While birds are the most visible migrants, many insect species also travel hundreds of kilometers or more between breeding and non-breeding grounds. Credit: USFWS.
While most discussions about migration and migratory routes focus on vertebrates (particularly birds), many kinds of invertebrates are known to migrate as well. Besides the iconic monarch butterfly, species of both moths and dragonflies can be considered migratory over a range of distances. But these species are seldom, if ever, examined under the same lens as avian migrations to examine how connected populations really are.
A new review applies the concepts of migratory connectivity commonly found in bird studies to invertebrates. Migratory connectivity can be thought of as how well populations “stick together” as they move between breeding and non-breeding territories. High migratory connectivity means that populations are insular and don’t mix much, whereas low migratory connectivity means that populations tend to spread out and mix with others as they move back and forth.
How well do these concepts work for invertebrates as compared to birds? Although there are some caveats for insects, such as shorter generation times, less discrete populations, and differences in what “high” and “low” really mean, the migratory connectivity framework has potential to apply.
Monarch butterfly (Danaus plexippus). Credit: USFWS
The most well-known example may be monarchs in North America, whose western and eastern populations migrate long-distances between breeding and non-breeding grounds. It’s likely, though, that these populations are not as discrete as previously suspected, and migration routes are becoming more complex as individuals move towards new regions and mix more. There is still a lot to learn about how connected these populations are, not only for monarchs but for hundreds of other migratory butterfly species.
Other examples of migratory insects include noctuid moths such as the Bogong moth of eastern Austealia. Bogong moths migrate up to 1000 km and have the potential for high migratory connectivity, but little is known how much mixing occurs when they move between inland breeding areas and alpine caves. In another example, fall armyworm in eastern North America migrate in a way similar to monarchs, but likely have much higher connectivity as populations are kept separated by the Appalachian Mountains. Since many noctuid moths are agricultural pests, understanding the migratory connectivity of their populations could help better manage them.
Green darner (Anax junius). Credit: Dawn Kopp
In addition to lepidopterans, dragonflies are regular long-distance migrants. However, little is known about their migratory routes and how well connected they are. The green darner in North America is a rare exception, and is known to migrate across multiple latitudes in the eastern United States. But there is no solid evidence determining how well-connected migrating populations are, and this lack of knowledge holds true across most dragonfly species.
In contrast to these examples, most of the world’s migrating insects are actually small and rely on the wind for transport. Because movement is passive and dependent on wind currents, the degree of migratory connectivity is likely to be low and generally undirected. However, so little is known about these species’ movements and migratory routes that it’s difficult to determine what level of connectivity might exist.
In general, the high degree of migratory connectivity seen in birds is unlikely to exist in migratory insects. While more research into insect migrations may not impact insect conservation, many migrants are agricultural pests and could be better managed. Understanding connectivity in insect populations could lead to unexpected findings about how populations mix and relate to each other.
