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Monarch Butterfly

Milkweed Leaf Beetle Survival Mechanism

Many insects use splashy colors and color patterns to defend against being eaten.  (This practice is called “aposematism” from the Greek for “away” and “sign.”) If you spend time in a milkweed patch, you’ll notice that several of the insects you see have bright orange and black coloration.  Milkweed contains defensive chemicals known as cardiac glycosides and Monarchs as well as several other insects (many of which are black and orange) that feed on milkweed can tolerate them and store these chemicals as a defense. When avian predators consume a Monarch butterfly containing these chemicals, a bird suffers digestive upset.

Once a bird has gotten sick after eating a poisonous black and orange insect such as a Monarch, it tends to avoid any and all insects with similar coloration, regardless of their toxicity or lack of it.  Milkweed Leaf Beetle larvae and adults do not absorb the cardiac glycosides in milkweed like a Monarch, so they have no toxic compounds in them and will not poison a predator.  Insect-eating birds don’t know this, however, and the beetles successfully deter predation through their coloration.

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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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Survival Through Mimicry: The Viceroy Butterfly

The survival of Viceroy butterflies in all of their life stages is significantly enhanced by mimicry.  A Viceroy egg resembles a tiny plant gall.  Both larva and pupa bear a striking resemblance to bird droppings.  And the similarity of a Viceroy to a Monarch is well known. For years it was thought that this mimicry was Batesian in nature – a harmless organism (Viceroy) mimicking a poisonous (Monarch) or harmful one in order to avoid a mutual predator.  However, recently it’s been discovered that the Viceroy butterfly is as unpalatable as the Monarch, which means that  mimicry in its adult stage is technically Mullerian – both organisms are unpalatable/noxious and have similar warning mechanisms, such as the adult butterfly’s coloring.

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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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Viceroy vs. Monarch

8-18-18 MONARCH vs. viceroy1To clarify yesterday’s post on mimicry, here are the Viceroy and Monarch, side by side. Note the horizontal black line across the hindwings of the Viceroy.  The (larger) Monarch lacks this line.

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Maybe A Great Year For Monarchs?

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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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Stink Bugs Preying On Insects

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Shield-shaped stink bugs (Pentatomidae) are fairly distinctive looking and smelling. Their common name comes from the presence of scent glands that open on their thorax, discharging pungent compounds over a wick-like surface near the bug’s hind legs. These compounds serve as a chemical deterrent for would-be predators, and also as an alarm for other stink bugs.

About a third of the 200 North American species of stink bugs are predaceous – they actively hunt and kill their prey. (The other two-thirds are plant feeders.) After spearing the prey with its piercing mouthpart, or beak, the stink bug injects digestive enzymes into its victim. These enzymes help liquefy tissues in the body of the prey and a muscular pump in the head of the stink bug enables the bug to suck nutrient rich liquid from its prey. Many predaceous stink bugs are major predators of webworms, tent caterpillars, Colorado Potato Beetles and a number of other insects that are considered garden pests. Unfortunately, non-pest species, such as the monarch caterpillar (see photo) are also subject to stink bug predation.

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First Monarchs Have Arrived At Wintering Sanctuaries

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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

7-19-12 monarch eating eggshell IMG_4494

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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Milkweed Tussock Moth Caterpillars Feeding

milkweed tussock moth2 038Female milkweed tussock moths lay their eggs in masses on the underside of milkweed and dogbane leaves, which their larvae will eat. The hatching caterpillars are gray and hairy, but in no time they have developed the tufts of hairs that give them their name and make them resemble little mops. When still fairly young, the siblings stay together, skeletonizing the leaves they consume, leaving only the strongest veins that contain sticky latex. As they mature, the caterpillars tend to wander, and it’s unusual to find large groups of them on a single leaf. At this point they often cut through a vein in order to prevent the latex from reaching the area of the leaf where they are feeding. (Older monarch caterpillars use this same tactic.) Like monarchs, milkweed tussock moths, because they’ve consumed the cardiac glycosides contained in milkweed and dogbane leaves, are toxic to predators.

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