Insect migration is amazing
Migration is fascinating, but few people realize that the vast majority of migratory animals are insects. Four to six billion dragonflies were seen migrating in Argentina. 100-200 million monarchs move across eastern North America. Contrast that with “only” 1.5 million wildebeest. Migration happens in all the major kinds of flying insects, especially grasshoppers, aphids and planthoppers, and butterflies and moths. Some make amazingly long voyages – scaled to body size, the longest are 25 times longer than that of large birds.
Insects migrate wherever temperatures are warm enough. Most of that migration is dependent on wind. Powerful fliers can move closer to the ground, but most fly hundreds of feet high where they can take advantage of very fast winds, and reach migration speeds similar to that of birds. But aside from the monarch, we actually know almost nothing about where these insects go, when and how they get there, or even why they do it.
Insect migration is unexplored
Our lack of knowledge here should alarm you. Many migrants are serious agricultural pests responsible for considerable economic damage (although some are parasites of those pests). Some can spread disease to plants, people and livestock. On the other hand, many migrants, including hoverflies, are pollinators and we rely on their periodic arrival for our crops. Others represent seasonally important food sources for other animals, including bears and bats.
We have no idea of the true extent of negative or positive effects of migratory insects, and we’re barely starting to even ask the research questions that will reveal the answers. And because insect migration is based on weather, it is very likely to be disrupted in unpredictable ways by climate change. Insect numbers are already dropping worldwide and we don’t know why, and so we may lose these enormous migrations before we even understand them.
Insect migration is unique
Besides the sheer volume, insect migration is also special because it differs fundamentally from familiar migrations, such as birds, in a fascinating way: each migration round-trip spans multiple generations. In monarch butterflies, for example, each annual 2,000+ mile journey involves 3-4 different generations, and no individual makes a repeat journey. This means they had to evolve innate ways of knowing when to leave, which way to travel, and when to stop. Reproduction is also tricky, and many species delay mating until after they arrive at a suitable place to lay eggs.
These aspects of insect migration mean that scientists studying them can’t rely on what they know about other kinds of migrants. It also means that insect migration is likely to have unique conservation requirements, since breeding habitats aren’t restricted to summer and wintering grounds. And for most of these insects, migration routes cross international borders, which further complicates conservation efforts. But without basic understanding of their migration biology it is very challenging to understand how best to protect them.
Origins of insect migration
Migration has evolved many times in insects, despite their ability to wait out bad conditions using diapause (a little like hibernation). You might ask why insects don’t use diapause instead of migrating, which is surely dangerous and expensive. We do have some insights on the answer to that question for one species, the Silver Y moth, which travels 375 miles between England and the Mediterranean basin in as little as 3 nights on high-altitude favorable winds.
Migration allows the Silvery Y moth to reproduce year-round, and simulations based on wind movements show that 80% of migrants would be able to find optimal habitat at their destination, a surprisingly high survival rate. Migrants have more offspring and higher population growth rates, and lower rates of parasites and disease. For the Silver Y moth, this meant the evolution of complex behavior: taking off to assess favorable winds hundreds of feet aloft and returning to the ground if the winds aren’t right, and adjusting headings to compensate for drift if the winds are slightly off. Only 4% of moths in the same family (Noctuidae) have evolved such long-distance movements, so most must rely on diapause to survive hard times, but the benefits for migration are clear in this case.
While most insect migrants are elusive, some are quite common. Here’s a great resource on common migratory butterflies and dragonflies in North America and how you can use your own garden to help them.
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