Even the most diehard bacon lovers might not consider rubbing rancid pig lard on themselves to keep mosquitoes away. But researchers at New Mexico State University discovered that rancid fats and oils of animals such as shark and alligator, among others, are effective insect repellents.

Texas archaeologist Gus Costa reached out to NMSU biology professor Immo Hansen's lab to conduct experiments to test the viability of multiple written accounts about rancid fat insect repellents documented by Spanish conquistadors. Explorers in the Gulf Coast in the 1700s were convinced of the effectiveness of rancid fats that Native Americans used to protect themselves against mosquitoes in the coastal bend of Texas and adjacent states, which host some of the densest mosquito populations in the United States.

"Gus started procuring fats and making them rancid and sending them to us," said Hansen. "That took about five years as we went back and forth. We also sourced our own oils and did the same experiments. Last year, it all came together, and we wrote a paper on it."

The results were published in August in the journal PLOS ONE, with Acosta as the senior author. The article is titled "Rancid rumors or Native wisdom: Evaluating the efficacy of animal fats as insect repellents attributed to historic-period Native Americans." Hansen is a co-author along with Hailey Luker, NMSU biology Ph.D. candidate, and Claudia Galvan, who manages the lab for co-author Omar Holguin, NMSU associate professor in the Department of Plant and Environmental Sciences.

Few traditional Native American mosquito repellents have been examined using controlled laboratory methods for repellency testing. In this study, Hansen and his team tested the repellent efficacy of fats derived from alligator, cod and shark along with bear, beef tallow and pig lard that were aged to various stages of rancidity. The researchers made sure that all fats were sustainably and ethically sourced.

The first step was to test each sample's rancidity score. On a scale of one to five, where one means no odor and five is a high level of odor, seven people were asked to rank various substances based on smell.

"When a fat or oil becomes rancid, that doesn't necessarily mean it smells bad, it just means that it has an odor," said Luker. "These fats and oils are made up of long fatty acids, which don't have a scent, but once they get broken down through oxidization, through sunlight, through temperature, those fatty acids get cut down into smaller pieces and that's what you smell."

"What we found that is interesting is if they're not rancid, they had lower levels of repellency, if they are rancid, they're at higher levels," Galvan said. "We can use that information moving forward to potentially make a targeted, maybe a more efficient insect repellent in the future that might not be so smelly."

Using yellow fever mosquitoes (Aedes aegypti), the team performed an arm-in-cage assay to measure the complete protection times resulting from these fats when applied to human skin. They then used a Y-tube olfactometer assay to evaluate long-distance repellency and tested tick-repellency in a crawling assay.

The results suggest that rancid animal fats from cod, bear and alligator are potent but short-lived mosquito repellents. The findings prove the validity of traditional ethnozoological knowledge of Native American people and support aspects of the ethnohistorical record. They also discovered both rancid and fresh fats do not repel ticks.

At this point, the tests leave the team with a list of substances with varying levels of effectiveness as mosquito repellent that can be the subject of further research.

"For the rancid fats, we don't yet know the active ingredient that repels mosquitoes," Hansen said. "We really want to find out about the active ingredients in this big mixture of different chemicals that Claudia has produced. We have a nice list here."

"There are a lot of traditional remedies that go back centuries from either the Spanish or different types of Indigenous populations and we utilize them here today," said Galvan, who plans to pursue a career at the Centers for Disease Control and Prevention. "So, I think that the full circle message of this study is definitely something that we can really learn from, but then we can also improve."

The full article can be seen at https://newsroom.nmsu.edu/news/nmsu-researchers-test-300-year-old-insect-repellent-strategies-used-by-native-americans/s/dcfa4b07-b40d-4adf-9ba1-7a43c676d31c