Mosquitoes are prevalent, two-winged insects belonging to the family Culicidae, recognized primarily for the nuisance of their bite. Only the female requires a blood meal to develop her eggs. Understanding when these insects are active is key, as their schedule is governed entirely by environmental factors like temperature, light, and humidity. This predictable timing allows people to anticipate periods of peak activity throughout the year and across the 24-hour day.
Seasonal Emergence and Dormancy
The appearance of mosquitoes in temperate regions is dictated by the annual temperature cycle, marking the beginning of their active season. Mosquitoes cannot survive sustained cold as active adults, so they employ various survival strategies to endure winter. Many species survive the cold months in a state of arrested development called diapause, often as cold-tolerant eggs or as hibernating adult females sheltered in places like hollow logs, basements, or culverts.
The “mosquito season” commences when temperatures become consistently warm enough to trigger development and emergence. For many species, this threshold is a sustained temperature of around $50^\circ\text{F}$ ($10^\circ\text{C}$), which signals the start of breeding activity. Once this warmth arrives, overwintered eggs hatch or hibernating females emerge to seek a blood meal. Activity tapers off in the fall as temperatures drop below this threshold, causing the insects to enter diapause or die off.
Peak Activity Hours
For the majority of common mosquito species, such as those belonging to the Culex genus, activity follows a crepuscular and nocturnal schedule. These species, often called house mosquitoes, seek hosts primarily around dusk and dawn. The peak biting period frequently occurs from sunset through the early hours of the night, sometimes peaking between 10:00 PM and midnight.
This timing is a direct adaptation to avoid the harsh conditions of midday. Direct sunlight and high daytime temperatures cause rapid desiccation, or drying out, which can be lethal to the insects. During the day, nocturnal mosquitoes retreat to cool, shaded, and humid resting sites, such as dense vegetation, under decks, or inside sheds. They emerge once the sun has set and the air cools, providing the necessary moisture and lower light levels for safe flight.
Environmental Conditions That Drive Activity
A mosquito’s willingness to fly and bite is heavily influenced by immediate environmental conditions, even within peak hours. Temperature defines their optimal window, with activity accelerating when the air is between $64^\circ\text{F}$ and $93^\circ\text{F}$ ($18^\circ\text{C}$ and $34^\circ\text{C}$). Activity is highest in the range of $80^\circ\text{F}$ to $90^\circ\text{F}$, as this warmth accelerates their metabolism and reproductive cycles.
High humidity is another significant driver, as moisture prevents the insects from drying out during flight. Activity is ideal when humidity is at $42\%$ or higher, which is why recent rainfall or proximity to standing water increases mosquito presence. Conversely, activity is suppressed by high winds, as the small insects are weak flyers and cannot easily navigate in air currents. Temperatures exceeding $90^\circ\text{F}$ ($32^\circ\text{C}$) also cause activity to slow down as the heat becomes detrimental to their survival.
Why Different Mosquitoes Keep Different Schedules
Not all mosquitoes adhere to the nocturnal schedule, as some species have evolved distinct behaviors based on habitat and host preferences. A primary exception is the Aedes genus, which includes the Asian Tiger Mosquito (Aedes albopictus) and the Yellow Fever Mosquito (Aedes aegypti). These species are known to be daytime biters, exhibiting peak activity during the morning hours, often two hours after sunrise, and again in the late afternoon.
This daytime schedule is largely a result of their adaptation to breeding in small containers of water close to human dwellings, often in urban or suburban settings. Since they live in close proximity to hosts, they feed throughout the day, often avoiding intense heat by biting in shaded areas. This flexibility means protection from bites must be a concern during daylight hours, not just after sunset.