Mosquito Almanac
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Heat, drought, and mosquito activity

By Season Updated Jul 2026 7 min read
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Short answer

Warmth speeds mosquitoes up. In warmer water they develop from egg to biting adult faster, and inside the mosquito the West Nile virus ripens faster too, so a warm summer generally means more mosquitoes and more transmission (Sources 2, 3). But the relationship is not a straight line. Past a certain point, extreme heat and drought can actually cut nuisance numbers, because breeding pools dry up and very high temperatures shorten mosquito lifespans (Sources 1, 4). The catch is that a dry, hot summer can still raise West Nile virus risk at the same time as it lowers the mosquito count. When water is scarce, the remaining pools tend to be stagnant and rich in organic matter, which favors the Culex mosquitoes that carry the virus, and both those mosquitoes and the birds that amplify the virus crowd together around the little water that is left (Sources 1, 5). So fewer bites does not reliably mean less danger.

Heat speeds development

Mosquitoes are cold-blooded, so temperature sets the pace of nearly everything they do. In warmer water, eggs hatch sooner and the larval and pupal stages finish faster. A controlled study of Culex tarsalis, a major West Nile vector in the western part of the country, found that immature mosquitoes reared at 31 degrees C developed fastest and reached adulthood soonest, while those held at 19 degrees C took the longest (Source 2). Warmer conditions produced smaller adults, which is a consistent pattern in insects reared at higher temperatures (Source 2).

Heat also speeds up the virus itself. West Nile virus has to multiply and spread inside a mosquito before that mosquito can pass it on, a delay called the extrinsic incubation period. That process is temperature-driven: modeling work anchored in California surveillance data estimated that the virus does not replicate in the mosquito below about 14.3 degrees C, and that above that threshold the incubation period shortens as temperatures rise, so warmer regions completed the cycle faster and sustained longer transmission seasons (Source 3). Put the two together and warmth acts on both clocks at once, the mosquito’s and the virus’s, up to a point.

That qualifier matters. Transmission does not simply keep climbing with the thermometer. A synthesis of laboratory data across several Culex vectors found that West Nile transmission peaks at intermediate temperatures, roughly 23 to 26 degrees C, and then declines at hotter temperatures (Source 4).

Extreme heat can reduce some mosquitoes

Above the optimum, more heat works against the mosquito. The same synthesis placed the upper thermal limit for West Nile transmission at roughly 32 to 35 degrees C depending on the vector, with transmission falling off toward that ceiling mainly because adult mosquitoes do not live as long at high temperatures, which leaves them less time to acquire and pass on the virus (Source 4). Development can suffer too. In the Culex tarsalis rearing study, the highest temperature tested did not just speed development, it also lowered the share of larvae that survived to emerge as adults in some strains (Source 2).

Then there is the water. Mosquitoes need standing water for the larval and pupal stages, and sustained heat with little rain evaporates the shallow pools, containers, and ditches they rely on. Drought reduces both suitable breeding habitat and the overall area of standing water on the landscape (Source 1). When those sites dry out before the immatures finish developing, that generation is lost. This is part of why a brutal heat wave or a deep drought can coincide with fewer mosquitoes biting in the yard, even though the season is otherwise favorable to them (Sources 1, 4). The nuisance count and the danger, though, are not the same thing.

The drought and West Nile paradox

Here is the part that surprises people. Drought tends to reduce the number of mosquitoes, yet several lines of evidence link drought to higher, not lower, West Nile virus risk.

A long-term study in Kern County, California, drawing on more than 500,000 trap nights collected from 2010 to 2023, found exactly this split. Wetter conditions were associated with more mosquitoes but a lower infection rate, while drier, drought conditions went the other way: fewer Culex tarsalis, but a higher share of them carrying West Nile virus (Source 1). In that dataset a shift toward wetter conditions raised Culex tarsalis abundance by about 9 percent while lowering the infection rate by about 6 percent, so drought pushed both numbers in the direction that concentrates risk (Source 1).

The leading explanation is concentration. Drought shrinks the total amount of standing water on the landscape, and the pools that remain are often stagnant and loaded with the organic matter and nutrients that Culex larvae favor. As open water disappears, both the mosquitoes and the birds that serve as the virus’s amplifying hosts are drawn to the same shrinking wet spots. West Nile virus is maintained in a cycle between Culex mosquitoes and birds, with humans as incidental dead-end hosts who do not pass it on (Source 5). Crowd the vectors and the reservoir birds together and the mosquito-to-bird-to-mosquito cycle runs hotter, so a larger fraction of mosquitoes end up infected even as their total number falls (Sources 1, 5). The size of the effect depends on the species and its breeding habits, which is one reason drought does not play out identically everywhere (Source 1).

