Telling apart male and female blacklegged ticks (Ixodes scapularis) has important implications for research and management efforts, but it’s difficult to do with tick larvae and nymphs. In 2025, researchers developed a genetic test for quickly determining sex in blacklegged ticks, and it works with both adult and immature stages. Shown here are blacklegged tick adults (male at top left, female at top right) and nymphs, near the head of a pin for scale. (Photo by Jim Occi, BugPics, Bugwood.org)

By Ed Ricciuti

Ed Ricciuti

In the case of ticks, the classic trope derived from a Kipling poem holds very true, since the female of the species is unquestionably deadlier than the male. Adult male ticks do not feed—they just mate and that’s it for them—while females latch on to hosts, feed on blood, and potentially spread disease. So do the larvae and nymphs, known together as “immatures.”

Knowing the sex of ticks is important to understanding their biology and controlling them. In the case of blacklegged ticks (Ixodes scapularis), it’s not that hard to identify the sex of adults because of sexual dimorphism, such as overt differences in genital openings, but those traits in immatures are arcane and not always reliable indicators. A recent breakthrough published in October in the Journal of Medical Entomology, however, makes it possible to figure out the sex of immatures at a molecular level and promises to open doors to great understanding of tick biology.

“Male and female nymphs also overlap in their body size, and given their small size, this makes identifying them morphologically nearly impossible,” says Cody Koloski, a Ph.D. candidate in the Department of Veterinary Microbiology at the University of Saskatchewan, Canada, and lead author on the new study.

Koloski and his colleagues assayed immatures with a DNA test recently developed by Isobel Ronai, Ph.D., of Harvard University and colleagues to distinguish between adult female and male blacklegged ticks. Ronai’s team published the test protocol in May 2025 in the Journal of Medical Entomology. Both advances will offer researchers a new tool to study the species, the main vector of Lyme disease and capable of spreading a half-dozen other ailments as well.

Koloski’s main research focus has been on how the immune system of mice that are hosts to the blacklegged tick affects the transmission and abundance of the Lyme disease bacteria, Borrelia burgdorferi, in ticks. He has found that female blacklegged nymphs acquire a higher infection rate than males.

When he read Ronai’s paper, Koloski says, “I immediately realized that I needed to see if their method could work on larvae and nymphs.”

The newly found ability to identify the sex of blacklegged ticks relies on a highly efficient method of DNA sequencing called a polymerase chain reaction. It can simultaneously detect two different DNA sequences in a single reaction, boosting speed of the research. One of the sequences targeted was specific to males.

The paper describes how DNA was extracted from the nymph leg and then sequenced. Nymphs were then allowed to mature into adults as a double-check on sex determination.

Until now, determining sex of immature ticks has required sensitive equipment operated by highly trained experts. Even then, results of testing have not always been reliable. Validation of a molecular tool to distinguish sex of immatures “could significantly benefit studies in tick-borne pathogen ecology,” Koloski’s team writes.

They raised hopes that the new test capability will facilitate future research into how sex impacts key traits of the blacklegged tick, including the intake of blood, how they seek out hosts, and survivability. They also noted that increased understanding of tick biology could lead to better control methods.

Increasingly, scientists who study ticks have anticipated that research on the genetics of ticks could lead to genetic control methods that have been aimed at other arthropod pests. A likely candidate is the sterile insect technique (SIT), in which sterile males are released in the wild to mate with females, which then fail to produce offspring. It has had success against several species, notably Aedes mosquitoes.

Males can be sterilized for SIT by radiation, the time-tested method, or genetic modification, which is the subject of research by the University of Nevada’s Monika Gulia-Nuss, Ph.D. Koloski was working at her laboratory when he initiated the test on immatures. The new DNA test, says Gulia-Nuss, “provides efficient methods to separate males from females, a necessary step for population suppression strategies like the sterile males release either using traditional radiation method or the genetic modification.”

The blacklegged tick is endemic to much of the nation’s northeast, west, and southeastern regions and is spreading. As it does, so does the incidence of infection by the Lyme disease spirochete, which it transmits. Among the other disease pathogens it spreads are those of babesiosis and ehrlichiosis.

Diseases transmitted by ticks now account for more than 80 percent of all cases of vector-borne illnesses reported annually. Almost a half million people contract Lyme disease annually in the United States, making the blacklegged tick a major health concern.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His most recent book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.