 when it was believed the purpose had been served. Cases of screw-worms, all of them in Florida and southern Texas, were reduced then to a small number.

Severe screw-worm outbreaks occurred in Florida, Georgia, Alabama, and southern South Carolina. Less serious outbreaks occurred in Mississippi, Tennessee, northern South Carolina, and North Carolina. Infestations have also been found in Kentucky, Virginia, and New Jersey.

Surveys have been made each year since 1943 to determine the incidence and relative abundance of screw-worms, ascertain the amounts of critical insecticides needed for their control, aid in the proper distribution of the insecticides, and advise stockmen of the approved methods and materials for treatment and prevention.

THE SCREW-WORM FLY, bluish green in color, has three dark stripes on its back. The area below and between the eyes is reddish or orange. In size and coloration, the fly is almost identical to a species of common blow flies, which is the adult of another species known as the secondary screw-worm.

The female screw-worm fly lays her eggs 10 to 400 at a time on the edges of wounds of warm-blooded animals. A female can lay 3,000 eggs, which she usually deposits in masses of 200 to 400 at 4-day intervals. The eggs of the screw-worm are placed in shingle-like masses and are cemented together. The eggs of ordinary blow flies are placed in haphazard fashion and are not securely cemented together.

The eggs hatch in 6 to 12 hours. The tiny whitish worms feed in clusters. They eat into the live flesh, in which they soon form a pocket. As they develop they assume a pinkish color. After about 3 to 10 days the screw-worms leave the wound, drop to the ground, and burrow into the soil. The outer skin hardens and forms a pupa. Seven to 14 days later the mature fly emerges. During cool weather the pupal stage may last 2 months. The flies are ready to mate and lay eggs 2 to 5 days after they emerge. The average life cycle from egg to egg is about 21 days. It may be shorter under favorable conditions or considerably longer under adverse conditions, especially in cool weather. If average daily temperatures lower than 54 F. prevail for 2 months or more, the pupae die in the soil. Therefore the screw-worm usually survives the winter only in Florida, southern Texas, and Mexico.

In the Southeast the overwintering area ordinarily is peninsular Florida, the northern limit of which is about 50 miles south of the Florida-Georgia line. In extremely mild winters, as in 1949-1950, the screw-worm has survived in Georgia, Alabama, South Carolina, and Florida.

In warm weather in spring and summer the screw-worm flies migrate at the rate of 35 miles a week. In the Southeast, the usual infested area in summer includes all of Florida, the southern two-thirds of Georgia, and the southeast corner of Alabama. In the Southwest the natural migration ordinarily encompasses most of Texas, southern New Mexico, southern Arizona, and southern California. When warm winters permit survival in a larger area, the subsequent migrations will extend over a proportionately greater area of the Southern States.

Screw-worm infestations frequently occur in the Central and Northern States, often in outbreak numbers. They are not due to the natural migration of screw-worm flies but to the importation of infested livestock. Severe and costly outbreaks, a direct result of the shipment of infested livestock to places where screw-worms were unknown and unrecognized and where stockmen were unprepared to combat them, have occurred as far north as South Dakota and New Jersey.

A NUMBER OF predatory beetles and ants destroy screw-worm larvae and pupae, but they do not effectively control screw-worms. No methods have been developed to propagate these beneficial insects in sufficient numbers so they can be utilized.

The screw-worm is a true parasite that lives only in the living flesh of warm-blooded animals. It is not found in cold-blooded animals or in decaying flesh or vegetable matter. The worms found in cold-blooded animals and decaying organic matter are common blow flies.

Any warm-blooded animal is subject to screw-worm attack. Infestations have been found on practically all kinds of wild and domestic animals, poultry, and man, but are more commonly found in cattle, hogs, sheep, and goats.

Occasional reports have been received that large numbers of wild animals, especially deer, were killed by screw-worms, but no intensive studies have been made and only a few of the reports have been verified. Outbreaks in wildlife apparently are associated with the development of large screw-worm populations in domestic animals.

Before an animal can become infested with screw-worms, some break must occur in its body surface. That may be a tick bite, the navel opening of a new-born animal, a scratch, a surgical operation, a cut, or some diseased condition of the skin or mucous membranes, especially around the natural openings. Any open wounds attract the female screw-worm fly, and around them she deposits the eggs. The worms cannot eat through the unbroken skin of a healthy animal.

An infested wound becomes increasingly attractive to the flies and constantly receives new batches of eggs. Screw-worms of different sizes and ages therefore are often found in a wound. As the larvae develop, there is a constant dropping of mature larvae from the wound to the ground, where they pupate and later emerge as flies, building up a population to infest new wounds and reinfest old ones.

INFESTED ANIMALS often Stray from the herd and hide in the underbrush or palmettos or in some isolated place. They appear nervous and make frantic efforts to scratch or lick the infested wound.

Most untreated infestations result in the death of the animals. Death may be caused directly by the screw-worms in destroying tissues or by complications following an infestation. The screw-worm fly is a carrier of a joint disease in calves. It must be remembered that a wound is being continually reinfested, that old infestations are often more attractive than are fresh wounds, and that repeated reinfestation means death to the animal. If the infestation is in the navel, death very likely will result more quickly than if the infestation is in some meaty and less vulnerable part.

The infested wound has a watery discharge of bloody exudate and a bad odor. If one wipes the bloody discharge from the wound with absorbent cotton, he will see the worms crowded together in pockets, with only their rear ends exposed. The heads are embedded in the living flesh and have two hooklike mouth parts that tear the tissues and cause bleeding. In large wounds, where some flesh is decaying, there may also be maggots of ordinary blow flies. The maggots are not embedded in the flesh but crawl on the surface of the wound.

IN DOMESTIC LIVESTOCK screw-worms can be controlled by the proper and timely application of remedies that kill the worms but do not harm the animal.

The mistreatment of wounds and infestations often means more costly methods of control or the loss of the animal. Creosotes, coal tars, and similar preparations aggravate the wound by destroying tissues, enlarging the wounds, and retarding healing. They also invite reinfestations and impair the health of the animal.

Cooperative effort among all livestock owners is essential for effective control. One stockman can do a fairly good job by frequent inspections and treatment of his animals, but labor and materials would cost much less if everybody in a community did the same.

SEVERAL EXCELLENT REMEDIES are available. The most effective material for killing screw-worms and protecting the wound against reinfestation is a development by the Bureau of Entomology and Plant Quarantine, known as "EQ 335 Screw-worm Remedy." The figures 335 represent the concentrations of the two main active ingredients, lindane (3 percent) and pine oil (35 percent). EQ 335 does not stain. It is not highly volatile. It will kill screw-worm flies that visit the treated wounds. It contains (in percentages by weight) lindane, 3; pine oil, 35; mineral oil, 40-44; emulsifier, 8 12; silica aerogel, 8 12.

EQ 335 is best applied with a 1-inch brush. It should be worked well into the wound. Special attention should be given any deep pockets. A coating should be applied completely around the wound and on areas contaminated with exudates.

Screw-worm adult.