Entomologist is decoding chemical signaling between ticks with an eye toward controlling Lyme disease

Having grown up in China, entomologist Andrew Li didn’t see his first tick until 2000, when he began working at a U.S. Department of Agriculture lab in Kerrville, Texas, studying tick species that transmit livestock disease. But by 2013, Li’s knowledge of the bloodsucking parasites was so extensive it landed him at USDA’s Invasive Insect Biocontrol & Behavior Laboratory, in Beltsville, Md. His research now focuses on monitoring local populations of deer ticks—which can transmit Lyme disease—and lone star ticks—which can impart a meat allergy to humans—and developing ways to better control the tiny arachnids.

Monitoring and controlling ticks in the U.S. is increasingly important: Cases of tick-borne illnesses reported to the U.S. Centers for Disease Control & Prevention more than doubled between 2004 and 2016. Each year since 2013, the agency has recorded more than 25,000 cases of Lyme disease, plus several thousand more probable cases. Part of Li’s work centers on identifying the chemicals that ticks use to communicate with one another. Emma Hiolski spoke with Li to learn more about the signaling molecules, called semiochemicals, and their potential to slow the rising tide of tick-borne illnesses.

Which tick-borne illnesses are of most concern for humans, and how do ticks spread them?

Though there are an increasing number of tick-borne pathogens other than the Lyme disease bacterium—and more occurrences of these illnesses—nothing compares to the scale of Lyme disease. Lyme disease is caused by a bacterium, Borrelia burgdorferi, which is transmitted by black-legged ticks (Ixodes scapularis), also known as deer ticks. The deer tick has larval, nymph, and adult stages, and each stage needs to take a blood meal from an animal or human. The larvae are actually clean because females cannot pass the Lyme disease pathogen to their eggs. But when larvae take a blood meal from a rodent—like a white-footed mouse, which can carry the pathogen—the larvae can pick up the pathogen. After feeding, larvae molt into nymphs, which can carry the bacteria and spread it to humans when they bite people.

Why might tick-borne diseases be on the rise?

They’re boosted, in part, by wildlife that supports tick populations: mice, other small animals, and deer. White-tailed deer populations have grown in the past 20 or 30 years. Urbanization also plays a role: We share space with wildlife more than we did in the past. Climate change may also contribute. For at least one species, the lone star tick, the northern boundary of its range has expanded in recent years, possibly due to warming temperatures.

How do ticks use semiochemicals, and what do we know about them?

The two types of tick semiochemicals we know most about are arrestment pheromones (or arrestants) and sex pheromones. Arrestment behavior occurs when a tick senses a chemical signal from other ticks and stops moving. Arrestants are commonly purines—namely, guanine and xanthine—found in tick excrement.

One particular compound—2,4-dichlorophenol—has been identified as a tick sex pheromone. It is released by female ticks and received by male ticks, bringing them together for mating. The same compound has been identified in several different tick species, including the lone star ticks. But the deer tick is a different story: Nobody has identified a specific sex pheromone yet. When it comes to semiochemicals and pheromones in tick research, I think we’re behind compared with other insects.

Ticks are different from other blood-feeding insects—they do not fly. They just clamp on to a blade of grass, or a bush, and patiently wait for a deer, human, rabbit, or other animal to pass by. Because ticks cannot move long distances by themselves, males and females rely on their meals to bring them together. Once on the host, they use shorter-range, low-volatility chemicals to connect. Each species has a unique biology, and some species go through very complex behaviors. There are mating pheromones, mounting pheromones, and a variety of chemicals are involved in some of the tick species that have been studied.

How do you go about identifying and studying tick semiochemicals?

For pheromone semiochemical research, it is a combination of chemistry and behavior work. You have to observe and test whether chemical communication is involved in certain behaviors. For example, you can put a piece of filter paper into a jar with a hundred ticks for a couple days, until the filter paper is soaked with tick odor. Then, you test how other ticks respond to the paper. That is how the arrestment pheromone was found. When such a filter paper was placed in a Petri dish with two dozen nymphs, the nymphs all stayed on the paper in an arrested behavior. After studying behavior, you move to chemistry and try to isolate the compound. Once you’ve analyzed the compound, you test it and see how the tick responds. It’s kind of a back-and-forth.

Is there a particular compound you’re especially intrigued by?

Because I’m working on deer ticks now, and because nobody has identified this, I still have questions about the deer tick sex pheromone. Is there indeed no such thing as a sex pheromone in deer ticks, as some have suggested? If not, why not? Even though people have not identified a typical, standard sex pheromone, it does not mean there are no other chemicals dictating or affecting their behavior.

How can learning about tick semiochemicals help us in the future?

The use of semiochemicals is for control. For other insects, like fruit flies, weevils, and agricultural pests, you have pheromone traps that use semiochemicals as lures in various ways to monitor, control, and suppress populations. In the field of tick control, we do not really have a commercial product on the market. Chemical communication is such an important part of tick biology. If we can identify the chemicals that play a critical role in tick behavior, we can use the chemicals to design control products to suppress the tick population or reduce human exposure to ticks.

Emma Hiolski is a freelance writer. A version of this story first appeared in ACS Central Science: cenm.ag/li. This interview was edited for length and clarity.