Megan's Nature Nook
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This time of year, we hear about Monarch and Painted Lady butterflies during their great migration of thousands of miles. Well, butterflies aren’t the only insects that migrate! Various species of butterflies, moths, beetles, and dragonflies partake in seasonal migration including the Common Green Darner Dragonfly (Anax junius). The basics are known about their migration, but there is still more to learn. Tracking Dragonfly Migration Dragonflies can be hard to track since they’re fast fliers making them hard to catch, they don’t travel in swarms like some butterflies so they can be hard to spot, and they’re small so putting a radio transmitter or leg band on isn’t easy. Since scientists cannot use these traditional ways to track Green Darners and their migration patterns, they use something else to track and learn more about them – water! This is going to get a little science-y, so bear with me. Water is made of 2 hydrogen and 1 oxygen atoms. Most hydrogen atoms have 1 proton and no neutron. A very small fraction of hydrogen atoms have 1 proton and 1 neutron, this is called deuterium. The amount of deuterium in water varies roughly by latitude in North America. So, the water in Manitoba has a different deuterium-to-hydrogen ratio than water in Mississippi. Dragonflies start their lives in the water as eggs then nymphs (the teenage stage of dragonflies). Since they are growing and developing in the water, the local deuterium ratio is incorporated into their tissue. Scientists can test adult wing samples for three hydrogen isotopes, which each have their own chemical signature. Each isotope varies geographically, so by identifying the isotopes, they are able to narrow down where the dragonfly was born. This is how scientists are able to tell that Green Darners migrate. They were finding adults with isotopes that didn’t match the region that they were found, but instead a region farther away. Think Texas vs. Minnesota, they would have different isotopes found in their wings if they grew up in different areas with different deuterium-to-hydrogen ratios. Green Darner Generations & Migration Green Darners partake in an annual, multi-generational migration with three generations traveling more than 1,500 miles. Although Green Darners migrate, not all of them do. There are two populations, resident and migratory. The resident population doesn’t migrate. They breed in the north in the summer and the nymphs overwinter in ponds or lakes in the north, emerging in the spring and spending summer as adults where the cycle then starts over again. The migrating population is a bit more complex. First Generation In February to March, the first generation emerges from ponds and lakes in the southern United States, Mexico, and Caribbean. In May they will arrive in New England or the upper Midwest, laying eggs and the dying. Second Generation The second generation is split into two group, some will overwinter as nymphs in northern waters, emerging as adults in the spring. Others will transform into adults and fly south July through October. Once in the south, the second generation will lay eggs and die. Third Generation The third generation will live as adults on the coast for the winter, laying eggs in the spring then dying. Then the process will start over again with the first generation of the great migration heading north. This graphic shows how it takes 3 generations to complete the year long migration. The red represents many dragonflies and gray means no dragonflies. Continued Research There is still more to learn about Green Darner migration, but they have found out some information through studies and data collected through citizen science projects. Temperature plays a big role in triggering migration. Scientists have found that they don’t start migrating until 9.5°C or about 49°F. Then they progress northward as the temperatures increase in the spring. Climate change poses a big concern for Green Darners, among other species as it would greatly affect their migration and lifecycle. Dragonflies are a key part of the food web, since they eat many insects, like mosquitoes, and help to keep pests in check. They also are a food source both as nymphs and adults to waterfowl, fish, toads, birds, and more. Losing such a key component in the ecosystems could cause many problems. Although dragonflies may not be the easiest to track and research, scientists are busy at work learning as much as they can about these interesting creatures! If you’re interested in learning more about dragonfly migration or participating in dragonfly citizen science, check out the Migratory Dragonfly Partnership by clicking the button below. Keep your eye on the sky and you may be lucky enough to spot some of these dragonflies on their way south!
1 Comment
Elana
10/2/2020 03:49:40 pm
I love two (plus more) things about this! First, I learned some amazing dragonfly facts, and second, now I have a bunch of dragonfly questions that I'm excited to look into :)
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