With paper-weight wings and spindly legs, the mosquito hardly seems built to handle the cold. The secret to its survival is eggs built to withstand freezing temperatures. Even if some eggs die off during extreme cold, mosquito populations rebound quickly. The same holds true for ticks that can wait out a cold snap far below a forest’s layer of leaves.
Researchers at Tyson Research Center, the environmental field station for Washington University in St. Louis, offer insight into how both insects are surviving the polar vortex that has gripped most of the Midwest and eastern United States.
In temperate regions, mosquitoes overwinter as eggs in natural or artificial containers that hold water during warmer months.
“There is substantial variability among and within species in the ability for these eggs to survive winter temperatures, but some species produce eggs that can survive extreme cold,” said Kim Medley, director of Tyson Research Center. Medley is a biologist who studies the ecology and evolution of vectors of human and wildlife disease — especially mosquitoes.
“For instance, the Asian tiger mosquito — a dominant mosquito in urban and suburban St. Louis — can produce eggs that overwinter in a state of hibernation called diapause.”
“As day length declines into the fall, female Asian tiger mosquitoes shift from laying directly developing eggs to laying diapause eggs,” Medley said. “Females will allocate more resources to the embryo inside the egg, including lipids that insulate the embryo from cold. These diapause eggs also have a reduced metabolism, allowing them to utilize the resources inside the eggs over long periods of cold and desiccation.”
Snow, ice and leaf litter may also help insulate mosquito eggs from extreme temperatures.
“Adaptation to cold temperatures can occur rapidly, as some of our work on the Asian tiger mosquito has shown,” Medley said. “Eggs of Asian tiger mosquitoes from the northern U.S. survive cold northern winters significantly better than eggs from more southern latitudes.”
A 1995 study showed that more than 50 percent or more of Asian tiger mosquito eggs die off in temperatures lower than 8.6 degrees Fahrenheit. But Medley cautioned that these observations are dated and were made in a laboratory study, not based on actual field observations.
“Even if we see die-offs of mosquito eggs during this extreme cold snap, populations are likely to quickly rebound this summer,” said Katie Westby, a postdoctoral fellow in the Medley laboratory.
Ticks in forests across the Midwest are vulnerable to freezing when they come into contact with ice. However, they have developed certain physical adaptations and tricks to deal with the cold.
“Like many other organisms, ticks can produce antifreeze proteins to help them tolerate cold temperatures,” said Solny Adalsteinsson, staff scientist at Tyson Research Center. “In fact, some researchers are exploring whether these proteins can be used to protect mammal tissues from frostbite.”
“Beyond temperature thresholds measured in the lab, habitat likely plays an important role in buffering ticks from winter temperatures,” Adalsteinsson said.
“Most ticks are not active on very cold days in winter here, so they are down below the leaf litter layer,” she said. “Leaf litter and snow may provide insulation so that the temperatures stay warmer where the ticks are compared to ambient air temperatures.”