A Wasp Smaller Than a Grain of Rice Is Being Tested Against Screwworm

The screwworm fly has one reliable weakness, and the country has been leaning on it since the 1950s: males sterilized by radiation, released by the million, so that wild females mate once and produce nothing. That program is why Cochliomyia hominivorax was pushed out of the United States in 1966. It is also why the current outbreak, at 59 confirmed U.S. cases with 54 of them in Texas according to the USDA figures cited this week, has been handled as an engineering problem — build the flies, harden the ports, watch the wounds.

A paper published this spring in the Journal of Medical Entomology proposes a second, much smaller weapon. Tachinaephagus zealandicus is an encyrtid wasp about the size of a grain of rice, and in laboratory trials it killed secondary screwworm larvae at rates comparable to the fly species it already parasitizes in the field.

The work was led by Chris Geden at the USDA Agricultural Research Service's Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, Florida, with co-authors at the ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville and at the screwworm research laboratory USDA runs with the Panama-U.S. commission at Pacora. The journal issue is dated May, the paper went online in June, and Entomology Today wrote it up Friday.

What makes the study awkward is what it could not study. New World screwworm is a quarantine pest, which means working with it directly carries a level of regulatory and biological risk most labs do not want. So the team used the congeneric secondary screwworm, Cochliomyia macellaria, as a proxy — a fly that lives in the same genus, occupies similar carrion and wound niches, and can be reared without the containment apparatus.

The wasp does its work at the one moment the fly is exposed. Wandering-stage larvae drop out of an animal's wound, burrow into the ground, and pupate. That is when T. zealandicus finds them, injecting eggs into larvae more than twice its own size. The offspring eat the host from the inside and the fly never becomes an adult.

In the trials, host-to-parasitoid ratios ran from 5:1 to 40:1. At the most crowded ratio, a single female killed as many as 18 hosts, and five wasps working a dense patch of larvae cut the population by roughly half. The wasps also showed no preference between screwworm larvae and Sarcophaga bullata, a flesh fly they already use as a host, which is the finding that matters for a biocontrol agent: an indiscriminate hunter will keep itself alive between outbreaks.

Soil turned out to matter in a way that will shape any release program. Clay did not stop the wasps. Moist soil did not stop them. Dry soil gave the larvae some protection, because a larva on hard, dry ground can thrash and escape an attack that would land on softer terrain. Pastures baked by a Texas October are, in other words, a different proposition from irrigated ground.

"Combatting New World screwworm requires a multi-pronged approach," Geden said. "Wasp releases could be helpful for treating localized hot spots where NWS populations are gaining a foothold."

That framing is deliberate. Nothing in the paper suggests wasps replace sterile flies, which suppress populations across whole regions and which the USDA is scaling up at a facility under construction at Moore Air Base in Edinburg. What a parasitoid offers is the piece the sterile insect technique cannot deliver: a self-sustaining predator that keeps working in the scrub along a creek bed where nobody is releasing anything.

"It fits into our approach of integrated pest management," said Phillip Kaufman, head of Texas A&M's entomology department, who was not involved in the study. "It's sort of like death by a thousand cuts. Everything you do takes a little bit more of the population away."

The gap between this and a field program is the gap Geden names himself. Geden said more work is needed to see whether the results hold outside a lab and against the actual New World screwworm rather than its proxy, and his lab has already developed methods for economical mass production of the wasp. The unanswered question is whether mass-rearing and release measurably reduces fly populations in the field.

For Texas, the timeline is unforgiving. The state's cattle herd, the largest in a country that produces roughly 12 million metric tons of beef a year, is now sharing rangeland with a parasite that has already been confirmed in wildlife — a hunter-harvested elk in Pecos County, the first such case in the country.

Sterile flies will carry this fight. A wasp the size of a rice grain is being asked, for now, only to help at the edges.