Signs of Summer 6: Milkweed and Monarchs

Female Monarch (photo by K.D.Harrelson (Wikimedia Commons))

Female Monarch (photo by K.D.Harrelson (Wikimedia Commons))

Last year I wrote about the importance of milkweed to the biology of the monarch butterfly. Various species of milkweed are the only plants on which the monarch can lay its eggs.  The chemicals that the monarch caterpillar accumulates from feeding on the milkweed make it and also the adult butterfly it will turn into poisonous to, and thus protected from, most potential vertebrate predators. Human impacts on old field ecosystems where milkweeds can flourish are greatly reducing the distribution of this important plant and are at least one of the factors that are causing the observed population declines of this beautiful butterfly.

Over the past twenty-five years both out on the campus nature trail and also around my house I have tried to encourage the growth of milkweed. I have opened ripe milkweed seed pods and let the fluffy, floating seeds drop into the butterfly garden and succession plots that are located on the entrance edge of the nature trail. I have not subsequently, however, observed any thriving milkweed plants in these plots! The USDA describes milkweed propagation as an easy task. I have not experienced the ease of it!

My best ‘gardening” of milkweed has come around my house. Whenever I see a milkweed plant I let it grow. This has led to some discussion about the aesthetics of several of our flower beds, but playing the “monarch card” usually results in allowing the milkweed to remain. One incredibly persistent set of milkweed plants comes up through a crack in my driveway. They have been growing, flowering and thriving for at least fifteen years!
Just outside my back room where my writing desk is located there is a stand of nine very hearty milkweed plants. They started flowering a week and a half ago and have been absolutely covered with honey bees and bumble bees. I can hear their buzzing as I sit at my desk!

I mentioned last year that in spite of the presence of milkweed around my house, I have yet to see significant numbers of monarchs and have yet to observe any monarch caterpillars munching away on the milkweed leaves. I did see a monarch fluttering around the milkweed last week, though, and I have been watching the plants closely for any signs of eggs or caterpillars.

The life cycle of the monarch can be examined from two different perspectives: the local cycle of an individual and the year-long cycle of the migrating population. The local cycle typically takes six to eight weeks from egg to senescing adult, while the migrating cycle may extend the life span of an individual to up to nine months.

The local cycle begins with the adult butterflies emerging from their cocoons (their “chrysalises”). These adult may live for two to five weeks depending on temperature and other weather conditions and also on the availability of their food supply (flower nectar). The emerged females release pheromones which attract males. Females that have not mated release more pheromones than previously mated females and, thus, attract more males. Males fly after the females and force them to the ground to mate. Only about one third of these mating attempts, though, actually result in the transfer of the male’s packet of sperm (the “spermatophore”).

Monarch caterpillar (photo by D. Ramsey (Wikimedia Commons))

Monarch caterpillar (photo by D. Ramsey (Wikimedia Commons))

Females lay their eggs on the leaves of milkweed plants. A female can lay a total of three hundred to four hundred eggs and will spread these eggs over many milkweed plants. The eggs hatch in three to five days depending on the temperature. The emerging larvae feed first on the egg capsule and then begin to eat the milkweed leaves. They molt five times during this larval life stage and increase their body mass more than two thousand times. The eggs and the larvae (the “caterpillars”) are under intense predation pressure. More than ninety percent of the eggs and caterpillars will fail to survive. Eggs are eaten by ants, earwigs and snails, and larvae are eaten by beetles and other insects (like paper wasps) or killed by parasitoid wasps, bacteria, or fungi. Since the larvae are feeding exclusively on milkweed leaves they are accumulating the milkweed’s cardenolides (a cardiac glycoside that can cause the heart of a vertebrate to stop its contractions) in their body tissues. These cardenolides make the larvae (and, eventually, the adults, too!) poisonous to most vertebrates. Relatively few monarch caterpillars or adult butterflies, then, are consumed by vertebrate predators.

The end stage caterpillar then forms a cocoon (“chrysalis”) within which the tissues and organs of the larvae dissolve and are reformed into the structures of the butterfly. This metamorphosis takes between nine and fifteen days. The emergence of the butterfly from the chrysalis then starts the cycle all over again.

In the migrating life cycle there are great differences in life span and timing of reproduction especially in the Fall migrating forms. This migrating life form does not mate when it emerges from its chrysalis, instead it begins its long flight toward its frequently far distant over-wintering habitats. In these particular habitats (described below) the migrating life form enters a hibernating condition called “diapause” which can last many weeks or even months. Emergence from this diapause state then triggers mating and the beginning of the return migration back to the Spring and Summer ranges. These migrating monarchs may live up to nine months but spend much of this time period in its inactive, diapause state.

Monarchs overwintering in Mexico (Bfpage (En. Wikipedia))

Monarchs overwintering in Mexico (Bfpage (En. Wikipedia))

The two migrating populations of monarchs in North America are separated by the Rocky Mountains. The larger area east of the Rockies supports a much larger population of monarchs. All of these butterflies overwinter in the coniferous forests in the mountains of the Mexican states of Michoacán and Mexico. For the monarchs that reach the northeast states of the United States and the southeast provinces of Canada, this migration to and from this very specific overwintering site in Mexico covers several thousand miles. The monarchs that live in the smaller area west of the Rockies, on the other hand, overwinter in in coastal sites in Southern and Central California. Their migratory route only measures hundreds of miles at the most. In both overwintering sites, however, the numbers of monarchs covering the trees and shrubs while waiting out the winter months in their diapause states can be truly staggering!

Following the eastern population through their cycle blends together the local and the migratory aspects of the monarch’s life cycle. Between February and March the monarchs who have spent possibly four or five months in their diapause state, re-awaken, mate, and then begin their flight north. They fly as far north as Texas and Oklahoma and out across the southern states. With luck, they have timed their arrival in these areas with the emergence of the new, Spring crop of milkweed. The overwintering migrants then lay their eggs on the milkweed and die. The next generation then undergoes a local life cycle and the adult butterflies mate on emergence and then continue their fight northward in late March and early April. This cohort of adults then gets further north into the Midwest and mid-Atlantic states. This cohort again has ideally timed their northernmost arrival to the emergence of the new crop of milkweed. This first, post-migrant generation then lays their eggs on the milkweed and dies. The second post-migrant generation then undergoes a local life cycle sequence and the emerging adults in June or July head into the northern most states and southern Canada. Again, they lay their eggs near the end of their brief lives and die. The next generation (the third, post-migrant generation) can have two different types of individuals. One type continues on its collective northern flight while the other type turns to the south and gets a head start on the Fall return migration. The northward flying cohort lays its eggs on the northern edge of the milkweed plants while the southward flying cohort lays its eggs on the southern mass of milkweed. Out of these eggs are hatched the larvae that metamorphose into the adults that will be the long-lived migratory life forms that will then attempt to fly all the way back to the coniferous forests in the mountains of Mexico.

The migrating monarchs stop at nectar sites to drink and re-fuel. They follow a variety of cues to stay on their course including polarized light patterns, UV light patterns and the Earth’s geomagnetic fields. They also utilize weather fronts and prevailing winds to give them a flight boost and save a great deal of wear and tear on their delicate wings.
The monarch I saw on my milkweed was probably part of the third, post-migrant generation cohort. He (and he did look like a male) was probably looking for a female with which to mate and then would either fly off to the north or turn back to the south. His offspring will be the long distance migrators that will make the long trek back to Mexico.

We will see more monarchs over the next two months. Let’s keep planting and conserving milkweed (and as many natural nectar sources as possible!) so that they can sustain themselves on their long extended cycles!

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