|With wings open|
|With wings closed|
This year may turn out to be one of the worst, if not THE worst, Monarch butterfly migrations in history! A 20% to 30% decline in the Monarch population is expected this year in Texas. It seems that many of the nectar sources used by the Monarchs as fuel along the migration routes did not reach their full blooming potential this summer due to the existing drought that is plaguing much of the US (beginning its third straight year in Texas). Drought and heat stress have significant influence on a plants developmental processes resulting in stunted growth and delays in or absence of flowering. High heat can even melt the butterfly larvae.
The Monarch population began its decline long before the current drought began. It has been in decline for the past 8 years according to researchers at Texas A&M University. The reason is thought to be the increasing destruction of their environment as millions of acres of land are cleared and converted to crop use each year.
I should also mention that not only the drought is killing the plants upon which the Monarchs feed. Monsanto’s weed filler, Roundup, kills many of the plants relied on by the Monarchs. The milkweed is one of these plants killed by Roundup. The larval stage in the Monarch’s life cycle requires milkweed for the larvae to survive. No milkweed, no Monarch butterflies.
Now, about the migration itself. First of all, the Monarch butterfly is the only butterfly known to make a two-way migration as birds do. So, why does a Monarch butterfly migrate up to 3,000 miles from Canada to Mexico and back again? Is it because they don’t like cold weather? That is partially true. The fact is that they are not able to survive the cold winters of most of the United States. In a previous post I mentioned that the Monarch is cold-blooded. They are unable to fly if their body temperature drops below 55 degrees. So why don’t they stay in Mexico? Because the milkweed plant that the butterfly larvae feed on does not grow in Mexico. As a result, for the species to survive, it must move back and forth between Canada and Mexico.
Let me try and explain how this migration works. Since it is a continuous cycle I will start in Canada in the spring. Female Monarchs arrive in Canada during March and April and lay their eggs on milkweed plants. It takes about a month after the eggs hatch to produce an adult butterfly. Call these new adults the “First Generation”. These new adults will live only 2 to 6 weeks so they waste no time in mating and a “Second Generation” is born in May and June and a “Third Generation” is born in July and August.
The “Fourth Generation” is born in September and October and is very, very special. These little guys don’t die in 2 to 6 weeks, as did the previous generations. Although they look the same as the other generations they are biologically and behaviorally different from those earlier generations. They are not sexually mature for one thing, so they don’t mate right away. These changes are triggered by the cooler temperatures and shorter days. They will live 6 to 9 months and fly all the way to Mexico and spend the winter in the mountain reserves of Michoacán (southbound Monarchs begin crossing the Red River into Texas by the first week in October).
The Monarchs that return to Mexico each fall are the great-great-grandchildren of the Monarchs that left Mexico the previous spring. Not one butterfly lives long enough to complete the entire 2-way migration. So how do any of these butterflies know where to go? It can’t be a learned trait. Who would teach them? Therefore, it must be some innate sense they possess. No one really knows. What makes it so weird is the fact that they even go to the same trees in Mexico every year.
When spring arrives, these special Monarchs leave Mexico and fly through Texas, mating and laying their eggs in milkweed plants along the way. These special Monarchs then die, letting their offspring continue the northward migration to the northern US and Canada.
Next, what kind of navigation system enables the Monarch to migrate to the same trees, as did their great-great-grandparents? That’s for tomorrow! Meanwhile, here are a couple of new photos of one of the guys from yesterday (Soldier or Queen?).