As already mentioned, the Monarchs make massive southward migrations from the United States and southern Canada, starting in August until the first frost, and in the spring there’s a northward migration. This is the only butterfly that migrates both north and south as birds do. One such return trip involves three to four generations of the butterfly. No individual butterfly completes the round trip. Female monarchs lay eggs for the next generation during these migrations. The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer.
The migration route was determined by Canadian entomologists* Fred and Norah Urquhart after a 38-year research, aided by naturalists Kenneth C. Brugger and Catalina Trail who identified the butterflies’ overwintering** sites in Mexico; a discovery which has been called the “entomological discovery of the 20th century”. An IMAX film’ Flight of the Butterflies, tells the story of this long search by the Urquharts, Brugger and Trail. There is also evidence that eastern North American populations of the monarch butterfly migrate to south Florida and Cuba.
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries in Mexico. The western population of the butterflies overwinters in various coastal sites in central and southern California, notably in Pacific Grove, Santa Cruz, and Grover Beach.
We know that the second, third and fourth generations return to their northern locations in the United States and Canada in the spring, but how the new generations manage to return to and from the same overwintering spots, is still a subject of research. The flight patterns appear to be inherited, based on a combination of the position of the sun in the sky and a time-compensated Sun compass that depends upon a circadian clock*** based in their antennae.
New research has shown they can use the earth’s magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the spectrum. In presence of violet or blue light, it can function as a chemical compass, which tells the animal if it is aligned with the earth’s magnetic field. However, it cannot tell the difference between magnetic north or south. The complete magnetic sense is present in a single antenna. This information is so fascinating!
Monarch butterflies are one of the few insects that can cross the Atlantic too. They are becoming more common in Bermuda, due to increased use of milkweed (the only food the larvae eat) as an ornamental plant in flower gardens. The ones born in Bermuda remain there year round due to the mild climate.
What a powerhouse of an insect! 🙂
*An ‘entomologist’ is someone who studies insects
**Overwintering means to spend the winter
***The Circadian Clock is a biochemical mechanism that oscillates with a period of 24 hours and is coordinated with the day-night cycle