Natural Compound Deters Lone Star Tick Larvae
By Leslie Mertz
A naturally occurring botanical compound found in anise, fennel, vanilla, and cranberries is effective in deterring one of the major human and livestock pests, according to research by scientists at the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS).
The study showed that the botanical compound called p-anisaldehyde fends off the larva of the lone star tick (Amblyomma americanum), which is one of the most prevalent ticks in the southeastern and south-central United States. Not only does the lone star tick parasitize cattle, horses, goats, dogs, and many other animals, but it is also the most frequently reported tick species to bite humans in those regions of the country, and it can transmit diseases such as erhlichiosis, spotted-fever rickettsiosis, tularemia, and protozoan infections, according to Allan Showler, Ph.D., research entomologist at the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas, and lead author of a study on the subject published in October in the Journal of Medical Entomology.
“This tick is problematic anywhere there are a lot of host animals. For instance, on the east Texas coastal plains, you have turkey and white-tailed deer, as well as other large ungulates like feral nilgai antelope, all running around hosting ticks. That’s in addition to the domesticated livestock, such as cattle, that are ranging out there. So, anytime humans go hunting or hiking or otherwise go out into these areas, they might pick up a tick,” Showler says.
Making matters worse, all three life stages in three-host tick species—the larvae, nymphs, and both adult males and females—bite. The larval tick climbs onto low vegetation and waits (the wait is called “questing”). When a host animal happens by, it gloms on, finds a vulnerable spot, and sinks its mouthparts into the skin to begin feeding. “Once it becomes replete with blood, it will drop off and become a nymph,” Showler describes. The process repeats with the nymph questing, feeding and dropping off to become an adult, and with the adult repeating the process. When the adult female drops off the host, she lays her eggs. (In one-host tick species, such as the southern cattle fever tick, all three mobile life stages feed on the same host without dropping off.)
Showler has been interested in “more organic, greener, less-toxic-to-humans approaches for tick repellency and tick control, but when it comes to many of the botanicals, it’s kind of a crapshoot,” he says, explaining that many have not been thoroughly tested on different pest species. “I started out with a number of different botanicals that might bear something out in pilot studies. And while the other ones weren’t so interesting, p-anisaldehyde was.” He found studies showing that this compound disoriented mosquitoes but was an attractant for certain other insect species and was toxic to others. “It was really curious, so we decided to just give it a go on these ticks and see what happened,” he remarks.
Specifically, he and co-author Jessica Harlien, who is also a USDA-ARS researcher, tested various formulations and treatments of p-anisaldehyde on lone star ticks (Amblyomma americanum). “What jumped out at us more than anything else was its deterrence or repellency,” he says. The two are different: A deterrent causes the target species to avoid a compound, while a repellant actually drives the tick away. “We’re pretty sure that p-anisaldehyde is a deterrent, at least on lone star tick larvae.” They also found that the compound is lethal to the ticks if they are immersed in certain concentrations and that exposure of adult females to sublethal concentrations reduced egg production.
So far, they have conducted all of their deterrence and repellency tests of p-anisaldehyde on larval ticks. “Part of the reason is the larvae tend to be more vulnerable because they’re smaller, so it’s a good place to begin,” he says, noting that they will start testing repellency on nymphs and adults soon.
“When it comes to botanicals, I know some people like to say it’s been done to death, but it’s the opposite. Actually most of the studies stop where we stopped: We make a discovery, but now someone else has to take the reins and develop it into a commercial product,” he explains. “What we’ve done in this study is report that p-anisaldehyde looks to have lethal (contact) properties on lone star tick larvae, and it has deterrent properties against larvae, but, as far as how it might be formulated and applied in an economically feasible way for use on humans or other animals, that is something that has yet to be developed.”
Beyond lone star ticks, the researchers are also running tests on other pest species, notably house flies and selected biting flies. “We’ve got some really good preliminary information,” he says, adding that they hope to publish those results soon. As another teaser of his work, he adds, “Without divulging anything, we’re working right now on something that, at least in the laboratory, is knocking these lone star ticks dead really fast. I can’t give any more details because we’re still in the middle of the study, but hopefully that’s coming soon.”
Journal of Medical Entomology
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.