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Hungry, Hungry Larvae: How Entomologists Design the Diet for Mass-Reared Screwworm Flies

screwworm fly larvae at COPEG

At COPEG, the Panama – United States Commission for the Eradication and Prevention of Screwworm, approximately 20 million sterilized screwworm flies (Cochliomyia hominivorax) are reared every week. All those larvae (such as those being handled by COPEG workers here) need to eat, and a new article in the Journal of Insect Science details COPEG entomologists’ work to refine their latest formulation for the mass-reared screwworm diet. (Photo credit: USDA-APHIS)

When screwworm flies (Cochliomyia hominivorax) showed up in the Florida Keys in 2016, for the first time in more than 50 years, government agencies were ready to respond. They used the sterile insect technique, suppressing the wild population of flies with an overwhelming number of mass-reared, sterilized flies.

Those flies came from a facility in Panama maintained by the U.S. and Panama governments known as COPEG (the Spanish-language acronym for Panama – United States Commission for the Eradication and Prevention of Screwworm). COPEG’s mass-rearing program serves a dual purpose: providing a steady supply of sterilized screwworms to maintain a permanent sterile fly barrier in eastern Panama and standing ready to supply sterilized screwworms for outbreak response elsewhere.

And “mass-rearing” is no exaggeration: COPEG’s target output is approximately 20 million screwworm pupae per week. (Though it has the capacity to exceed 100 million per week!)

By now, maybe you’re wondering (or maybe you’re not wondering at all, but we’re going to answer it for you anyway): How do you feed all those baby screwworms?

The answer involves a whole lot of cow blood.

In the 1950s, when the sterile insect technique was first effectively operationalized, screwworm fly larvae were raised on a diet that included raw ground beef, but the drive for cost efficiency drove an evolution in the diet toward dry powdered ingredients. By 2007, the diet consisted of spray-dried whole bovine blood, spray-dried egg, dry powdered milk substitute, and cellulose fiber, all mixed with water. But, in the last couple of years, the availability of spray-dried whole bovine blood has dwindled, leading the scientists at COPEG to once again revise their screwworm-rearing diet. And their latest formulation is documented in an article published in June in the open-access Journal of Insect Science.

The change was driven largely by simple market dynamics, says Joshua Thomas, entomologist with the U.S. Department of Agriculture-Animal and Plant Health Inspection Service at COPEG and lead author on the report. “Simply put, companies have moved away from the sale of whole blood over the much more flexible byproducts, optimizing cost-to-benefit ratios,” Thomas says. “As technology advanced, the ease of partitioning the whole blood into its plasma component and cellular fractions became more and more efficient.”

So, Thomas and colleagues moved to a formula that replaced the whole-blood portion with a mix of hemoglobin and plasma as separate ingredients. They chose a hemoglobin-to-plasma ratio of 8:2, but they also tested ratios of 9:1 and 7:3 and measured the results in terms of hatch rate, larval weight, pupal weight, and pupal quantity.

The results show that the diets with more plasma (8:2 and 7:3 ratios) produced more larvae but not greater overall larval or pupal weight than the 9:1 ratio. In terms of cost efficiency, however, the 9:1 ratio came out about 3 percent more expensive than the other two ratios, which ran nearly even.

At least for now, COPEG will continue with its 8:2 hemoglobin-to-plasma ratio in its screwworm -rearing diet, Thomas says. Future research may elucidate why, exactly, the varying ratios result in differing yields.

“Though both are high in protein content and low in fat, plasma and hemoglobin contain a few very specific differences,” says Thomas. “For example, our Quality Control department commonly finds the pH of plasma between 9-10, while our powdered hemoglobin stays between 7-8. As well, the amino acid profiles of both are not exactly alike, nor are the specific nutritional values.”

In 1964, a study by Richard E. Gingrich in the Annals of the Entomological Society of America outlined the specific proteins required for screwworm larvae. “As a personal curiosity, I am interested in replicating the research of Dr. Gingrich, utilizing the technology and techniques we now have available more than 50 years later,” Thomas says. “I think the study would result in a clear and direct understanding of the exact amino acids, elements, and vitamins needed to make minute changes to our diet at the most precise levels for any situation.”

The new report in the Journal of Insect Science offers the public a glimpse into a government entomology program that they might take for granted or otherwise not often think about—even though it saves U.S. an estimated $1.3 billion in damage to livestock every year by keeping the pest at bay.

“We wanted to give our readers a bit of insight into how a mass rearing program, such as COPEG, evaluates and scrutinizes all changes both scientifically and financially. Multiple, slow, deliberate steps ensure overall yield sustainability and population stability,” says Thomas. “On the other hand, proven cost benefits in the field are often what drives administrative change.”

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