Down the Garden Path: Fall Insect Migration

Each year, many of us enjoy the fall bird migrations. It is a great time to view many interesting and rarely seen birds as they travel on their long journeys south. But there are also other hidden migrations happening all around us, they are just not as easy to see.

Many species of dragonflies are traveling across North America, flying more than 800 miles on a two-inch wingspan. Green darner dragonflies (Anax junius) migrate from Canada to the Gulf of Mexico. But unlike songbirds or monarch butterflies, dragonflies and other insects are much more discreet.

How in the world to scientists even track dragonfly migrations?! Well, in a 2018 study, scientists assessed museum collections from over the last 140 years to extract chemical samples from 800 dragonfly wings, and then tested them to link the animal to its birthplace. Knowing where the dragonfly was collected from, and then linking it back to its birthplace, scientists can see how far these tiny creatures traveled.

And much like monarchs, dragonflies have multi-generational migration. Dragonfly eggs are laid in wetlands and water, and the larvae develop underwater. The larvae are signaled to emerge by natural cues, such as temperature. In Late February and into March, the first generation of adult dragonflies emerge from lakes and ponds across the southern US, Mexico and Caribbean. Then those adults set off on a migration for hundreds of miles north, arriving in New England and the upper Midwest by May. When they arrive, they mate, lay their eggs, and die. For this second generation many will overwinter in ponds and lakes and emerge the next spring, but some species will reach maturity and head south between July and October. When those dragonflies reach the south they will mate and lay eggs that will overwinter and emerge in the spring to begin the cycle again.

Not all insects migrate together in mass, but when they do some of these migrations can be seen on radar returns. As the insects take off, if there are enough of them, the radar signal can pick them up. On June 4,2019, a cloud of lady beetles, also known as lady bugs, was seen on radar over California. The millions of lady beetles occupied an area of about 80 miles by 80 miles. This is a similar phenomenon that can be seen with bird flocks during the spring and fall peak migration times.

Some moths also migrate seasonally in multi-generational migrations. Pink-spotted hawkmoths (Agrius cingulata)are primarily centered in Central America and Mexico, but during the summer they migrate up to areas from the Rocky Mountains into the Upper Midwest and up into British Columbia.

Large milkweed beetles (Oncopeltus fasciatus) also have mass multi-generational migrations. Many of these striking orange and black beetles follow the spring emergence of milkweed plants from Mexico into the southern US. Large milkweed beetle adults live for about a month. The first generation travels as far north as Canada where they lay eggs and die. The next generation spends the summer up north. The last generation of the year then migrates back down south to continue the cycle in warmer areas during the winter.

Migration plays an important role in redistributing energy and nutrients among geographic regions. Plus, many migrating insects, such as moths and butterflies and even some beetles, are important pollinators of wildflowers and agricultural crops. It is fascinating to explore how these diminutive creatures have such daring and adventurous life cycles.

Karly Tumminello is a certified Master Naturalist volunteer with University of Illinois Extension in DuPage, Kane, and Kendall counties.