I Will Survive: Researchers Comb the Literature for Larval Tick Questing Data
By Melissa Mayer
Remember the time the U.S. Centers for Disease Control and Prevention tweeted a photo of a poppy seed muffin sporting five seed-sized ticks? The idea was to show just how small ticks can be, but those were nymphal ticks. Ticks in the life stage before that—larval ticks—are about half the size of a poppy seed, and their tiny size is a big problem when it comes to sampling them in the field. As a result, tick larvae are often overlooked in the literature.
— CDC (@CDCgov) May 4, 2018
A team of researchers from the University of Texas Rio Grande Valley and the U.S. Department of Agriculture’s Cattle Fever Tick Research Laboratory wanted to change that, so they compiled the available data on factors that affect these hard (ixodid) tick larvae, presented in a review paper published in June in the Annals of the Entomological Society of America.
Surely You Quest
The team focused on the larval ticks’ host-seeking behavior, called questing. Since they can’t really run or jump, the ticks climb up a plant and wait, stretching out their front legs so they can grab on to an animal as it passes by. This is a particularly important activity for three-host ticks. These parasites seek hosts during three distinct life phases (larva, nymph, and adult) but drop off to molt between phases and also to lay eggs.
For larval ticks, questing is an incredibly vulnerable time. Their high surface area to volume ratio means that they can dry out quickly, especially when relative humidity dips below the optimal 80–85 percent. In fact, the odds that one of these larvae will find a suitable host, successfully attach, and complete its life cycle (from one generation of larvae to the next) are around 5,000 to 1.
May the Odds Be Ever in Your Favor
Fortunately, ticks have a few tricks up their evolutionary sleeves that help them quest. For one, they have a suite of sensory receptors that enable them to detect and respond to microclimatic changes. These include eyes on the sides of their heads that register changes in light (such as a shadow when a host passes over them) and help the ticks know which way is up.
Haller’s organs, situated on the ends of the ticks’ front legs, are helpfully exposed when the ticks extend those appendages in the questing pose. These pick up sensory cues—like heat, humidity, carbon dioxide, ammonia, and vibrations—that indicate the approach of a potential host.
Ticks also work together to survive. “When the host approaches them, they will link arms and do a monkey chain so that if one of the ticks can grab on to the host, a whole bunch of them can link right along up into the host,” says one of the paper’s authors, Donald Thomas, Ph.D. This clustering serves another important function: When the questing larvae form clusters, they are less likely to dry out.
Ixodid larvae can survive surprisingly long spells without feeding—up to 9 months. And they can enter a dormant period if things get too dicey. “When they’re questing and the conditions are not optimal … they can completely slow down their metabolism and stop themselves from questing until the conditions are favorable,” says co-author Brenda Leal.
More Data, Better Tick Management
In addition to humidity, the authors compiled available data for other factors that influence questing. These include temperature, precipitation, photoperiod, seasonality, habitat considerations, and the presence of natural enemies.
“It really depends on multiple things,” Leal says. “My recommendation for anybody who wants to study or sample larval ticks is to really look at what season you’re in, the temperature, humidity, location, things like that.”
The team points out serious gaps in the available knowledge of larval tick biology—especially when it comes to what’s known about predation during off-host stages and species-specific data for how long larvae can survive off-host. They also say entomologists need more efficient techniques for sampling those miniscule larval ticks.
This is all crucial data because ticks are responsible for 77 percent of vector-borne disease in the United States, and they are second only to mosquitoes worldwide. The costs associated with this disease are steep, and understanding tick behavior is vital to tick management.
“Because it’s such an essential life stage and they’re able to transmit disease, if we’re able to focus on the larvae and get them when they’re at their most vulnerable stage, this can really help tick management because you’d be preventing them from proceeding into adulthood and reproducing,” says Leal.
Annals of the Entomological Society of America
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: firstname.lastname@example.org.