Snow-Covered Tires Help Invasive Mosquitoes Survive Cold Winters
By John P. Roche, Ph.D.
The Asian tiger mosquito (Aedes albopictus) is a vector of dangerous diseases including Zika virus, dengue fever, and chikungunya. The mosquito is an invasive species in the U.S., and its range is expanding. To help predict the spread of the species, entomologists want to know which factors limit the species’ range. Previous research established that the Asian tiger mosquito has a lower temperature limit of minus 0.5 degrees Celsius to minus 2.5 C, and the prediction was that they could not survive north of the latitudinal line in the U.S. that has that minimum temperature in January. But the species is indeed sometimes seen north of this line. What could explain this?
One possibility is that the species uses its ability to adjust its physiology to different conditions—something called phenotypic plasticity— to adjust to the low temperatures. Another possibility is that the mosquitoes still have the same January temperature threshold but they use insulated microhabitats as refuges to survive. In a new study published this month in Environmental Entomology, Katie Susong, Lyric Bartholomay, Ph.D., and colleagues at the University of Wisconsin-Madison tested if insulated car tire microhabitats could increase survival of Aedes albopictus in the northern Midwest of the U.S. and if snow cover further increased survival by providing additional insulation.
The investigators set up a transect of field sites spanning about 300 kilometers from Milwaukee, Wisconsin, in the north to Champaign-Urbana, Illinois, in the south. The transect spanned the line, or “isotherm,” where minimum temperatures were minus 0.5 C to minus 2.5 C. They placed three tires on the ground at each of five sites along this transect, and outfitted each tire with internal and external temperature sensors.
Aedes albopictus survives winter conditions by laying eggs that go into a suspended state called embryonic diapause. Susong and colleagues used mosquitoes from two colonies: one colony of Aedes albopictus that was established from adults gathered from the wild in Champaign-Urbana, Illinois, and one lab-raised colony of a species native to the upper Midwest, the eastern treehole mosquito (Aedes triseriatus).
The investigators generated diapausing eggs by placing pupae into environmental chambers with a photoperiod set to stimulate diapause. They then placed the prepared eggs on sheets. In each tire, they placed two sheets of diapause eggs and one sheet of non-diapause eggs from each species. They left the eggs in the tires from November 2018 to April 2019. As controls, they also stored sheets of diapause eggs from each species in the lab at 4 C and sheets of non-diapause eggs from each species in the lab at 27 C.
At the end of April, the team collected all the sheets of eggs from the tires in the field. Then they submerged all sheets of field-maintained eggs and lab-maintained eggs in water to trigger hatching. The next day, they counted the number of larvae that hatched.
Averaging across all sites, the mean temperature inside tires in January was minus 3.3 C, whereas the mean external temperature was minus 4.8 C (a difference of 1.5 degrees). At sites where eggs survived, the mean January temperature inside tires in January was minus 1.6 C and the mean external temperature was minus 5.3 C (a difference of 3.5 degrees). Thus, the insulating effect was higher for the sites where eggs survived than for the average across all sites.
At sites where eggs survived, the longest period with temperature below minus 12 C was 11 hours. At sites where no eggs survived, the longest period with temperature below minus 12 C was 43 hours. In addition, the mean January snow depth was 35.5 millimeters deeper at sites where eggs survived than at sites where eggs did not survive.
As expected, the investigators found that more eggs survived in tires with higher internal temps. Surprisingly, though, higher internal tire temperatures were found at more northerly sites. This was due to the insulating effects of the deeper snow cover present at higher latitudes. Climate change is predicted to reduce snow cover but increase extreme low temperature events in the midwestern U.S.
“As we think about climate change and the predictions about significant decreases in sustained snow cover,” Bartholomay says, “there isn’t a clear relationship between warming temperatures and more readily hospitable conditions for range expansion and establishment of invasive species like Aedes albopictus.”
It is even possible that climate change could end up rolling back the northerly expansion of the Asian tiger mosquito in the U.S.
In summary, Susong, Bartholomay, and colleagues found that the internal temperature inside all tires was greater than the external temperature and that snow cover increased insulation for tires if the snow cover was deeper than 100 millimeters. These findings support the hypothesis that Asian tiger mosquitoes can overwinter north of their isotherm limit through the use of insulated microhabitats. “Our study revealed that tires,” Bartholomay says, “and particularly tires covered in snow, create a microhabitat that buffers mosquito eggs from the cold—even cold experienced during a so-called polar vortex that brought arctic temperatures to Wisconsin and surrounding states during 2019.”
“Therefore,” Bartholomay says, “the Asian tiger mosquito can invade northward into states where we would not have previously predicted it could survive. This species is a daytime feeder with an annoying bite, and it is a human health threat because it is a competent vector for a number of mosquito-borne viruses.”
These findings can be valuable for improving control efforts. One obvious control step would be to reduce the number of abandoned tires available for use by mosquitoes. The study’s results also are valuable for improving models of ecological suitability of microhabitats and their effects on the distribution of Asian tiger mosquitoes. “We know that tires are one such microhabitat,” Bartholomay says, “so urban density, proximity to roadways, and proximity to tire service centers could be interesting indicators to estimate the likelihood of the Asian tiger mosquito invasion into new spaces; in fact, other labs have noted this connection.”
The investigators found that insulated tire microhabitats can increase survival of Asian tiger mosquitoes, but these refuges may also influence other aspects of the mosquitoes’ life histories, and there may be other types of insulated microhabitats used by the mosquitoes. Future research can delve into these intriguing possibilities.
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.