Soil Ecology: Critical But Understudied in the Fight Against Ticks
By Leslie Mertz, Ph.D.
After spending 10 years combing the fields and forests for ticks that carry Lyme and other diseases of medical and veterinary importance, researcher James Burtis, Ph.D., has something he’d like to say: Look to the earth.
The reason is that vector ticks spend most of their lives in the soil and leaf litter in a period of dormancy and only a small fraction of their time seeking out and feeding on hosts. Even in tick species that engage several hosts in their life cycle, more than three-quarters of their lives are spent off-host in the soil or among the leaf litter. That includes the infamous blacklegged tick (Ixodes scapularis) that bites and feeds on the blood of three different hosts over its life cycle and can pass the Lyme disease-causing bacterium Borrelia burgdorferi to humans.
Although vector ticks spend so much more time in the soil than attached to hosts, most research into these invertebrates centers on tick-host relationships, according to a new review article published in July in the Journal of Medical Entomology for which Burtis was lead author. The paper examined more than 130 vector-tick studies conducted over the past three decades and found that the vast majority focused solely on the seeking and biting phases; consequently, “we don’t know much about what they do off-host and during these long inactive periods, which can last for many months,” Burtis says. Burtis conducted much of the review as part of his doctoral dissertation in Cornell University’s Department of Natural Resources. He is now a postdoctoral associate in Cornell’s Department of Entomology.
The spotlight on ticks’ host-seeking (called questing) and host-attachment makes sense when one considers the background of the researchers who are conducting the work, Burtis says, noting that most either have a public-health background that emphasizes the infection aspect or an ecology background with a vertebrate focus that leads them to study how infection passes between a host and a tick, such as a deer and a blacklegged tick.
“What I’d like this review paper to accomplish is to push these tick researchers to spend time with soil ecologists and to think more about what’s happening when the ticks aren’t in contact with hosts,” Burtis says.
For instance, the review shows that many of the field experiments in tick research have centered on how to control ticks once they have already become a nuisance; while that is a worthwhile area of study, an understanding of why the ticks are there in the first place might also present helpful information for developing an effective plan to manage them.
“It would be great to know if there’s an off-host time when ticks are particularly vulnerable,” Burtis says. “From our own work [at Cornell], for example, we have found that blacklegged tick nymphs are not dying at high rates over the winter, but a lot of them do die in the spring and early summer, so if we can figure out what’s killing them off-host in the spring and what factors vary from year to year [to cause higher or lower mortality], that might lead to some type of management application that will allow us to increase mortality during that time period.”
Through work as part of both the Cary Institute of Ecosystem Studies in Millbrook, New York, and Cornell, Burtis has been gathering information on how soil-living arthropods and other organisms affect tick populations and on the impact of overwintering conditions on blacklegged ticks. He is now participating in the Northeast Regional Center for Excellence in Vector-Borne Diseases at Cornell and addressing both tick and mosquito-borne diseases in the region, while also investigating the potential for pesticide resistance to emerge in tick populations that have been managed for long periods of time.
In addition, he and others at Cornell are now planning experiments to learn how soil organic matter is related to tick survival. “The organic matter in the soil can vary quite a bit from place to place,” he says, “so it creates a nice little microclimate—it retains moisture, it cuts down on variability and temperature—and we want to see how much that microclimate affects their survival.”
While rooting around in tick territory for the past decade, Burtis acknowledges that he often finds ticks on his clothes, and even though he tries to protect himself by tucking his pant legs into his socks, the little blood-suckers sometimes make their way to his skin and latch on. To catch these crafty ticks, he does a regular body check after each outdoor foray, a habit that allowed him to notice very early signs of an infection with Lyme disease and have it quickly and successfully treated.
The religiousness of tucking and tick-checking is especially paying off now, he notes, because he has seen an increase in tick numbers over the past decade in central New York, where he does most of his field work. “It’s gone from being very hard for me to find a tick-collection site 10 years ago to being very hard for me now to find a site that I can’t collect from,” he says. No one is certain why, although he suspects land use change and growing deer numbers may be partly to blame. It’s just one more reason to learn more about ticks through their entire lifespan, including the time they spend off-host, Burtis says: “Ticks are basically soil animals, so we need to look down.”
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.