
We have a potted lemon tree on our deck, which spends its warmer months outside and the frosty months indoors. It is actually quite productive with very good fruit. Its blossoms are exquisitely fragrant. During the first week of October we noticed monarch butterflies on the flowers, extracting nectar. I noticed that the monarchs were a little larger and brighter than the ones I had seen earlier in the summer. It dawned on me, these are the migratory generation! They are probably headed to Mexico!
Monarch butterflies are common in many parts of the world, but only a few groups are migratory. There are three groups in the US, and all three are migratory. On the extreme east coast they have a range from Florida to eastern Virginia. West of the Rocky Mountains, they overwinter in southern California and migrate all the way north into southern Canada and back. The population east of the Rocky Mountains is the largest and most spectacular. The most extreme distance is from the wintering grounds in Mexico all the way to Canada north of the border of Maine. That is over 3000 miles (5000 k.). If that weren’t mind boggling enough, it is astounding to learn that the monarchs that return to Mexico are the great-grandchildren of the ones that begin the migration in the spring! It takes four generations to complete this cycle, and the overwintering generation finds the same few acres occupied by their great grandparents the year before. A monarch may not have contact with either its parents or its offspring—no learned behavior passed on. This is truly one of God’s creations meant to “confound the wise,” much too intricate to have happened randomly.
Before 1975, scientists were unaware of this awesome story. It was assumed that some eggs survived the winter and hatched out like other insects. The tribal Mexicans thought that the monarchs were spirits of the dead that came every fall and left every spring. The migration was discovered when scientists began tagging monarchs to track them. This led to the link between the overwintering Mexican population and the summer population in the US. Look up monarch migration on YouTube. There a number of videos with good pictures of monarchs huddled tightly on oyamel fir trees in the mountains of central Mexico.
The monarchs arrive in their wintering habitat early in November. They are in an attenuated state for the winter, leaving their huddled masses only occasionally for a drink of water on a warm day. They are “ coldblooded,” expending almost no energy for body functions. They don’t eat all winter, relying on lipids and sugar stored on the southern migration. This stored food also has to last for the first leg of the journey north around the beginning of March, when nectar producing plants begin to be available. The overwintering generation moves north as far as Texas and the southernmost tier of states until it finds milkweed in bloom. Monarchs lay their eggs on milkweed plants only! Then the first generation dies. The eggs hatch into the larva or caterpillar stage. At maturity, the caterpillar spins a beautiful chrysalis. The chrysalis opens a little later, transforming into an adult butterfly, which promptly heads further north looking for fresh blooming milkweed. The subsequent three summer generations of monarchs each have a complete life cycle of five to six weeks. The fall generation is a whole new ball game! In the northernmost range, monarchs “born” after mid-August become the long migrating generation. There are no more fresh milkweed. They enter diapause; they will not lay eggs until they complete the first leg of the spring migration. They will live for eight to nine months.
The long migration to Mexico is obviously fraught with many challenges. The monarchs “know” to head south. They “know” how to rely on wind currents to travel in the right direction, since they are not able to travel such a great distance by their own strength. It is interesting that the prevailing winds mostly don’t blow toward Mexico! As they travel, they feed on nectar from flowers, both to give them energy for the journey and to build body reserves to last through the winter and early spring. A severe drought over a wide area directly in the path of the migration could prevent them from building adequate energy stores for the winter. Interstate highways kill monarchs, since they are flying at low altitude in search of nectar sources. There are also parasites and predators that could increase mortality. In the wintering grounds, a rare freeze can lower the rate of survival. In 2016 a freak snowstorm killed many of the overwintering butterflies.
Wintering populations are estimated annually based on the land area occupied by the massed butterflies. There has been a long decline in the overwintering population from its peak in 1996 ( 380 million on 45 acres) until its low point in 2013 (14 million on about 2 acres ). They have rebounded somewhat since. There is concern among some scientists that something must be done to save the monarchs from extinction. Some theorize that herbicide use has led to a drastic decline in available milkweed. The summer populations appear to be resilient in rebuilding to normal summer population counts in the US despite the winter numbers. The fall nectar supply for the migration is being studied as is mortality from vehicle traffic. Some are advocating a large effort to plant more milkweed. There is one proposal to plant nectar producing flowers and milkweed along interstate highway corridors that match migration routes. Is that a good idea? One project involved planting a milkweed species in Texas that bloomed all summer. Returning monarchs were confused by these plants and some tried to stay instead of continuing the migration. They were also infested with a parasite hosted all summer by those milkweeds.
The milkweed is an interesting study in itself. By the way, I had never smelled a milkweed flower until about three years ago. I broke one off to show my granddaughter the milky latex from which comes its name, and we accidentally smelled the blossom. It was like a fine perfume! This obviously helps to attract pollinators including monarch butterflies. One would assume that the monarch butterfly and the milkweed have wonderful symbiotic relationship. The milkweed is actually not a very friendly host. It is toxic to most insects and even vertebrates. It is also somewhat toxic to monarchs, but there are some peculiar benefits as well. It has a bitter taste, therefore it is not disturbed by foraging animals. The milky latex can entrap insects that dare to chew its leaves, but the monarch caterpillar “knows” how to consume the leaves while minimizing exposure to the latex. The toxin level does, however, build up in the body of the caterpillar and carries over even to the adult butterfly. This makes the monarch toxic to predators in all three stages of its development although it does lose toxicity over time. Birds learn to associate the distinctive colors of each stage to a belly ache and leave them alone. Some birds do successfully feed on monarchs.
Many mysteries remain in the monarch story. When did they have their largest population? There is no mention of monarch butterflies in colonial writing. The northeastern US may have been too forested to have enough milkweed habitat to support migration into that region. It would seem that the maximum milkweed habitat in America could have been around 1900. At that time the amount of actively farmed land, abandoned farmland, and cut-over timber land could have been at a peak level. We only have monarch population estimates since 1975. How do the monarchs “know” what they are supposed do? How do they “know” which generation they are? What are the signals for migration? How do they navigate? Obviously that information is all a part of their DNA. It is an amazing story. If God cares for the sparrows, surely He counts the monarchs. They are an inspiration of faithfulness in glorifying their Creator. They give us pause to reflect on God’s infinite wisdom and artistry in creation.
Bon voyage, monarchs! Safe travels! May you have a restful winter.