What it means for you

Do not read a dry summer as a safe summer. It is easy to assume that a hot, rain-starved stretch with fewer mosquitoes around means less to worry about, but the evidence points the other way for West Nile virus: the years and places where mosquito numbers thin out under drought can be the same ones where the mosquitoes that remain are more likely to be infected (Sources 1, 5). Judge risk by local surveillance, not by how many bites you are getting.

The practical response does not change with the weather. Keep emptying standing water weekly, including the stagnant, scummy pools and containers that Culex mosquitoes prefer, since those are exactly the sites drought leaves behind. Keep using an EPA-registered repellent and screens, and pay particular attention around dusk and dawn, when many West Nile vectors bite. For how the virus spreads and who faces the most serious illness, see the disease cluster (Related, below).

Sources

  1. Sambado S, et al. “The paradoxical impact of drought on West Nile virus risk: insights from long-term ecological data.” Proceedings of the Royal Society B: Biological Sciences, 2025. Analysis of more than 500,000 trap nights in Kern County, California (2010 to 2023) found drought lowered Culex tarsalis abundance but raised infection rates; a shift toward wetter conditions raised abundance about 9 percent and lowered infection rate about 6 percent. The paper states that drought reduces suitable breeding habitat and the overall area of standing water, with the proposed mechanism being spatial aggregation of mosquitoes and competent bird hosts around remaining water. https://pmc.ncbi.nlm.nih.gov/articles/PMC12404801/ (accessed 20 July 2026).
  2. Dodson BL, Kramer LD, Rasgon JL. “Effects of larval rearing temperature on immature development and West Nile virus vector competence of Culex tarsalis.” Parasites & Vectors, 2012. Higher rearing temperature (31 degrees C) shortened time to pupation and emergence and produced smaller adults; the highest temperature also reduced emergence success in some strains. https://pmc.ncbi.nlm.nih.gov/articles/PMC3480948/ (accessed 20 July 2026).
  3. Hartley DM, et al. “Effects of Temperature on Emergence and Seasonality of West Nile Virus in California.” American Journal of Tropical Medicine and Hygiene, 2012;86(5):884 to 894. Modeled the extrinsic incubation period with a thermal minimum near 14.3 degrees C for virus replication, above which the incubation period shortens as temperature rises; warmer districts had longer, more efficient transmission seasons. https://pmc.ncbi.nlm.nih.gov/articles/PMC3335698/ (accessed 20 July 2026).
  4. Shocket MS, et al. “Transmission of West Nile and five other temperate mosquito-borne viruses peaks at temperatures between 23 and 26 degrees C.” eLife, 2020;9:e58511. Across several Culex vectors, transmission peaked at intermediate temperatures (about 23 to 26 degrees C) and declined toward an upper thermal limit near 32 to 35 degrees C, driven largely by shortened adult mosquito lifespan at high temperatures. https://pmc.ncbi.nlm.nih.gov/articles/PMC7492091/ (accessed 20 July 2026).
  5. Centers for Disease Control and Prevention, “Transmission of West Nile Virus.” West Nile virus cycles between Culex mosquitoes and birds, which act as amplifying hosts; humans, horses, and other mammals are dead-end hosts that do not pass the virus on; amplification runs from spring emergence through fall. https://www.cdc.gov/west-nile-virus/php/transmission/index.html (accessed 20 July 2026; page returned HTTP 403 on direct fetch, statements taken from the CDC search result summary of this URL).

Related

  • [Summer: peak mosquito season] (placeholder)
  • [Spring: the season mosquitoes wake up] (placeholder)
  • [Fall: when mosquito season ends] (placeholder)
  • [West Nile virus explained] (placeholder)
  • [Standing water: the backyard breeding checklist] (placeholder)
  • [How weather and climate shape mosquito seasons] (placeholder)

Educational note: This guide is general reference information about mosquito biology and mosquito-borne disease risk. It is not medical advice. If you have symptoms of a mosquito-borne illness, or questions about repellents, vaccines, or local risk, consult a licensed healthcare professional or your local health department.

Last reviewed 20 July 2026.