dragonfly on window screenThe Question: Someone recently told our young son that if you see a dragonfly near water (such as a pond), then it is safe for humans to drink the water. Is this true?

Submitted by: Jean, Minnesota, USA

(click on photos and graphics to expand)

dragonfly by John FlanneryThe Short Answer:  Dragonflies are indicators of water quality, but that relates to the ecological health of a stream, or pond, or lake, not to its safety for people to drink.  Public health experts will tell you that you can’t assume any lake, pond, stream or river in the United States is safe to drink.  On the other hand, some hikers claim they drink water from high mountain streams all the time with no resulting illness.  You can never be sure about that, however.  That’s why most hikers either boil, chemically treat, or filter water before drinking.

Dragonflies can be found flying over water with an active infestation of Giardia, for example. Or salmonella. And virtually all water with animals in it will have E. coli and other bacteria. And if there are people swimming in the pond, that adds another potential source of disease-causing microorganisms. For most people with a healthy immune system, the bacteria and parasites floating around in your typical lake or pond won’t cause a problem. After all, we all get water in our mouths every time we swim in a lake, pond, or stream.

Deliberately drinking pond water, however, is probably pushing your luck.

dragonfly larva - Matt Keevil

More Information: Field ecology programs teach people to use a Biotic Index Card, as a relatively accurate method of assessing the overall level of ecological health or disruption of aquatic systems that a non-scientist can use with just a little training. The biotic index method is based on the fact that aquatic insects, both those that spend their entire lives in water, and those whose larval stages take place in water, have differing levels of sensitivity to pollution and disruption. For example, mayflies (Ephemeroptera), are rarely found in polluted or disturbed water. They need fairly pristine conditions or they won’t thrive. Flies (Diptera) that have aquatic larvae, on the other hand, are generally tolerant of dirty, polluted water. So if you find mayflies, that’s a good sign that the body of water is relatively healthy. If all you find are fly larvae, that’s a bad sign.

Dragonflies and Damselflies (Odonata) fall somewhere in between. They’ll grow in super clean water, but they’ll also do okay in water that isn’t quite perfect. They generally won’t grow in badly polluted water, however. That’s why there’s some truth to the statement about dragonflies and clean water. If you have dragonfly larvae in the pond or lake, it can be taken as a rough indicator that the body of water is not badly polluted.

Note, however, that this has little to do with whether the water is safe for humans to drink. For people, in addition to worrying about chemical pollution, you also have to worry about disease-causing microorganisms.

Also, the Biotic Index Card is an overall guide. There are several hundred species of dragonflies in North America and some are more fussy about water quality than others.

A study in Rhode Island categorized hundreds of bodies of water as to the level of pollution and disruption. The researchers then matched up adult dragonflies with bodies of water and noted whether they were found in disturbed bodies of water or not. They created a score from 0-10, where 0 means the dragonfly species is only found at badly disturbed and polluted bodies of water, and 10 means the species is only found at bodies of water that are very clean and undisturbed. So 10 means very fussy about water quality, and zero means not at all fussy.

damselfly by Charles J SharpFor the 135 species of dragonflies found in Rhode Island, the average score of all species was 6.4, suggesting that adult dragonflies are indeed somewhat fussy. They are mostly associated with less disturbed bodies of water. But the scores range from 10 for half a dozen species (such as Dorocordulia libera) that were only ever found in bodies of water characterized as “least disturbed,” all the way down to a score of 0 for the damselfly Ischnura ramburii, which was found only in the “most disturbed” bodies of water.

So unless you know your dragonfly species and their exact correlation with clean water, the presence of adult dragonflies over a body of water would give only the roughest of gauges. Certainly not anything I’d recommend as a guide to the safety of drinking.

Sources: Kutcher, T E, Bried, J T., 2014, Adult Odonata conservatism as an indicator of freshwater wetland condition. Ecological Indicators. 38:31-39.

Zimmerman, M. C. 1993. The use of the biotic index as an indication of water quality. Pages 85-98, in Tested studies for laboratory teaching, Volume 5. Proceedings of the 5th Workshop/Conference of the Association for Biology Laboratory Education (ABLE), 115 pages. Viewed on 7/8/2016 at http://www.ableweb.org/volumes/vol-5/6-zimmerman.pdf.

William E. Sharpe, William G. Kimmel, and Anthony R. Buda. Biotic Index Card. Viewed on 7/8/2016 at http://www.pspb.org/water/media/BioticIndexCard.pdf.

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