Organic Dust Takes a Bite Out of Ticks
By Leslie Mertz, Ph.D.
Two organic dusts are extremely effective in battling lone star ticks, a major human-biting and disease-carrying tick in the United States, according to a new study published in September in the Journal of Medical Entomology by scientists at the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS).
Not only that, but the dusts may also be effective against cattle fever ticks, which carry a protozoan that causes up to 90 percent mortality in cows not previously exposed, according to the Texas Animal Health Commission.
The researchers’ primary interest in conducting the study was actually the tick species Rhipicephalus (Boophilus) microplus and R. (B.) annulatus, both sometimes known as cattle fever ticks, but because these ticks are under quarantine, the researchers turned to lone star ticks (Amblyomma americanum) as a model, says senior author Allan Showler, Ph.D., research entomologist at the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas. A permanent quarantine zone runs along the Lower Rio Grande Valley on the Texas border to help keep cattle fever ticks contained to that area and prevent more from spreading north out of Mexico, where they are abundant. A few outbreaks of cattle fever ticks in Texas herds in 2016 have led to additional quarantines in several southern Texas counties.
The lone star tick is a good model for two reasons, Showler says. For one, he has access to a large colony of these arthropods in his research center, which is well north of the quarantined areas. The other benefit of the lone star tick is that the larvae are larger and more robust than those of cattle fever tick larvae, he says, so measures that work against lone star tick larvae are anticipated to be at least as effective against cattle fever ticks.
Get Them Young
The researchers wanted to catch the ticks before they get onto the host animals; for cattle fever ticks, that means the larval stage. Unlike most other ticks, cattle fever ticks are one-host parasites: Once a cattle tick larva crawls onto a host animal, it stays there through adulthood, Showler explains. In contrast, many other ticks utilize multiple hosts and can be found in vegetation at all of their life stages: larva, nymph, and adult.
The researchers’ next consideration was the type of control measure to study. “Since much of the quarantined area is made up of wildlife refuges, we started to think about larval tick controls that are inert, non-toxic and safe environmentally,” Showler says. Two came to mind. One is a clay dust called kaolin, which is sold under the trade name Surround and works by glomming onto and abrading the protective, waxy cuticle that enshrouds the larval tick body. The other is a silicate dust sold as CimeXa, which works by destroying the cuticle. Both actions cause the larva to lose water and shrivel up.
Although each of the two dusts showed high mortality in larval lone star ticks, CimeXa showed a slight advantage in dry-dust applications. CimeXa killed almost every single tick larva within 24 hours, regardless of whether the researchers applied the dry dust directly onto the larva or coated the substrate with the dust and allowed the larva to simply crawl across it. Separate tests of the dust on lone star nymphs (instead of larvae) showed a nearly 100 percent kill rate with direct application of CimeXa by 24 hours, and more than 80 percent dead with the substrate test at the 48-hour mark.
After the lab tests, the research group followed up by trying CimeXa on ticks in a natural environment. Specifically, they sprayed the dry CimeXa dust onto grass clumps holding heavy populations of wild ticks, in this case Gulf Coast ticks (Amblyomma maculatum). Although problematic high winds made dust application less than ideal, the dry dust knocked down the number of tick larvae by 96 percent and nymphs by 100 percent. “Even though CimeXa is really light and fluffy, and we lost a lot of it in the wind, it decimated those ticks,” Showler says.
With these results on lone star and Gulf Coast tick larvae, Showler and his colleagues are now planning additional experiments to determine CimeXa’s impact on non-target species, especially beneficial insects and other arthropods. The researchers are also in the midst of a study to test CimeXa on ticks that are already on cattle. Preliminary data are beginning to come in, and he hopes to publish those results by the end of the year.
“The whole idea of all this work is to test a variety of different tactics against cattle fever ticks, as well as other ticks, both on- and off-host. And, in particular, we’re looking for organic alternatives to conventional synthetic pesticides, which will be useful in protected areas, such as the very significant percentage of wildlife refuge land on the South Texas coastal plains and Lower Rio Grande Valley,” he says. “We need something that will be acceptable to the stewards of those lands, and we need something that works.”
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.