Why are there worms in my swimming pool?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

Often during the late spring and summer, we receive samples of worms people have collected from their swimming pools. These worms measure up to 14 inches in length and are usually tan to dark brown in color. Their occurrence obviously concerns people. However, if you experience this phenomenon don’t fret, almost always, these are horsehair or Gordian worms. They are parasites of some insects, but do not harm mammals.

Horsehair worms develop as parasites in the bodies of grasshoppers, crickets, cockroaches and some beetles. When mature, they leave their hosts to mate and lay eggs. This occurs in water, so they are often found in puddles, ponds, livestock watering troughs, swimming pools or any container with water. In water, these worms wiggle slowly, often contorting their bodies into intricate knots. They are called horsehair worms because they resemble the hairs of horses’ manes or tails and are often found in areas where horses drink. It was believed as these hairs fell into water, they came to life.

Adult worms mate in water and females lay long gelatinous strings of millions of eggs. The eggs hatch usually within a few weeks and the young nematodes crawl onto vegetation near the water’s edge to be consumed by crickets or grasshoppers. It is believed as the nematodes mature inside their hosts’ bodies and get ready to emerge, the insects become very thirsty, thus seeking out water. If they jump into your swimming pool, the result is one to several horsehair worm adults swimming around the pool.

Since horsehair worms are harmless, except to insects, no control is necessary. For identification, worms can be collected and placed in sealed containers with water or preferably alcohol and delivered or mailed to
MSU Diagnostic Services. There is a $10.00 fee for their identification.

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