How will this year’s harsh winter affect mosquitoes and ticks?

A severe winter means fewer troublesome insects in summer? The link between weather and the survival of mosquitoes and ticks exists, but it is not as simple and obvious as it seems. Snow cover thickness and microclimate matter much more than thermometer readings.

Mosquitoes and ticks waiting for spring warmth

To better understand the relationship between mosquito and tick survival and weather, it is worth looking at how insects spend the winter.

Ticks burrow into the soil, but not very deep. Thanks to a slowed metabolism, they can survive in this state for a very long time.

Mosquitoes spend the winter somewhat differently. In their case, water is essential for survival. According to Brian Lovett, an entomologist at Cornell University, drought is the natural killer of mosquitoes. Low temperature has a less significant effect.

Referring to studies on American insect species, Lovett notes that mosquitoes spend the winter in frost-resistant developmental stages. Adult mosquitoes can survive by obtaining nutrients from the stored blood of their hosts. Some mosquitoes also survive as larvae, but the greatest chance of surviving the winter is for females fertilized in autumn. In cold conditions, their metabolism almost stops so that in spring they can start laying eggs as quickly as possible.

Temperature, snow cover thickness, local microclimate – these determine mosquito and tick survival

A cold winter does not necessarily mean a decrease in insect populations. Negative temperatures are often accompanied by snow, and it is the snow cover that increases the chances of mosquitoes and ticks surviving.

For ticks overwintering in the leaf litter, snow acts as thermal insulation. Research on the Canadian species Ixodes scapularis has shown that a thick layer of white fluff along with leaves protects against even very low temperatures. At the same time, unstable, often melting snow increases tick mortality in winter.

Results from another study conducted in Germany on the winter activity of two tick species, Ixodes ricinus and Dermacentor reticulatus, showed a link between survival and temperatures. The authors analyzed insect presence by observing and counting them in experimental plots and by collecting information from veterinarians about attacked animals (mainly dogs and cats). Ticks of the species Dermacentor reticulatus proved to be much more active. Scientists relate this to milder winters and the absence of frost.

Mosquitoes can be harmed by both too low and too high temperatures. Studies of the species Culex pipiens and Anopheles messeae showed that at 5–16°C, their mortality increases.

There is one more factor that affects insect survival in winter. It is the microclimate. Results from a study conducted in Maine, USA, show that ticks have a better chance of survival where it is humid and relatively warm. Favorable conditions can exist not only in forests. Mosquitoes readily use urban infrastructure. A species that has adapted exceptionally well to life among humans is the tiger mosquito (Aedes albopictus). Its eggs were found in Switzerland in sewer wells, where temperatures are higher than outside.

Mild winters invite invasive species

German scientists from the Bernhard Nocht Institute for Tropical Medicine point out another consequence of mild winters. Such conditions favor the development of Asian invasive mosquito species in our climate zone. Some mosquitoes bring threats in the form of diseases. Researchers point to the chikungunya virus transmitted by tiger mosquitoes (Aedes albopictus) and the Japanese encephalitis virus (dangerous to humans as well), transmitted by Japanese mosquitoes (Aedes japonicus).

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Sources:

Probst J, Springer A, Topp AK, Bröker M, Williams H, Dautel H, Kahl O, Strube C. Winter activity of questing ticks (Ixodes ricinus and Dermacentor reticulatus) in Germany – Evidence from quasi-natural tick plots, field studies and a tick submission study. Ticks Tick Borne Dis. 2023 Nov;14(6):102225. doi: 10.1016/j.ttbdis.2023.102225. Epub 2023 Jul 1. PMID: 37399628

Crandall KE, Millien V, Kerr JT. High-resolution environmental and host-related factors impacting questing Ixodes scapularis at their northern range edge. Ecol Evol. 2024 Feb 21;14(2):e10855. doi: 10.1002/ece3.10855. PMID: 38384829; PMCID: PMC10879908

Ravasi, D., Mangili, F., Huber, D. et al. The effects of microclimatic winter conditions in urban areas on the risk of establishment for Aedes albopictus. Sci Rep 12, 15967 (2022). https://doi.org/10.1038/s41598-022-20436-9

F G Sauer, E Timmermann, U Lange, R Lühken, E Kiel, Effects of Hibernation Site, Temperature, and Humidity on the Abundance and Survival of Overwintering Culex pipiens pipiens and Anopheles messeae (Diptera: Culicidae), Journal of Medical Entomology, Volume 59, Issue 6, November 2022, Pages 2013–2021, https://doi.org/10.1093/jme/tjac139

https://www.bnitm.de/en/news/news/world-mosquito-day-climate-change-and-virus-outbreaks