Two days ago, on November 6th, the first Monarch Butterflies arrived in their 73-mile-wide overwintering area in the Transvolcanic Mountains of Central Mexico. This miraculous flight, which takes a Monarch roughly two months, can be up to 3,000 miles long. Using the sun, and most likely the earth’s magnetic field, they head for specific stands of Oyamel Fir trees, where they will cluster and be protected, unless weather conditions are severe, from extreme temperatures, predators , rain and snow until next March, when their journey north begins. (These butterflies only get about half way back to New England, at which time they mate and lay eggs. The third or fourth generation of these monarchs will reach their eastern destination.) To track the migration of these remarkable insects, go to http://www.learner.org/jnorth/monarch/index.html.
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According to the Vermont Center for Ecostudies, this spring and summer there’s been an estimated drop of 90% in the overall monarch population in eastern Canada – the most dramatic decline ever recorded. Vermont (and most likely New England in general) is experiencing much the same situation. The low numbers of monarchs are due to several factors that they have encountered along their migratory routes the past couple of years, including extreme temperatures, record drought, low nectar production by flowering plants and a scarcity of their host plant, milkweed. The cold temperatures and record amounts of rain this spring undoubtedly added to their stress.
The Viceroy Butterfly closely resembles the Monarch Butterfly, but is smaller, and has a black line that runs across the veins of its back wings, which the Monarch lacks. While Viceroys don’t contain the poisonous cardiac glycosides that Monarchs do, they do contain salicylic acid due to fact that the larvae feed on willows. This acid not only causes the Viceroy to taste bad, but makes whatever eats it sick. So not only do these two butterflies look alike, but they discourage predators in the same way. This is not a coincidence. The fact that they are both toxic if eaten and are preyed upon by some of the same predators has led to their similar appearance. This phenomenon is referred to as Müllerian mimicry, and essentially it means if two insects resemble each other, they both benefit from each other’s defense mechanism — should a predator eat one insect with a certain coloration and find it inedible, it will learn to avoid catching any insects with similar coloration.