By Linda Weiford, WSU News
High temperatures forced the insects’ biological processes into slow mode, similar to hibernation in bears during winter, said Glen Scoles, a tick expert with the U.S. Department of Agriculture’s Animal Disease Research Unit at Washington State University.
While humans can escape the heat by donning fewer clothes, jumping in water and turning on air conditioners, ticks are driven into a physiological state called aestivation, characterized by stillness and a lowered metabolic rate, said Scoles, an entomologist who has studied the parasite for 18 years.
“During hot, dry weather, many ticks will die,” he said. “Others will undergo aestivation, a physiological process that’s evolved over millions of years to help them conserve energy and moisture so they can survive periods of heat and low humidity.”
Survival of the hottest
In, say, 60- or 70-degree weather, a tick searches or “quests” for a blood meal by perching atop a leaf, twig or grass blade and outstretching its hooked legs to snag an unfortunate animal or human. But when conditions turn hot and arid, the tick, sensing a threat of desiccation, burrows in brush or grass and enters a state of dormancy.
“If you found a tick in this condition, you’d see that its legs are pulled in and it pretty much looks dead,” said Scoles.
Existing on this outer edge of survival, “ticks are still out there but they’re not coming after you,” he said.
Does this mean we can camp, hike and tramp across meadows through August with no chance of being snagged by a hungry tick?
“When it comes to ticks, I never say never,” said Scoles. “There’s a small chance, especially in cooler areas with more humidity, that ticks are still active. Just as they need blood to survive, they also need moisture, which they acquire from humidity in the air if the conditions are right.”
Outlived the dinosaurs
Female ticks that gorged on a host before the mercury soared may produce as many as 5,000 offspring – that’s right, 5,000 – but only one or two will survive, said Scoles. Aestivation is an adaptive strategy that helps keep those few from dying.
“It’s a tough world out there for ticks,” Scoles said.
As autumn approaches, the lucky ticks (not females who’ve laid eggs, as they die afterwards) will emerge from dormancy and literally rise to the occasion in search of one last target to draw blood from before winter descends. Though many will perish during the freezing months, enough larvae survive to ensure a new generation of hungry ticks will emerge from the forests and fields starting in late spring.
Because of their physiological ingenuity, ticks, which date back to the Cretaceous period, still roam the earth, said Scoles.
“There were ticks feeding on dinosaurs before mammals even existed,” he said. “The dinosaurs are long gone but ticks are still with us. When it comes to figuring out how to survive, my bet is with the ticks.”