Alert readers across Iowa and in neighboring states are asking, "Why are there are so many dragonflies this summer?" I'm not sure what explains this larger-than-normal number of dragonflies but callers are reporting anywhere from "dozens" to "hundreds" of dragonflies flying in swarms during late afternoon to early evening.
Excessive rainfall in this year does not explain the abundance. Dragonflies develop as nymphs in rivers, streams and lakes. Most take at least one year to develop from the egg to the adult stage, and some take 2 or 3 years. So the swarmers you see now are at least one year old and probably two. These are the offspring of last year’s adults (if not the adults that were flying back in 2008 or 2009). To me, that means this year’s abundance is related to what happened 1 to 3 years ago, and not what happened 1 to 3 months ago. In fact, I predict that dragonfly numbers will be down in the next 1 to 3 years as flooding of 2010 may have been detrimental to nymphs in flooded streams. More water in the stream, and especially flooding, would seem to work against the dragonflies, not for them.
What we do know is that dragonflies are more numerous in high-quality water, so abundance is an indicator of healthy aquatic ecosystems, and that's a good thing.
Dragonflies are often observed long distances from the nearest water. It appears they travel long distances and then congregate (“swarm”) in areas where there is a plentiful flying food source such as emerging winged ants, mosquitoes, etc. Yes, dragonflies eat mosquitoes, but it’s apparent they are not keeping up with this year’s bumper crop.
More information about dragonflies
- Horticulture and Home Pest News
- Iowa Insect Information Notes
- BugGuide (dragonflies in general)
- Iowa Odonata Survey (a volunteer group surveying dragonflies and damselflies in Iowa).