Thoroughly modern migrants: moths and butterflies—round-trip tickets not necessary

Science News, June 26, 2004 by Susan Milius

Hugh Dingle is gracious about geese and robins. They may be the most popular icons of migration, winging south in the fall and lifting people's spirits in spring, when they finally honk or bob-bob-bob their way back to the same territory. For decades, a bird-based idea of migration dominated popular and scientific thinking: Individual animals went somewhere each year, then came back. In the 1960s, however, entomologists began to free themselves from the traditions of bird behavior and consider migration in broader terms, says entomologist Dingle of University of Queensland in Brisbane, Australia. Today's view puts more emphasis on the behavior of the organism and less on the route.

An individual migrant, such as a monarch butterfly, doesn't have to make round-trips. It can leave the return to future generations. But the migrant does have to behave in distinct ways. It ignores a lot of interesting stimuli, even neglects some of the normal chores of living. Depending on the species, the migrant may shun sex or food as it just keeps on traveling. It can't act like a vacationer, open to new experiences on its journey.

This broader definition of migration, embracing the habits of certain moths and butterflies, has raised some old questions in new ways. Weather has always mattered in migration studies, but entomologists are now working to understand movements of air masses over continents to predict arrivals of hitchhiking populations of moths, including some of the most vexing of agricultural pests. Butterflies being taken up as model organisms for studying navigation, and that research is moving to the molecular level.

NO DISTRACTIONS Butterflies have set the standard for fluttering around, foraging with plenty of zigzags and detours. So, a butterfly that keeps a fairly consistent heading, especially when flying more than 8 feet off the ground, is a suspected migrant.

This relatively straight movement counts as one of the five typical signs of migratory behavior, says Dingle, who has relied on his own findings and the data of others to write extensively on migration. Another migratory characteristic is unusually prolonged travel.

Suppression of some normal appetites is another item on Dingle's list. The migrating butterfly, for example, might flap over patches of otherwise-enticing flowers.

Migrants also typically have distinct behaviors for starting and stopping their journeys. As the final sign of migration, they often reallocate their assets before the trip. A monarch butterfly, under good conditions, can store up to 125 percent of its lean, dry weight as fat. Also, a monarch going south in the fall stays in a stage of arrested sexual development.

Those characteristics show up not only in recognized migrators but also across a broad range of species. In Dingle's 1996 overview, Migration: The Biology of Life on the Move (Oxford University Press), he extended the term migrant to seeds hitchhiking in an animal's gut and fungal spores wafting on the breeze.

Just how many moths and butterflies fit this definition remains an active area of research. One thing entomologists will say for certain is that some of a farmer's least favorite moths migrate.

BLOWING IN THE WIND The first inkling of a windblown distribution of migrants in northern North America came from farmers who had linked certain pest outbreaks with winds from the south. Later, in the 1920s and 1930s, when planes captured high-altitude air samples, scientists routinely found insects, spiders, and mites 1.5 kilometers off the ground. As radar and wide-scale weather monitoring improved, researchers figured out that many teensy animals fly high into the atmosphere and catch the wind for a long, fast ride.

One agricultural pest, the fall armyworm moth, blows from the south to the northeastern and north-central United States and bursts out in abundant populations in the new territory, according to Dingle. Many of these moths die there, but fall weather patterns let enough offspring escape to head back to the warm south to perpetuate the species.

Scientists called radar entomologists, working with meteorologists, have developed several types of radars for detecting incoming masses of insects, such as the corn earworm moth. Similar airborne migrants bedevil farmers in Asia, Africa, and Australia. The radar entomologists are examining how the weather affects the migration patterns.

"High-altitude migration is very common, probably much more prevalent than the more-visible daytime migration of butterflies at low altitude," says Alistair Drake of the University of New South Wales in Canberra, Australia. Drake points out that in much of the world, such as temperate North America, weather patterns change seasonally, so an insect population can often catch a ride on the wind for both parts of the round-trip.

Entomologists have argued that only a round-trip journey for the population makes evolutionary sense in climates where it can't live year-round. Otherwise, the genetic drives for migratory behavior would just be taking a one-way ride out of the gene pool.