Butterfly Diapause and Migration

The life of a butterfly is hard. Sometimes it is too cold to go out and other times it is too hot. Still other times there is not enough to eat or drink around. Luckily, butterflies have evolved some tricks to survive these periods of adverse weather and food/water shortages. Butterflies have adapted to survive adverse environmental conditions in two major ways: migrationand diapause. Butterflies that migrate will cross large physical distances to reach areas of more suitable climate. Butterflies that go through diapause enter into a hibernation-like state, where physiological processes are suspended. After adverse weather conditions clear, they will come out of this state of suspended animation and return to normal functioning. 

Migration

Some butterflies opt to travel to warmer regions during the cold seasons. Butterflies will migrate thousands of miles to find a warmer place to spend the winter. The most well known migrating butterfly in the U.S. is likely the monarch butterfly (Danaus plexippus). Monarch butterflies are known for their yearly mass migrations, in which large groups of the insect will relocate from central U.S and Canada to Mexico and Florida. During these migrations adult monarch butterflies travel thousands of miles. 

A cloud of monarch butterflies in their overwinter sites in Mexico. Credit: R. Kumra via WikiCommons CC BY 2.0

Monarchs make their migration for the simple reason that food availability becomes scarce during the winter. They cannot easily survive low temperatures, so every year starting around October, they move en massesouthwest across the continent. They reach their destination in November, and stay until the spring. Monarch migration is an inter-generational affair, as no individual butterfly completes the two way migration. Subsequent generations are laid during the ensuing northern migration beginning in March. 

Monarchs are not the only butterflies that migrate.  Red Admiral, Painted Lady, and Morning Cloak butterflies also migrate, though the trip is not as pronounced as that of monarchs. In general, the reasons for migration are the same: lack of food and an inability to survive freezing temperatures. 

Diapause

Instead of migrating in response to adverse conditions, some butterflies can enter into a state of delayed development called diapause, in which resource consumption slows drastically and physical development halts. Diapause in insects can be considered analogous to hibernation in mammals, though diapause can be a response to high temperatures too. 

The woolly bear caterpillar spends most of the calendar year in diapause. Credit: Wooley Booley via WikiCommons CC BY 2.0

During diapause the insects metabolic rate gets extremely low, which halts physical development, but lets them enter a state of suspended animation, almost like hitting the pause button on life. Insects enter into diapause in response to food shortages or unfavorable temperatures. Diapause involves many steps, beginning with the insect consuming large amounts of lipids and carbohydrates to store for energy. The metabolisms slows, and they remain in that state for as long as needed. Once environmental conditions clear up, they exit diapause and return to normal life. 

In general, the environmental stimulus that triggers the onset of diapause is a shortening of the length of the day. For example, in the pine caterpillar (Dendrolimus punctatus), diapause can be triggered when the length of night exceed approximately 10 h and 40 min at 25 °C. The colder and darker the winter months are, the longer the pine caterpillar will remain in diapause. During this time, they encase themselves in a silk cocoon and metabolism greatly slows. Physical development is essentially put on hold to conserve resources. The insect monitors for changes in temperature and light, which signals it to exit diapause and resume normal functioning.