It’s time for part 3 of the year-end report for the Dragonfly Swarm Project! This week: the conclusions. I’m going to divide this post up into sections to make reading through everything a bit easier, each with its own conclusion.
Conclusion 1: In the United states, dragonfly swarms are more common in the east than the west.
I made a similar conclusion last year, but the pattern has held true a second year and I feel it bears repeating. I still think that the reason for this pattern has to do with the availability of large bodies of water. Rather than writing it all out again, I’d like to direct you to last year’s conclusion #1 for more information on the importance of water for dragonfly swarms. If I’m right about a water-swarm connection, then I should expect to see the same patterns over the next few years. I therefore make a prediction:
Prediction 1. Dragonfly swarms will continue to be much more abundant in mesic/hydric areas with large bodies of water than in arid regions. Thus, dragonfly swarming will be most commonly reported east of the Missouri River in the US.
East vs. West – clearly one is more arid than the other!
Conclusion 2: Flooding and heavy rains are strongly associated with static dragonfly swarms.
People kept talking about flooding in their weather reports this year, so I added a specific question about flooding to the report form, even though the season was half over when I did so. Based on the data collected from that point on, over 2/3 of all dragonfly swarms are associated with floods or heavy rains that occurred sometime within the month prior to the swarm.
I think there is an obvious reason for this. Mosquitoes, other aquatic insects, and various terrestrial insects (including the flies commonly lumped into the “gnat” group) may take advantage of available water to increase their population sizes. Flooding and heavy rains in an area can leave big lakes/pools of water behind where none were previously, and these are prime breeding grounds for mosquitoes and other aquatics. Sodden soils can also lead to explosions of small flies. (“Bugnadoes” were reported in many areas along the severely flooded Missouri River this summer.) If things stay wet for a while, molds can grow and allow yet other insects to experience population booms. Prolonged wet conditions create perfect conditions for dragonfly swarms: lots of little flying insects (aka, dragonfly buffet!) concentrated in specific areas. Thus, heavy rains and floods lead to prey population booms, which in turn attract dragonflies to the area, which then leads to swarming.
There’s a lot of evidence to support this conclusion. Rains mixed with heavy snowmelt in Colorado and several rivers flooded in early to mid-June. In early July, there was a small surge in reports from Colorado where there had been hardly any the year before. The Missouri River flooded from late May until Mid-June and remained flooded for many months afterwards. There were many more reports in the Missouri River area this summer than last. Also, remember that huge swarming event that happened in the northeast, the one that resulted in hundreds of reports in Ohio, Pennsylvania, Virginia, and West Virginia? Hurricane Irene hit the mid- to north Atlantic coast August 27th and 28th, causing flooding in some of those areas. There were also heavy rains in Pennsylvania that created pools of standing water and sodden soils prior to the hurricane. The surge in swarm reports began only a week later. Over 400 reports came in from the four state area over the next three weeks. Based on this evidence, I make another prediction:
Prediction 2. Most swarms reported in future seasons will follow flooding or heavy rains.
Aerial view of Missouri River flooding in early September, over 3 months after the flooding began.
Conclusion 3: There were fewer swarms reported in the Midwest this year because they experienced flooding last year.
In 2010, there were major swarming events in Minneapolis, over large parts of Iowa, and in other areas of the Midwest that coincided with flooding. In contrast, this year there were comparatively few swarms reported in that part of the country. My hypothesis: flooding kills/displaces the developing nymphs so that relatively few dragonfly adults emerge the next year.
Flooding is generally bad for insects. Though some insects have figured out ways to avoid floods (including at least one species of giant water bug), flooding is dangerous for most aquatics, including dragonflies. A flood can decimate the nymphal dragonfly population by grinding them up in sediment loosened by the flood, washing them downstream, or leaving them stranded on land as the waters recede. The next generation of dragonflies can be effectively eliminated by floods, resulting in far fewer dragonfly adults emerging the following year. However, dragonflies are mobile creatures and some will find their way into areas where flooding has occurred in the past. They’ll go about their regular business of setting up territories and mating, generating a new population of nymphs that will emerge as adults sometime within the following few years.
I think this is what happened in the Midwest. Last year there was a lot of flooding, so this year there weren’t enough dragonflies in the area to form swarms. Thus, I observed a sharp decrease in the number of swarms reported along Lake Michigan, the Wabash River, and the Mississippi compared to 2010. I mentioned last week that I didn’t see the same migratory paths down the Wabash and Mississippi Rivers this year as I did in 2010. I think it’s because there was so little swarming in the northern Midwest this year that there simply weren’t enough dragonflies to form the big migratory swarms that people actually notice.
Of course, I could be totally wrong! It could be that the lack of flooding itself in the northern Midwest this year compared to last means that fewer swarms were forming and reported there. It might not have anything to do with nymphal dragonfly populations at all and everything to do with the amount of water itself. To test my hypothesis that flooding the year (or two) prior leads to a decrease in swarming in an area, I predict:
Prediction 3. There will be more dragonfly swarms reported from the northern Midwest (Minnesota, Michigan, Wisconsin, northern Illinois, Iowa, etc) in 2012 than in 2011. Similarly, there will be very few reports of swarms from eastern Ohio, Pennsylvania, Virginia, and West Virginia in 2012.
Midwestern field. This is the kind of place I would expect dragonfly swarms to form, but will there be swarms here next year? It’s close to the Mississippi River, which flooded horribly in spring 2011.
Those are my three main conclusions for the year! Ultimately, I think dragonfly swarming all comes down to weather. Migrations are certainly weather driven, but the static swarms are weather related too. Perhaps not directly in many cases, but weather patterns can lead to localized increases of prey in an area, which in turn leads to static swarm formation. It will be very interesting to see how weather patterns contribute to swarm formation over the next 3+ years!
Next week, I’m going to do one final post about the Dragonfly Swarm Project for 2011. I want to share some more of those swarm stories I get and I’ve got a few last thoughts to wrap up the season. Then, I’m going to start doing something else on Sundays, at least until the 2012 swarming season begins. Look for big changes in a few weeks!
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