Mosquito Control Colorado: Late Summer Is The Peak

Most people expect mosquitoes to fade as summer winds down. In Colorado they do the opposite. Late August into September is when the mosquito that matters here is at its peak, and it is also when human West Nile virus cases concentrate.

So the season that feels like it is ending is the season when mosquito control actually earns its keep.

Why do mosquitoes get worse as summer ends?

Three things line up at once.

The population is cumulative. A mosquito goes from egg to adult in roughly a week. Every week of standing water since spring has added another batch on top of the last. By late August a yard that has quietly held water all season is not producing its first generation, it is producing its tenth.

The species that matters here is still going. West Nile virus is carried in Colorado by Culex mosquitoes, most commonly Culex tarsalis and Culex pipiens. Culex tarsalis is most active from late June through the first frost, which in South Metro Denver is usually still weeks away when Labor Day arrives.

The water keeps coming. Late-season irrigation and late-summer rain keep gutters, saucers, tarps and low spots topped up exactly when the population is already at its highest. That is why mosquito control timed to late summer does more than the same work done in June.

Why are Colorado mosquitoes a West Nile concern?

The mosquito most common here is well suited to carrying the virus, and the state has one of the worst West Nile problems in the country. In 2023, Colorado recorded more West Nile cases than any other state. So a mosquito on your patio in late August carries more risk than one in a state with little West Nile activity.

The Colorado Department of Public Health and Environment notes that West Nile can spread any time mosquitoes are active. Cases have turned up from May to December. Most human cases, though, come from the late-summer peak in mosquito numbers, mainly in August and September.

That is the single most useful thing on this page. The risk window is open right now, and it stays open until the first hard frost. If the last time you thought about mosquitoes was in July, this is the month it actually counts.

None of this is a reason to panic, but when you act and which water you empty matter more here than the repellent you buy. In Colorado, mosquito control is a public-health question as well as a comfort one.

Most people who catch West Nile have no symptoms at all. A minority develop fever and body aches, and a small fraction develop a serious illness of the nervous system. Serious illness is uncommon, but Colorado sees more West Nile than most of the country, which makes the risk real here.

Where do mosquitoes actually breed around a house?

Mosquitoes breed in small amounts of water that most people overlook. A Culex mosquito can breed in any container that holds water for about a week, and a typical South Metro Denver yard has plenty of them.

The list, in rough order of how often it is the actual culprit:

Clogged gutters. The single most overlooked breeding site on most homes. Water sits in a blocked run all season and nobody ever looks.
Plant saucers and pot trays, indoors and out.
Tarps, covers, and toys holding water in folds and depressions after rain or irrigation.
Corrugated downspout extensions that hold water in the ridges.
Buckets, wheelbarrows, and trash can lids left upright.
Birdbaths and pet water bowls not changed weekly.
Low spots in the lawn that hold irrigation runoff.
Unused or poorly circulating water features, and neglected pools or hot tubs.
Tree holes and clogged French drains.

The rule that makes this manageable is the seven-day rule. A mosquito needs roughly a week of standing water to go from egg to adult. Empty, drain, or refresh every container on your property once a week and you break the cycle before it completes, regardless of how much rain fell.

It is the one piece of mosquito control that costs nothing, and it beats any spray a homeowner can buy.

Have Your Yard Checked for Breeding Water

What actually reduces mosquito bites?

Three things, in order of how much difference they make. Effective mosquito control is mostly these, in this order.

Eliminate breeding water. Covered above, and it is the largest lever by a wide margin. Mosquitoes have limited flight range, so the ones biting you were very likely produced within a few hundred feet.

Use an effective repellent at the right time. Culex mosquitoes are most active from dusk into the evening, which is when people are out on their patios. Use a registered insect repellent, follow its label, and cover up in the evening when you can. Repellent bracelets and ultrasonic devices do not have the evidence behind them that topical repellents do.

Cut back their daytime resting spots. Mosquitoes spend the day in cool, shady, damp places: dense ground cover, tall grass, ivy, shrubs against the house, and the underside of a deck. Thinning that growth and keeping the grass cut leaves them fewer places to rest.

What does not work well enough to count as mosquito control: bug zappers, which mostly affect harmless insects and few mosquitoes; citronella candles, whose effective range is very small; and standalone ultrasonic repellers.

When does professional mosquito control make sense?

Professional mosquito control makes sense when the source is not on your property, or not findable. That is the dividing line. If your standing water is gone and the biting continues, the mosquitoes are coming from a neighbor’s yard, a drainage, a greenbelt, or a detention pond. No amount of container-dumping on your side will fix it.

A professional treatment also earns its place in four cases:

The property backs onto open space, a drainage, or a pond.
You are hosting an event and need fewer of them for a day.
Someone in the house reacts badly to bites.
Your yard has shady, damp planting you cannot thin out.
What a professional can do that you cannot: Treat the shady spots where they rest by day, not just the ones flying. Treat standing water you cannot drain, with a larvicide product before they grow into adults. Repeat on a schedule that matches the season, instead of one spray in July.

Mosquito service is seasonal by nature. It runs through the active months and stops.

What Guardian does, and what we will tell you first

Guardian Pest Management is family owned by Sean and Michelle Meade, who have more than 30 years in pest control and founded the company in 2000. We serve Highlands Ranch, Littleton, Castle Rock, Parker, Centennial, Lone Tree, Castle Pines, Greenwood Village, Roxborough, and Cherry Hills.

On a mosquito control call, we start by looking for the water. If we find a source you can get rid of yourself, we will say so before we quote a treatment. Removing the breeding site does more than treating the adults it produces.

We use EPA-registered products applied according to label instructions. Home service does not require a long-term contract, so seasonal mosquito service is a seasonal commitment and nothing more.

The peak is running now and it does not end until the first frost, so there is still real season left to protect.

Frequently asked questions

No, and late summer is the point at which it matters most. Colorado’s human West Nile cases concentrate in August and September, and Culex tarsalis stays active until the first frost. Mosquito control in late August protects the part of the season with the highest risk, not the tail end of a quiet one.

Because the population is cumulative. Each batch takes about a week from egg to adult, so every week of standing water since spring has stacked on the last. Late-season irrigation and rain keep those containers full at exactly the wrong time.

August and September. The Colorado Department of Public Health and Environment notes that people can catch it any time mosquitoes are active, with cases as early as May and as late as December. Most, though, happen in the August to September peak.

Culex species, most commonly Culex tarsalis and Culex pipiens. Culex tarsalis is most active from late June through the first frost. Culex breed in standing and stagnant water rather than moving water.

Roughly a week from egg to adult. That is why emptying, draining, or refreshing every water-holding container on the property once a week breaks the cycle before it completes. Weekly is the interval that matters.

Clogged gutters most often, plus plant saucers, tarps and covers, corrugated downspout extensions, buckets, birdbaths, pet bowls, low spots holding irrigation water, and neglected water features. A container is enough; a pond is not required.

Not well. Zappers mostly affect non-target insects and few mosquitoes. Citronella candles have a very small effective radius. Eliminating breeding water and using a registered topical repellent at dusk do much more.

Often yes, because mosquitoes have limited flight range and mosquito control addresses the shady resting spots in your own yard. If the source is a neighboring property or a drainage, treatment manages the pressure rather than removing the source.

No. Mosquito activity runs from roughly late spring through the first frost, so service is seasonal.

To get your property looked at while the season is still running, call Guardian Pest Management at (303) 346-7087, or start at myguardianpest.com.

Get Your Property Checked Before the First Frost