THE SCREW-WORM

A, Life-history group showing a, egg mass; b, young larva, c, mature larva; d. pupa; e, adult female fly laying eggs. B, Part of an infested wound showing egg masses, very small larvae, and the posterior ends of mature larvae. (All about twice natural size.)

THE SCREW-WORM

The screw-worm, a very destructive livestock pest, is the larva of a fly that is about two or three times larger than the house fly. From eggs laid on wounds of livestock and other warm-blooded animals hatch maggots that feed on the flesh. Bites by ticks and flies also promote infestations. Newborn animals are susceptible to attack on the navel.

An adult fly deposits 200 to 300 eggs, which hatch in about 12 hours. The maggots reach maturity in about 5 days, when they are about one-half inch long. They crowd together in the wound with their heads (the small end) down. One hundred mature maggots will cause a wound about an inch in diameter and almost an inch deep. Infested wounds develop a straw-colored or bloody discharge, which smells bad. An infested wound is more attractive than a fresh wound. So in a few days several flies may have deposited eggs, which may have hatched; instead of the 200 or more maggots from the first fly, there then may be a thousand or more maggots of all sizes in the wound. Unchecked, the feeding by hundreds or thousands of maggots will soon kill the animal.

Mature maggots leave the wound, drop to the ground, enter the soil, and transform into the pupal or resting stage. The flies emerge from the pupae in 8 to 30 days, depending on the temperature. The adult flies mate a few days after they emerge, and the females seek an animal with a wound in which to repeat the life cycle.

The screw-worm can survive most winters only in the southern parts of Texas, Florida, or California. In spring it spreads northward. The adults are strong fliers and may spread as much as 25 to 35 miles per week. Thus before frost the pest usually spreads through parts of Texas, Oklahoma, Arizona, New Mexico, California, Florida, Georgia, and Alabama. The insect often is transported also into other parts of the country through shipments of infested livestock; if that happens in the spring, the insect may increase to outbreak proportions before fall.

Control: An animal must have a wound before it is susceptible to screw-worm attack. Any measures that will reduce the number of wounds in animals will therefore reduce the number of screw-worm cases. Handle animals carefully at all times. Take measures that will reduce chances for accidental wounds. Avoid surgical operations if possible during screw-worm seasons. Control ticks and flies.

Examine all livestock carefully twice each week. Treat any wound, whether infested or not, promptly with a good screw-worm remedy. EQ 335, which contains lindane and pine oil, is excellent. If wounds are infested, use EQ 335 to destroy the maggots; keep the animal available for re-treatment at least once each week until the wound is healed. Other screw-worm remedies such as Smear 62 and similar preparations containing diphenylamine and benzol have been used successfully for many years.