New World Screwworm: What Organic Farmers need to know and what their treatment options are

By Lia Sieler, Executive Director, Western Organic Dairy Producers Alliance (WODPA), Chico, CA

New World Screwworm (NWS, Cochliomyia hominivorax) is recognized as a highly destructive pest. NWS fly larvae, also known as maggots, invade the tissue of living animals, resulting in severe and often fatal injuries. This species can infest warm-blooded hosts, including livestock, pets, wildlife, humans, and even birds.

The term "screwworm" is derived from the larvae's characteristic feeding behavior, where they burrow into wounds in a manner like a screw penetrating wood. Maggots inflict significant harm by tearing at host tissue with their sharp mouth hooks; consequently, the wound may enlarge and deepen as additional larvae hatch and feed on viable tissue. The impact of NWS infestations can be substantial, frequently leading to life-threatening conditions for affected animals. Adult screwworm flies are comparable in size to common houseflies or slightly larger and are distinguished by orange eyes, metallic blue or green bodies, and three dark stripes along their backs.

In August 2025, the USDA introduced a five-part strategy for New World Screwworm:

1. Innovating Our Way to Eradication
2. Protect the United States Border – Construction of a Domestic Sterile Screwworm Production Facility
3. Wildlife Migration Prevention
4. Stop the Post from Spreading in Mexico and Ensure We are Full Partners in Eradication
5. United States Food Safety is of Utmost Importance.

As a follow up, the sterile fly facility in Edinburg, Texas was recently finished. This facility expands USDA’s ability to disperse sterile flies along the border and into the United States, if necessary. Sterile insect technique, combined with surveillance and movement restrictions, is an effective method to control and eradicate NWS. Female NWS flies mate only once, so mating with a sterile male prevents reproduction. Sterile flies are released by air or ground, with aerial dispersal preferred for covering large areas. The USDA produces sterile flies at the COPEG facility in Panama and is funding Mexico’s Metapa facility renovation to double capacity. Production is expected to start in summer 2026. Additionally, USDA is building a new facility in Texas with a target of 300 million sterile flies per week, aiming for a total output of 500 million sterile flies weekly across international facilities.

What can we, as Producers, do?

So, what can we, as producers, do to prepare for the possibility of New World Screwworm on our farms? We reached out to the National Organic Program who suggested we look to the USDA Animal and Plant Health Inspection Service (APHIS) for direction. However, there are no specific answers on their website for organic producers. So, we moved on to reaching out to individual certifiers. Where Food Comes From Organic as well as California Certified Organic Farmers (CCOF) were both responsive to our questions and provided resources to further understand NWS. California Department of Food and Agriculture (CDFA) also responded and directed us to their website where you can find general information about NWS and an infographic on NWS disposal and cleaning for livestock and horses (New World Screwworm Disposal & Cleaning for Livestock & Horses). However, the biggest help came from discussing with Texas A & M’s Agrilife Entomologist, Dr. Sonja L. Swiger and their Research & Extension Organic Program Specialist, Bob Whitney. Upon further discussion, we concluded that it may be a wise choice to add PyGanic Specialty as an approved substance for our farms. PyGanic Specialty is OMRI approved, labeled for animal contact and has been used for years for adult fly control. At this point, we do not know if it would be effective on larvae, but it may work. A wound treatment option is still needed but, as of now, we do not know what the best option would be. It is important to note that although it is not listed as a treatment option on the USDA website, PyGanic is in the same class as the majority of those listed (Pesticides for Control of New World Screwworm).

Some steps farmers can take to prevent infestation:

• Watch for signs of NWS in livestock.
• Handle livestock carefully and inspect pens and equipment for sharp objects that can cause wounds.
• Treat the umbilical cords of newborn animals and all wounds immediately with an approved insecticide.
• Protect livestock from other wound-causing parasites such as ticks.
• DO NOT bring in animals from Mexico or Central America.

If you think you have found a screwworm, report it immediately to your State animal health official and APHIS office. This will allow APHIS and partner agencies to respond quickly and remove the screwworms before a population becomes established.

Lia Sieler, Executive Director, Western Organic Dairy Producers Alliance, Chico, CA can be reached at Lia.wodpa@outlook.com and www.wodpa.com.

Posted: to Organic Production on Tue, Mar 3, 2026
Updated: Fri, Mar 6, 2026