Seasonal migrations are exciting times for us birders. In the spring, we anxiously await return of our breeding birds from their southern wintering grounds. In the fall, we’re sad to see them leave. While birds follow that classic concept of animal migration, insects (having shorter lifespans than a migration period) were historically excluded from consideration. Their small size and weight were also believed to restrict movement to the whim of the wind, and they weren’t believed to have specific or regular winter destinations.
So, do insects migrate? ABSOLUTELY!! Fortunately, recent studies are improving our understanding of insect migration. A key differentiator from other animals is that insect migration is multi-generational. Some species reproduce through 6 generations along their path and wintering area.
Large movements of a few species of butterflies and dragonflies have been known since the mid-1800s. But mostly, insect migrations remain mysteries. Their small size makes them hard to detect, they often move short distances, they don’t congregate to be witnessed, and there has been little research on most species (excepting agricultural pests).
The largest systematic study to quantify insect migration was by UK entomologists in 2000-2009. Medium to large insects were recorded by radar at 150 m and 1200 m heights and smaller insects estimated using balloon sampling. They calculated that 3.5 trillion insects migrate above the southern UK annually (vs 2.1 billion passerines) representing 3200 tons of biomass. That’s a lot of bugs!!
Why do some insects migrate? Like other animals, reasons include availability or quality of food, breeding or survival improvement, and seasonal adaptive response to temperatures and light since some species cannot survive freezing conditions.
How do insects migrate? New research is showing that migrating insects aren’t just dispersed by the wind. They take advantage of specific directional and speed winds with the ability to make some corrections. Insects often take advantage of the same migratory flyways as birds. The UK radar study showed some insects will wait for the right wind opportunities and fly at even greater speeds by attaining higher altitudes. Researchers suspect insects use similar mechanisms as other animals for navigation like celestial cues and magnetic fields, but don’t know how such capability passes between generations.
Who are these migratory insects? Butterflies, moths, dragonflies, bugs, aquatic beetles, aphids, flies, and more. In Pennsylvania, 22% of our 156 recorded species are migrants. They include the beloved Monarchs, Painted and American Ladies, Red Admirals, Variegated Fritillaries, and Question Marks.
Migratory insects need the same conservation support as birds. Go native in your own yards, and support open space preservation and restoration projects that embrace native vegetation. As you watch for migratory hawks this fall, look for tinier travelers in the air and you’re likely to find some Monarchs and dragonflies. If you find yourself at the shore, they tend to fly along the dunes.
Karen Campbell, a member of Lehigh Valley Audubon and FocusOnNatives.com provided the content and images for this article.