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.
Naturally Curious is supported by donations. If you choose to contribute, you may go to http://www.naturallycuriouswithmaryholland.wordpress.com and click on the yellow “donate” button.
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.
We think of monarch larvae as being impervious to the ills of milkweed, but they are very vulnerable when it comes to the sticky latex in the sap of their host plant. The mandibles of young monarch caterpillars are often glued together by this latex, preventing them from eating. Research shows that about 30 percent of monarch larval loss results from miring in this glue-like substance. One strategy young larvae use is to chew a near circle in a milkweed leaf, blocking the flow of latex to the enclosed surface area, which they then eat. If a monarch survives the first few stages, or instars, of its larval life, it uses yet another strategy to circumvent the latex. Older, larger larvae often cut through the midvein of a leaf they wish to consume, which dams the latex flow to the entire leaf beyond the cut. Look for limp leaves as you peruse a milkweed patch. If you find one, you may be rewarded with the nearby presence of a monarch caterpillar.
Unlike spiders, whose spinnerets, or silk-spinning spigots, are located at the tips of their abdomens, caterpillars’ spinnerets are located underneath their heads. The most prominent white structure with a black band around it is the monarch caterpillar’s spinneret, in which its silk glands are located. The smaller structures are called maxillary palps and are antennae-like sensory devices. Prior to metamorphosing into a chrysalis/pupa, the monarch caterpillar draws silk through its spinneret, and forms a small, well-anchored button of silk. The caterpillar clasps this button with a structure called a cremaster, located at the tip of its abdomen, from which it suspends itself upside down. Soon thereafter its skin splits, revealing a gold-dotted, green chrysalis from which an adult monarch butterfly will emerge in two weeks.
This 15-minute-old monarch butterfly that emerged yesterday will live for 2 to 5 weeks, long enough to mate and produce the next generation of monarchs. The generation of monarchs that emerges a month or more from now will live six to nine months, and not mate until next March or so – after flying to one of about a dozen locations in the Transvolcanic Mountains of central Mexico (a flight of up to 3,000 miles) and spending the winter. Late summer-emerging monarchs live longer than monarchs that emerge earlier in the summer because they do not immediately expend energy on breeding and the cool winter temperature in Mexico slows their metabolism, allowing them a longer life.