It’s that time of year when the bluebirds are singing, bees are buzzing—and ticks are questing for hosts in thick vegetation. In the United States, vector-borne disease incidence has been increasing over the past two decades, with tick-borne disease accounting for more than 75 percent of all reported cases.
One of the key ways to avoid contracting a vector-borne disease is to personally protect yourself from ticks by checking frequently for ticks, using insecticide-treated clothing, and correctly applying repellents. Currently, DEET represents more than 80 percent of all tick repellents in products found on store shelves in the U.S. Despite its strong safety record, DEET is one of the most commonly found chemicals in water, and its role in the environment has not yet been thoroughly studied. In the search for new options, a group of researchers at Iowa State University and the University of Wisconsin-Madison published a new study last week in the Journal of Medical Entomology that tested naturally derived tick repellents for their potential to protect us against these disease-carrying pests.
Naturally derived plant extracts have been tested for other medically important arthropods, namely mosquitoes, with varying levels of success. Wanting to expand the potential use of the putative repellents, the scientists in the lab of Joel Coats, Ph.D., at Iowa State University studied several commercially available plant-extract compounds for their ability to repel the blacklegged tick (Ixodes scapularis) and the American dog tick (Dermacentor variabilis).
One of the main pitfalls of using naturally derived plant extracts is that they only work for a short time. One way to extend their repellency is to attach them to other naturally derived chemical compounds. This provides a little more stability to the repellent, allowing it to repel ticks and mosquitoes a little longer. “We started with two natural [chemicals], and we joined them with a biodegradable chemical joining,” says Colin Wong, Ph.D., lead researcher on the laboratory repellency assays who is now a postdoctoral researcher with the U.S. Department of Agriculture’s Agricultural Research Service in Byron, Georgia. The idea is that the new, semi-synthetic repellent will quickly biodegrade into its naturally derived component parts and potentially cause less environmental impact than other repellents currently on the market.
In an earlier study published last fall in Advances in Arthropod Repellents, laboratory-reared ticks were dropped into the center of a petri dish container. Naturally, ticks will either try to escape the space or climb up the walls to seek hosts. However, to do either, the ticks in the study had to climb onto a piece of filter paper treated with a naturally derived repellent that was placed in the dish. The longer it took the tick to venture onto the repellent-laden filter paper, the greater the repellent effect that the plant compound had on the tick.
In the end, the researchers found a few plant-derived compounds that were more repellent than others and warranted further testing. These compounds were synthetically created but are very similar to those naturally found in lavender and sage, geraniums and roses, and lemongrass and verbena.
The next step, covered in the new study in the Journal of Medical Entomology, was to test these repellents in the field on wild ticks. The scientists in Coats’ lab reached out to Susan Paskewitz, Ph.D., and members of her lab at the University of Wisconsin-Madison to design a way to test these promising repellents in the field.
Xia Lee, Ph.D., lead author on the study and now a public health entomologist with the Department of Health Services in Wisconsin, led a group of field technicians out to Big Eau Pleine County Park in Marathon County, Wisconsin, during the warm summer months of June and July when there are typically high numbers of tick nymphs questing for hosts in the thick vegetation.
Lee and his team carried with them long pieces of canvas cotton attached to dowels. They heavily sprayed one side of the cloth with one of the three naturally-derived repellents, the industry standard DEET found commonly in tick repellents on store shelves, or the acetone control in which all of the repellents were diluted. (They also tested untreated cloth drags as a second control.)
Each repellent-laden canvas cloth drag was pulled through a stretch of vegetation, repellent side down, for 40 passes. The researchers quickly hung up the canvas so they could count and record all ticks to species. Then, they continued to observe ticks for a three-minute period and counted how many ticks detached within that period.
Most of the ticks observed were blacklegged tick nymphs, which was typical for the time of year they were testing. Surprisingly, none of the repellents (including DEET) resulted in fewer ticks attached compared to controls at the time of the first count. But after the first count, the researchers noticed that the ticks began to detach quickly before the three-minute observation period was up. At a low concentration, DEET caused more ticks to detach within the three-minute window than the compounds derived from geraniums and verbena, but the compound derived from lavender had equal levels of tick detachment as DEET. At a high concentration, all repellents caused equal tick detachment, which was significantly greater than both the controls.
Despite this, there were still ticks remaining on the canvas drags after the observation period. Says Lee, “[This] speaks to risk in the real world. If ticks are still grabbing onto you, they may fall off. Only 70-80 percent of the ticks are falling off, so you still have 20 percent of ticks that are still attached after a three-minute period. There’s still a possibility that those ticks may fall off or move to an untreated area, like from your arm to your armpit area.” This is why it is important to frequently check for and remove ticks when working or recreating in an area with high levels of ticks.
So far, these naturally derived repellents show promise as repellents worth stocking on store shelves next to DEET. Before that, though, several further tests need to occur to ensure that these naturally derived compounds are non-toxic to humans and the environment and that the repellents work as well on human clothing and skin as they do on canvas cloth drags.
Journal of Medical Entomology