When it comes to invertebrates, very odd ideas can circulate even in scientific circles. One of such is called 'The Pied Piper' effect, and states that many insects migrating to northern latitudes in the spring do it hopelessly, as their offspring will be incapable of making the return trip to the region of origin, where they would be able to successfully overwinter: a one way migration trip. Of course this makes no evolutionary sense. If in a population some individuals were genetically predisposed to migrate north each spring - while the rest stayed put - and the offspring of such migrants had no chance to make the return trip successfully, natural selection would select against migratory behaviour: migrants will quickly be purged out of the population, while the offspring of sedentary individuals would be more successful and propagate their sedentary tendencies resulting in a non-migratory species.
No matter how ridiculous this idea might seem to us, it has persisted for decades, in a similar way that Aristotle believed that swallows and other birds hibernated in the mud at the bottom of ponds or that Redstarts turned into Robins in the winter. An enormous body of research has confirmed that birds are capable of migrating back and forth to distant regions and have a magnetic compass that allows them to navigate even in adverse meteorological conditions. Bird migration is now widely accepted. Could it be that our prejudices with invertebrates made us find impossible to believe that some of them also could be able of regular back-and-forth seasonal migration? It appears so. But an online early paper by Jason Chapman and co-workers, from Rothamsted Research Station demonstrates that migration in an insect is an adaptive behaviour. Migrating insects use resources that are geographically and seasonally partitioned: in the summer they are available in more northern latitudes than in the winter and a two way migration is the most profitable way to use these resources. They collected data on the Silver Y, a common noctuid moth which is known to migrate from the Mediterranean to temperate latitudes in northern Europe. This species breeds continually in up to five generations a year and cannot survive the winters of northern Europe. The very active adults (above, adults often vibrate their wings when resting and when feeding) feed on nectar-rich plants and their caterpillars grow on a range of plants, including crops, where they can become a pest.
Building on a large body of previous work of the group on insect migration, some of it I already covered in BugBlog, these researchers analysed long term moth trap monitoring data at a national scale and entomological radar data. Both sets of data agree beautifully, complementing each other. The authors estimate that between 10 and 240 million Silver Y moths migrate annually to the UK, There strong inter annual variation in numbers and three years between 2000 and 2009 with exceptionally large immigration events in 2000, 2003 and 2006, likely due to benign ecological conditions in their wintering grounds. Their results provide solid evidence of the astounding population increases - and therefore potential reproductive benefits - experienced by migrating Silver Y moths in their northern breeding grounds. As the summer progresses, adult populations increase on average four-fold and for each moth that arrives in the spring, three will start the autumn return flight. There is a clear two-way movement: northern in the spring and southern in the autumn, with the autumn migration being of larger magnitude. But are they able to migrate all the way back? Simulations of expected moth return flights using flight and wind speeds and orientation of flying moths detected using the radar support the hypothesis that most of these moths could reach to their wintering grounds in the Mediterranean successfully. The Silver Y now joins the Painted Lady and the Monarch butterfly as models of insect migration, banning the 'Pied Piper effect' to the realm of fairy tales.
Chapman JW, Bell JR, Burgin LE, Reynolds DR, Pettersson LB, Hill JK, Bonsall MB, & Thomas JA (2012). Seasonal migration to high latitudes results in major reproductive benefits in an insect. Proceedings of the National Academy of Sciences of the United States of America PMID: 22927392