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Keeping Insects Out of Swimming Pools

By Chris Williams on December 30, 2011.

Q. Hello. I’ve been meaning to write to you about a problem we had this past summer with bugs in our swimming pool. I think they’re what you call midges, little flies that look kind of like mosquitoes. We get big swarms of them that drown in the pool and make a real mess. Is there anything we can do? We can’t cover the pool every day to keep them out.

A. Midges are small flies that have seasonal “hatches” or swarms where huge numbers of flies will emerge from their former aquatic habitat and take flight all at the same time (usually at dusk). Adult midges live for only hours or days and dead insects can pile up under lights or along roadsides during big hatches. You probably live in a water-side community or have a lake or pond nearby?

swimming-poolThe wormlike larvae of midges live in lakes, ponds, and streams and are important food for fish and other animals. The adult midge does not live in water but is found near water. Midges have wings and will fly to lights at night, including lights around a swimming pool or deck. They will also rest on light-colored walls and surfaces during the day.

It’s not advisable to use insecticides to kill midges in most cases. Killing the flying adults does little good since new midges emerge every day during a hatch. You can’t use standard insecticides in water bodies to kill the larvae because you will also kill fish and other aquatic organisms. In some cases, biocontrol agents can be used in small water bodies to control the larvae. Never use insecticides in your pool.

Besides midges, there are several other aquatic insects that will accidentally invade swimming pools. Some of these are insects, like midges, whose immature form lives in water. The adult does not live in water but is attracted to lights near the water where it emerges. This group includes caddisflies, dobsonflies, mayflies, midges, and stoneflies. Another group is adult insects that do live in water but will leave the water and fly to lights. These insects include water boatmen, backswimmers, giant water bugs, predaceous diving beetles, and water striders. Water boatmen sometimes migrate looking for a new body of water to inhabit and end up in pools. Aquatic insects are a nuisance and a mess in large numbers, but do no actual harm (except some will bite if handled).

For midges and other nuisance aquatic insects, outdoor light management and pool sanitation are the best controls.

·        Place bright lights 20 to 30 feet away from a swimming pool. Turn off underwater pool lights when not in use, or set pool and yard lights to turn on one hour after sunset. This will avoid many insects like caddisflies that fly mainly at dusk.

·        Direct or shield lights so that they shine only where needed. Especially avoid bright lights shining on light-colored exterior walls.

·        Use lower wattage bulbs in yard, pool, and deck lights or substitute sodium vapor lamps or others with a lower UV output. Place a brighter decoy light like a mercury vapor light 50 feet away from the pool to draw insects.

·        Make sure the pool is properly filtered, cleaned and treated with chlorine. Use a dip net to skim insects and debris from the swimming pool. Clean skimmer baskets regularly. [In New Hampshire and some other states, the Extension Service actually asks citizens to check their swimming pool skimmers for certain invasive insects like the Asian longhorned beetle. Swimming pool detection provides an early alert when a serious new pest enters the state.]

·        Use a pool cover when the pool is not used for extended periods. When leaving on vacation for more than two weeks, ask a friend or neighbor to take over pool maintenance.

·        Keep shrubs and grass next to a swimming pool trimmed closely.

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