Mosquitoes are a familiar presence in Illinois during warmer months. While their bites cause irritation, some species can transmit illnesses to humans and animals, such as West Nile virus, various forms of encephalitis, and heartworm disease in dogs. Their seasonal appearance and activity levels are closely tied to environmental conditions.
Seasonal Emergence and Peak Activity
Mosquito activity in Illinois typically begins in late spring, around April or May. Their emergence is triggered when average overnight temperatures consistently reach at least 50°F for about a week. This temperature threshold allows overwintered eggs to hatch and dormant adults to become active, marking the start of the mosquito season.
As the weather continues to warm, mosquito populations increase, reaching their highest levels during the hottest months of summer. Peak activity is commonly observed in July and August. While some species are active throughout the day, many, including the West Nile virus vector Culex mosquito, are most active during dusk and dawn.
Environmental Factors Influencing Mosquito Presence
Temperature plays a significant role in mosquito development and behavior. They thrive in temperatures ranging between 50°F and 95°F, with warmer conditions accelerating their life cycle. Higher temperatures lead to faster egg hatching, quicker larval development, and increased activity among adult mosquitoes.
Precipitation is another important factor, as all mosquito species require standing or slow-moving water for their eggs to develop. Female mosquitoes lay between 100 to 300 eggs at a time on or near stagnant water sources. Common breeding sites include puddles, ponds, birdbaths, clogged gutters, and discarded tires. While rain creates new breeding habitats, excessive rainfall can sometimes flush out larvae from certain areas like storm drains.
Humidity also influences mosquito presence. High humidity is beneficial for their survival and egg-laying processes. Mosquitoes prefer humidity levels of 42% or higher, with activity increasing significantly as relative humidity approaches 90%.
When Mosquitoes Disappear in Illinois
Mosquito activity in Illinois begins to decline as temperatures drop in the fall. Their populations diminish significantly when temperatures consistently fall below 50°F for an extended period, typically around mid-October. The arrival of the first hard frost generally signals the end of the active biting season.
Many mosquito species survive the colder months by overwintering as eggs, which enter a dormant state called diapause. Other species, such as Culex, can overwinter as adult females. These adults find sheltered locations like sewers, basements, or hollow logs, slowing their metabolism until warmer conditions return. Some mosquito larvae can also enter diapause in moist soil or water until spring.