Screwworms are killed by antiparasitic drugs, topical insecticides, physical removal from wounds, and on a population level, a massive sterile-insect program that has eliminated them from all of North and Central America. Whether you’re dealing with an active infestation in an animal or trying to understand how these pests are controlled at scale, several proven methods exist.
How Larvae Are Killed in Wounds
Screwworm fly larvae burrow into open wounds on living animals (and occasionally humans), feeding on tissue and growing rapidly. The primary way to kill them is applying a larvicide directly to the wound. Permethrin-based sprays are among the most common products registered for screwworm control in the U.S., sold under names like Catron IV and Prozap Screw Worm Aerosol. Coumaphos, another insecticide, is available as a flowable liquid and as an ingredient in wound powders like Tanidil, which the USDA has authorized under emergency exemption for use on cattle, swine, goats, sheep, horses, and even temporarily captured wild animals.
Topical treatments applied directly to larvae in the wound include ivermectin, cypermethrin combinations, and spinosad, all of which are highly effective against larvae that are two to four days old. In most cases, the wound is treated with the larvicide and then left open to heal rather than sutured closed, since closing an infested wound risks trapping dead larvae or bacteria inside.
For smaller infestations, particularly in pets or humans, suffocating agents can force larvae out or kill them in place. Mineral oil, liquid paraffin, petroleum jelly, beeswax, and even adhesive tape placed over the wound opening cut off the larvae’s air supply. Hydrogen peroxide, isopropyl alcohol, and chloroform in oil also work as larvicides and help ensure no larval fragments remain behind.
Injectable and Oral Antiparasitics
Systemic drugs that circulate through an animal’s bloodstream can kill screwworm larvae from the inside. Ivermectin and doramectin injections are authorized for cattle, sheep, and swine, both for treating active infestations and for prevention. These drugs belong to a class called macrocyclic lactones, which paralyze and kill the larvae feeding in tissue. For dogs and cats, newer oral antiparasitics, including isoxazoline-class drugs like lotilaner and sarolaner, have been used successfully alongside wound cleaning and manual larva removal. Oral spinosad has also shown effectiveness in companion animals.
Physical Removal
Manual extraction remains a critical step, especially for individual animals or human cases. Larvae are removed from the wound with forceps, often after flushing with saline. The challenge is ensuring every larva comes out intact, because fragments left behind can cause secondary infection. That’s why physical removal is almost always combined with a chemical larvicide or suffocating agent rather than used alone.
The Sterile Insect Technique
The most dramatic thing that kills screwworms isn’t a chemical. It’s a biological strategy that wiped them out across an entire continent. The sterile insect technique works by flooding an area with lab-raised male screwworm flies that have been sterilized with gamma radiation. These males survive and mate normally, but they can’t produce viable offspring. Because female screwworm flies mate only once in their lifetime, any female that mates with a sterile male will lay eggs that never hatch. Over successive generations, the wild population collapses.
This approach eradicated screwworms from the United States in 1966 and has since pushed the boundary of eradication south through Mexico and all of Central America. A production facility in Pacora, Panama, can generate more than 100 million sterile pupae per week. These are dyed with a fluorescent marker so field inspectors can distinguish them from wild flies under UV light. Most are released by aircraft along the Panama-Colombia border, creating a living barrier that prevents reinfestation from South America, where screwworms still exist. Ground releases from trucks or hatching crates supplement the aerial drops in harder-to-reach areas.
The technique proved itself again in 2017, when a small screwworm outbreak appeared in the Florida Keys. Sterile fly releases eliminated it before it could spread.
Prevention on Farms and Ranches
Screwworm flies target any open wound, no matter how small. A tick bite, a scratch from a fence, a newborn’s umbilical cord stump, or a surgical site from castration or dehorning can all attract egg-laying females. Prevention centers on minimizing wounds and treating the ones that do occur immediately.
The USDA recommends treating umbilical cords of newborn animals and all wounds right away with an approved insecticide. In areas where screwworms are present or could reappear, livestock producers should postpone procedures that create wounds, including dehorning, branding, shearing, ear notching, tail docking, and castration. Inspecting pens and equipment for sharp edges that could cut animals is basic but effective. Controlling ticks and other parasites that create small wounds also removes potential entry points for screwworm larvae.
For anyone traveling internationally with pets, inspection before returning home is essential. Screwworms remain active in South America, parts of the Caribbean, and tropical regions of Africa and Asia. A single infested animal brought across a border could reintroduce the pest to areas that spent decades eliminating it.
Why Screwworms Are Harder to Kill Than Other Fly Larvae
Unlike blowfly larvae that feed on dead tissue, screwworm larvae feed on living flesh, burrowing deeper into healthy tissue as they grow. This makes them harder to reach with topical treatments and more damaging the longer they’re left untreated. A single wound can host hundreds of larvae, and an untreated infestation can kill an adult animal within days. That aggressive feeding behavior is exactly why screwworm control relies on such a wide toolkit: fast-acting topical sprays for immediate kill, systemic drugs to reach larvae deep in tissue, physical removal to clear the wound, and the sterile insect program to suppress the population before infestations start.