Mosquitoes Along Crum, Ridley and Darby Creeks

Why creekside yard mosquitoes are so persistent

A fast-moving creek is not the main breeding ground. Mosquito larvae need still water, and moving current flushes them out. The trouble comes from everything around the creek: side pools left after storms, low spots in the floodplain, saturated soil, and the thick shade that keeps yards cool and damp all day.

Adult mosquitoes spend daylight hours resting on the undersides of leaves in dense, shady vegetation. Creek corridors in Delaware County are lined with exactly that kind of cover, so even mosquitoes that bred in a neighbor's gutter find ideal resting spots in a wooded backyard.

Crum, Ridley and Darby creeks at a glance

Crum Creek runs south through Marple, Springfield, Swarthmore and Nether Providence on its way to the Delaware River, passing the Crum Woods and Smedley Park. Ridley Creek flows through Ridley Creek State Park and the valley just outside Media before reaching the river near Chester. Darby Creek cuts through Haverford Township and the eastern boroughs, then joins the tidal marshes at the John Heinz National Wildlife Refuge at Tinicum.

Each corridor brings wooded banks, steep shaded slopes and backyards that end at the water. Along the lower Darby Creek, low ground and marsh add even more standing water late in the season.

Does Darby Creek flooding bring more pests?

It can. Darby Creek is known for flooding after heavy rain, and floodwater left behind in yards, basements and low corners of lots can produce a new wave of mosquitoes within about a week in summer heat. Some mosquito species lay eggs on damp ground that hatch only when flooded, which is why a burst of biting often follows a big storm.

Flooding also stirs up other pests. Wet basements draw camel crickets, centipedes and springtails, and displaced rodents look for dry ground, sometimes inside garages and crawl spaces. After high water recedes, drain what you can, dry out basements quickly and check the foundation for new gaps.

Where mosquitoes breed on a typical lot

Before any treatment, walk the property after a rain and empty anything holding water. These are the spots we find most often:

• Clogged gutters and downspout extensions that never fully drain
• Corrugated drain pipes and their buried outlets
• Tarps over firewood, grills and boats
• Plant saucers, buckets, toys and wheelbarrows
• Birdbaths and ornamental pots that are not refreshed weekly
• Tree holes and hollow stumps near the creek bank
• Low spots in the lawn and window wells that pond after storms

Do rain gardens and stormwater basins breed mosquitoes?

Only when they hold water too long. A rain garden built correctly drains within a day or two after a storm, which is faster than mosquito larvae can develop. Trouble starts when the soil compacts or silt clogs the bed and water sits for most of a week. If yours still has a puddle four or five days after rain, loosen the soil or regrade the low end.

Other stormwater features near creek corridors behave the same way. Sump pump discharge lines that dump into a low corner, French drain outlets buried under leaves, and grassy swales with a dip in the middle can all hold still water through July. Detention basins on homeowners association or township land are not yours to treat; report water that never drains to the municipality, which can pass it along to Pennsylvania's West Nile virus program run through DEP and county partners.

West Nile virus in Delaware County: what to know

Pennsylvania's Department of Environmental Protection runs a statewide West Nile virus surveillance program, and positive mosquito samples turn up in southeastern Pennsylvania most summers. The mosquito most associated with West Nile in this region, the northern house mosquito, breeds in stagnant, nutrient-rich water such as clogged catch basins, neglected pools and containers of rotting leaves.

The Asian tiger mosquito, the black-and-white daytime biter that has become common in the county, breeds in small containers and rarely flies far from where it hatched. Both species mean the same thing for homeowners: the water sources on and right around your lot matter more than the creek itself.

What cuts mosquitoes back in a creekside yard?

Source reduction comes first and never stops. After that, the most effective step for most creekside properties is a mosquito yard treatment applied to the shrubs, groundcover and the shaded undersides of foliage where adults rest. Standing water that cannot be drained, such as a decorative pond or a low corner that stays wet, can be treated with a larvicide made from naturally occurring bacteria that targets mosquito larvae.

Treatments are typically repeated every few weeks through mosquito season in PA, which runs roughly from May into early fall depending on temperature. Homeowners who prefer botanical products can read about natural mosquito control, and our mosquito control overview explains how yard work and treatment fit together. Thinning dense undergrowth and opening the yard to sun and breeze also makes a measurable difference.

Opening up shade on a creek bank without breaking local rules

Less shade means fewer resting adults, but creekside slopes need care. Thinning low branches and invasive undergrowth such as multiflora rose, Japanese knotweed and porcelain berry opens the yard to sun and breeze while leaving tree roots to hold the bank. Clearing a slope to bare soil invites erosion, and the sediment ends up in the creek.

Many townships along the Crum, Ridley and Darby regulate clearing and grading within a stream buffer, so check with the township before removing vegetation near the water. Mowing a path, trimming shrubs up off the ground and keeping leaf piles away from the patio usually stay well within those rules and still cut the cool, damp cover mosquitoes depend on.