New, Fast DNA Method Spots Pesticide-Resistant Ticks
By Leslie Mertz, Ph.D.
Invasive southern cattle ticks, also known as cattle fever ticks, not only carry parasites that cause the often deadly bovine disease of cattle fever, but they are now also becoming resistant to “just about every class of pesticide sold on the market,” according to research entomologist Donald Thomas, Ph.D., of the Cattle Fever Tick Research Laboratory (CFTRL) at Moore Field near Edinburg, Texas. The lab is part of the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS).
To help track the spread of resistant ticks, Thomas and colleagues at the USDA and several universities have developed a new and quick way to spot ticks that can survive pyrethroids, one of the most commonly used groups of pesticides. They described the new method in a report published Monday in the Annals of the Entomological Society of America, part of a new special collection on advanced genetic analysis of invasive arthropods.
The researchers are now also working on similar methods to identify resistance to other classes of pesticides.
A New Way
Thanks to a concerted eradication effort, the United States has been almost completely free of southern cattle ticks (Rhipicephalus (Boophilus) microplus) since the 1940s. The blood-sucking bugs still persist, however, in five Texas counties lying along the border with Mexico, a country where both the ticks and cattle fever disease are prevalent. About a decade ago, the CFTRL saw its first cases of resistance to pyrethroids in southern Texas, Thomas said. “We found them on just one ranch at first and then over the years, they spread to ranches in the next county and the next county.” The CFTRL tests those ticks found on cattle from the ranches, as well as on cows that make the short swim across the Rio Grande and enter southern Texas from Mexico. “Today, we find about 60 percent of the ticks coming into the lab are resistant to pyrethroids,” he said.
The traditional test for resistance—called the larval packet test—starts with adult females that are fully engorged with blood. “What we do is hold the fully engorged female tick in the lab until she lays eggs, then we hold those eggs until they hatch into larvae and wait a couple more weeks for the larvae to (reach the stage) where they start to move around and can grasp things,” Thomas says. At that point, the larvae are placed in small packets containing lethal doses of various insecticides, and those that survive are deemed resistant to that compound. The entire process takes a minimum of six weeks.
While the larval packet test works, Thomas and his group wanted something that would work on any cattle fever tick and do it much more quickly. To do it, they focused on genetics. Specifically, the researchers designed a DNA-based assay to scan the tick’s DNA for three mutations in the sodium-channel gene that confer some level of resistance, including two mutations that were previously known and one more that they identified in ticks. They focused on permethrin, which is one of the pyrethroids. The assay found that ticks possessing one of the three mutations have mild resistance, which means that one in 10 can survive a lethal permethrin dose; and, those with any two of the three mutations have high resistance, which equates to a survival rate of 90 percent.
Moreover, the assay provides results overnight.
The researchers’ next step was to learn more about how the ticks were moving from one site to another, such as via ranch-to-ranch trade or Mexican cattle crossing the Rio Grande. For this, they looked at DNA microsatellite markers, which are snippets of repeated DNA that can disclose the tick’s familial relationships and therefore the precise population from which they originated.
Based on this information, the researchers have been able to track tick movements in Texas. “Mostly in the past, by the time we would see ticks popping up here or there, they were already widespread. Now we are able to show the spread of the pesticide resistance year by year, county by county, and ranch by ranch,” Thomas says. “For example, if we’re suddenly seeing a tick infestation in Starr County and find that the DNA is very similar to ticks from an area over in Cameron County, we can do a traceback to find that a ranch in Cameron shipped cattle to Starr.”
The microsatellite information also revealed several genetically distinct populations hailing from Mexico and possibly arriving in the United States in the cattle trade “because we import more than a million head of cattle a year from Mexico,” Thomas says. By testing ticks the researchers had hoped to follow each family of ticks back to its distinct location of origin, but extensive trading and interbreeding of cattle in Mexico have made that impossible. “It’s a heterogeneous situation in Mexico, so the different families of ticks are just cropping up everywhere,” he says.
To continue tracking tick resistance, Thomas and his group are now developing DNA-based assays for other classes of pesticides—such as amitraz, organophosphates, and ivermectins—that still rely on larval packet tests. “We need to develop assays for the other classes of pesticides, so that we can find out whether they have the genes for resistance,” Thomas says. “That’s where our research is headed now.”
Annals of the Entomological Society of America
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.