Monarchs Starting To Leave Mexico
The Monarchs’ migration north has begun! We are not the only part of the world that is experiencing unusually high temperatures — there has been a heat wave in Mexico this spring where the Monarchs overwinter, and it has them on the move, leaving their sanctuaries and beginning the more than 2,000 mile journey to New England.
This overwintering generation of Monarchs lays eggs in northern Mexico and southern U.S. and then dies. When their eggs hatch and develop into adults, usually by late April to early June, they continue the journey north that their parents began, laying eggs along the way. They begin to arrive in northern U. S. and southern Canada in late May.
To follow their progress northward, go to Journey North’s site, https://maps.journeynorth.org/map/?year=2020&map=monarch-adult-first. Although we probably won’t see any Monarchs in New England until the end of May at the earliest, it’s fun to be able to see exactly how far they have gotten as spring progresses. Journey North citizen scientists also monitor mammals, amphibians and birds. To participate in their research or to see their observations go to https://journeynorth.org/.
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Monarchs Splitting Exoskeleton For The Fifth And Final Time
In the Northeast there appears to be an amazingly large number of Monarch larvae this year, and most of these larvae will complete their metamorphosis by transforming into a beautiful green chrysalis. Once mature, the larva, or caterpillar, wanders about and finds a suitable spot (usually protected and stable) to spend the next two weeks hanging precariously in the wind. It then spins a silk mat in this location, and puts a silk “button” in the middle of the mat. It clasps the button with its last set of prolegs (it has three pairs of true legs, and five pairs of so-called prolegs) and spends about 18 hours hanging in a “J,” with its head down, preparing to split its exoskeleton for the last time and reveal the chrysalis within it.
Ba Rea, a Monarch specialist (and publisher of my children’s book, Milkweed Visitors), informs her “Monarchchaser’s Blog” (https://monarchchaser.wordpress.com/about-monarchs/) readers that even though the visible changes between the larval and pupal (chrysalis) stages of a Monarch are sudden, inside the caterpillar these changes are taking place gradually and long before we can see them. “The parts that will transform the caterpillar into a butterfly are present from the time that the egg hatches. Inside the caterpillar are “imaginal disks.” As wonderfully fanciful as the word imaginal sounds, it is actually referring to the adult stage of the monarch which is called the imago. These disks are the cells that will become the butterfly’s wings, legs, proboscis and antennae, among other things. By the time the caterpillar is half an inch long its butterfly wings are already developing inside it. “
After eight to fifteen days, the adult Monarch emerges from its chrysalis and heads towards Mexico (butterflies that emerge after the middle of August migrate). It is the great grandchildren and great great grandchildren of these migrating monarchs that will return next summer. (Photo: Monarch hanging in a “J” from Jewelweed, also known as Touch-Me-Not — not the sturdiest of plants to hang from!)
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Caterpillars Eating & Molting
The larval stage of a butterfly or moth is spent doing little but eating. Only as a caterpillar will these insects have chewing mouthparts, and they waste no time in using them. As they eat, caterpillars increase in size and their skin (exoskeleton) becomes tighter and tighter, as it doesn’t grow larger. The caterpillar grows a new, larger exoskeleton underneath the outer skin and then sheds, or molts, the old one. Most caterpillars molt five times. At first, the new exoskeleton is very soft and not very protective, but it soon hardens. The shed exoskeleton is often eaten before the caterpillar ingests more plant food.
There are names for the caterpillar’s stage of development in between each molt, called “instars.” When the caterpillar hatches from its egg, it is referred to as a “first instar” caterpillar. After its first molt, the caterpillar is referred to as a “second instar,” and so on up until the exoskeleton is shed for the final time, revealing the chrysalis (if it’s a butterfly).
The Monarch in the photograph is a very new 4th instar instar caterpillar (see antennae which haven’t hardened). It has shed three times. Its third exoskeleton (which it has just shed) is on the milkweed leaf behind the caterpillar. To see a real-time video of a Monarch molting go to https://www.youtube.com/watch?v=EbHyq3RwtxI.
(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin. Also to Ba Rea ((www.basrelief.org) for her instar confirmation.)
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Monarch Butterfly Larvae Are Cannabalistic
The very first meal that a Monarch Butterfly caterpillar eats is its own eggshell. In order to hatch, it eats its way out of the egg, and then polishes off the remainder of the eggshell. It then starts to wander around the leaf and if it finds another Monarch egg, it will start to eat it.
Female Monarch Butterflies lay 300-500 eggs over two to five weeks of egg-laying. Normally, a Monarch only lays one egg at a time (on the underside of a tender, young milkweed leaf). It is fairly rare to find more than one egg on a leaf, or even on the same plant. After a female lays an egg, several seconds up to a minute goes by before she lays another egg (referred to as a refractory period). During this time she usually moves on and finds another milkweed plant on which to lay the next egg. This lapse of time between the laying of each egg probably evolved to discourage the laying of multiple eggs on one leaf and to encourage the dispersal of a female’s eggs on different milkweed plants so as to decrease the chances of cannibalism occurring.
According to Dr. Lincoln Brower, renowned Monarch entomologist, a cluster of Monarch eggs on any given milkweed leaf indicates that either milkweed is in short supply, or the female that laid the eggs is either sick, very old or she has been flying for a very long time and several eggs have matured.
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Maybe A Great Year For Monarchs?
I have no idea what the status of monarch caterpillars is in other parts of the country this year, but at least in parts of Vermont and New Hampshire, they are plentiful! Two on one plant — just like the old days! (Thanks to Sadie Brown for NH input.)
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First Monarchs Have Arrived At Wintering Sanctuaries
During the first two weeks of October, south winds prevented migrating Monarchs from making a lot of progress on their flight southward. Cold fronts were weak during this time, and wind blew from the north infrequently. On Oct.12 this persistent weather pattern broke, headwinds subsided and thousands of Monarchs were seen migrating through Texas. By Oct. 20 the first Monarchs entered Mexico and by the 23rd the first butterflies had reached their wintering grounds. Follow their progress as they continue to stream across northern Mexico, headed for their sanctuaries, at http://www.learner.org/jnorth/maps/monarch.html.
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Eggs Of Migrating Generation Of Monarchs Hatching
The Monarch eggs that are hatching now contain the larvae that will metamorphose into the butterflies that will migrate this fall to central Mexico. Unlike earlier-hatching generations that only live six to eight weeks, the Monarchs that result from late summer and early fall hatchings live six to nine months. Part of the reason for this difference in life span is that, unlike the earlier generations that mate soon after emerging from their chrysalides, late-hatching Monarchs postpone mating (reproductive diapause) until the end of winter, thereby conserving energy for their two to three thousand-mile, two-month migration. (Photo: monarch larva’s first meal – its eggshell.)
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