Humans are evolutionarily programmed to despise certain smells for good reason. Some clearly unpleasant odors, such as that of rotting meat, are often a byproduct of a bacterial infection, something we generally hope to avoid as a species. But while these unattractive odors may repulse people, other creatures are irresistibly drawn to them.

“We use these bacterial signals to stay away from certain things, for example not eating spoiled meat,” Jessica Metcalf, a microbial ecologist at the University of Colorado, recently explained. “Insects use them for a different reason. They say, ‘Yeah, bring it in, it smells like a great place to lay eggs.'”

Metcalf is currently focused on an insect whose penchant for stinky, unsanitary hot spots is once again causing problems in the United States: the New World screwworm blowfly (Cochliomyia hominivorax). The screwworm is attracted to open wounds on any warm-blooded animal, and this is particularly a problem for livestock, including cattle, sheep and goats. Eggs laid in these lesions will spawn worms that feed on living tissue and often become deadly to their hosts.

Environmentalists effectively eradicated screwworm populations from the continental US in 1966 using sterilization techniques developed in the mid-20th century. Unfortunately, the pests re-emerged in southern Mexico in 2024 and funding cuts overseen by the Department of Government Efficiency (DOGE) eliminated plans to combat the fly’s return. Texas confirmed the first new cases of screwworm infestation in the United States in more than 50 years in June. Within weeks, observers confirmed additional examples in livestock and dogs in other parts of the state, as well as in New Mexico.

The initial step in fighting screwworm is to monitor new outbreaks, and the easiest way to control the insects is with baited lures. Many popular configurations are based on the odors produced by bacteria in rotting meat. Unfortunately, screwworms are not the only organism attracted to the smell. A closely related, less dangerous and more common blowfly called the secondary screwworm will also find its way into traps, making it difficult for conservationists to precisely monitor their targets.

Unlike the New World screwworm, its botfly cousin prefers to eat dead meat. Knowing this, Metcalf’s team is designing improved traps packed with custom-made baits that reflect bacterial odors that are only emitted from living tissue. To do this, they are identifying specific volatile organic compounds that only attract New World screwworms.

It’s not just the host’s bacteria that attract insects. Screwworms carry bacteria that infect their hosts and then signal other screwworms to attach themselves to the food. By identifying those microscopic organisms, Metcalf and his colleagues are further customizing their lures to not only attract screwworms, but also to divert even more of them from livestock. The strategy relies on the same science used by forensic investigators to determine the time of a person’s death.

“It turns out that we can also take advantage of decomposition ecology, which relies heavily on blowflies and their bacterial partners, to improve surveillance and control of the New World screwworm,” Metcalf said.

Although it is still not as widespread a problem as it once was, these and other methods are necessary to keep the screwworm resurgence not only limited, but brief.

“As we had been working on the relationship between insects, bacteria and animal decomposition for the last 15 years… we were well prepared to address this problem,” he added.

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