Insect Winter Ecology - Migration

Migration

Migration in insects is different than in birds. Bird migration is a two-way, round-trip movement of each individual, whereas this is not usually the case with insects. The short lifespan of insects compared to birds means that the adult that made one leg of the trip will be replaced by a member of the next generation on the return voyage. As a result, invertebrate biologists have redefined migration for this group of organisms as consisting of three parts:

1. A persistent, straight line movement away from the natal area
2. Distinctive pre- and post-movement behaviors
3. Re-allocation of energy within the body associated with the movement

This definition allows for mass insect movements to be considered as migration. Perhaps the best known insect migration is that of the monarch butterfly. The monarch in North America migrates from as far north as Canada southward to Mexico and Southern California annually from about August to October. The population east of the Rocky Mountains overwinters in Michoacán, Mexico, and the western population overwinters in various sites in central coastal California, notably in Pacific Grove and Santa Cruz. The round trip journey is typically around 3,600 km in length. The longest one-way flight on record for monarchs is an astonishing 3,009 km from Ontario, Canada to San Luis Potosí, Mexico. They use the direction of sunlight and magnetic cues to orient themselves during migration.

The monarch requires significant energy to make such a long flight, which is provided by fat reserves. When they reach their overwintering sites, they begin a period of lowered metabolic rate. Nectar from flowers procured at the overwintering site provides energy for the northward migration. To limit their energy use, monarchs congregate in large clusters in order to maintain a suitable temperature. This strategy, similar to huddling in small mammals, makes use of body heat from all the organisms and lowers heat loss.

Another common winter migrant insect, found in much of North America, South America, and the Caribbean, is the Green Darner. Migration patterns in this species are much less studied than those of monarchs. Green darners leave their northern ranges in September and migrate south. Studies have noted a seasonal influx of green darners to southern Florida, which indicates migratory behavior. Little has been done with tracking of the green darner, and reasons for migration are not fully understood since there are both resident and migrant populations. The common cue for migration southward in this species is the onset of winter.