In 2016, Montana researchers used drones to drop peanut butter vaccine pellets for prairie dogs, hoping to protect the endangered ferrets that depend on them

Think of a swarm of drones flying low over the Montana prairies early in the morning, dropping pellet-sized, peanut butter-flavored bombs down prairie dog holes, as if they are delivering rodent snacks. Sounds like a bizarre fantasy, but it really happened. In 2016, according to the U.S. Fish and Wildlife Service, researchers used unmanned aerial vehicles to drop vaccine-filled baits on the Montana prairie to save the black-footed ferrets, the most endangered mammal in North America.

According to the U.S. Fish and Wildlife Service records, it wasn't even about inoculating the ferrets themselves; the objective was to keep alive the creatures that the ferrets feed on and live inside: the prairie dogs. Long before this 2016 drone program, the species had already been given up for lost twice over: first written off as extinct in the late 1970s, then again after a small rediscovered population in South Dakota died out in the mid-1970s, before a final wild colony turned up in Wyoming in 1981. Given that history, this use of drones seemed more like desperation than science fiction.

Why a ferret's survival depends on a rodent


Black-footed ferrets are strictly prairie dog obligates; they prey upon them, hunker down in their abandoned burrows, and rear their own young in tunnels excavated by the prairie dogs. Without prairie dogs, there are no ferrets. The issue is sylvatic plague, the same Yersinia pestis bacterium responsible for the medieval Black Death, which persists in rodents of the West even today and is capable of exterminating whole colonies of prairie dogs in weeks.

By the end of the 1980s, the population of black-footed ferrets dwindled to only 18 individuals until a captive breeding program turned the tide. Despite all of this progress, plague carried by fleas continued to reverse gains made. Wildlife teams had long been treating burrows by hand with insecticide dust to kill the fleas that carry plague, a method that works reasonably well at the scale of a single burrow or colony, but becomes slow and labor-intensive across the vast stretches of land a recovery program actually needs to cover.

Enter the drones and the peanut butter

Scientists at the National Wildlife Health Center of the U.S. Geological Survey have been studying what flavor the prairie dogs would really go for, and tried everything from blueberry to sweet potato until they finally settled on the clear victor: peanut butter, according to National Geographic. The peanut butter pellets with the oral sylvatic plague vaccine inside were to be distributed using a drone or all-terrain vehicle at a rate of about 50 per acre, allowing workers to get coverage that hand-dusting never could before.

The 2016 drone flights weren't a standalone stunt, either; they were one operational method used partway through a single, ongoing multi-year vaccine field trial that had already been running on the ground and by ATV since 2013, on the same Montana refuge, and would continue being tracked through 2017.

But here's the part that gets left out of the feel-good headlines

It is here that the narrative becomes somewhat more complex than what the 2016 press release would have us believe. In fact, early results from that same multi-year trial did yield some positive results: as described by Rocke et al., in their 2017 paper titled “Sylvatic Plague Vaccine Partially Protects Prairie Dogs (Cynomys spp.) in Field Trials,” published in EcoHealth, prairie dogs in vaccine-treated plots had significantly greater relative abundance than prairie dogs on placebo plots from 2013 to 2015.

However, a fuller analysis of that same trial told a different story once more years of data came in. Matchett et al., analyzing the complete 2013–2017 dataset from the same Charles M. Russell National Wildlife Refuge in Montana where the drone flights took place, published “Oral Sylvatic Plague Vaccine Does Not Adequately Protect Prairie Dogs (Cynomys spp.) for Endangered Black-Footed Ferret (Mustela nigripes) Conservation,” in the journal Vector-Borne and Zoonotic Diseases in 2021. It found that annual survival rates for prairie dogs on vaccinated plots were about five percent lower than prairie dogs on unvaccinated placebo plots for five years.

During two plague die-offs recorded in the study, survival rates dropped as low as 5–6% on both vaccine and placebo plots alike, and these figures could not be said to sustain a viable black-footed ferret population either way. Part of the explanation is logistical rather than biological: the vaccine works best when baits are distributed in late summer or fall, but each spring's new litter of pups is born unvaccinated and can double a colony's size, diluting how much of the population carries any protection. That timing gap is a separate problem from how densely an area gets baited, even a thorough 50-pellets-per-acre drop can only vaccinate the prairie dogs that are already alive and eating bait when the drone flies over, not the pups born, or the outbreaks that strike, in the months before the next round.

So was it all pointless?

No, but it was not quite the unassailable success initially heralded by the headlines, either. The vaccine seems to work on a larger scale in some cases, without acting as a fail-safe against an outbreak. Researchers now increasingly favor combining the vaccine with the older, labor-intensive flea-dusting method described earlier, each compensates for what the other struggles with, alongside newer techniques such as fipronil-laden bait pellets, which kill fleas systemically when prairie dogs eat them. If anything, this deserves more attention than it gets.