The Monarch Butterfly in Fall: Tennessee’s Front-Row Seat to a Continental Migration
Starting in September‚ millions of monarchs migrating through Tennessee will alight upon the mountain woodlands of central Mexico․ If your flower garden in late summer seems to boast more orange and black wings than it did in July‚ you’ve probably stumbled across the first stages of the world’s most famous insect migration․ Tennessee is right in the path of the eastern monarch migration flyway․
Why Fall Monarchs Are Different
Not all monarchs are the same: those that are born in June or July only live for a few weeks‚ mating‚ laying eggs and dying․ In late summer‚ the days are shorter and the weather is cooler․ Monarchs that emerge from their chrysalises in August and early September are different from the broods that emerge earlier․ The so-called ““super generation”” of monarchs‚ which survive to hibernate in Mexico as adults‚ will delay reproduction‚ have more fat reserves‚ live eight to nine months (compared to several weeks) and their only job is to fly to Mexico‚ survive the winter‚ and start the journey north in the spring․
If you find yourself in Tennessee in September or early October‚ odds are it’s the same generation․
Life Cycle‚ as it was in Fall
Though the monarch undergoes the stages of egg‚ caterpillar‚ chrysalis and adult‚ the stage at which the butterfly is ideally helped along in a Tennessee garden varies with the season․
The female lays a single egg on underside of a milkweed leaf․ The egg will hatch in three to five days‚ depending on the temperature․ The caterpillar eventually emerges and begins to eat almost continuously for two weeks‚ increasing in size to almost 2000 times its original size before forming a jade-green chrysalis with flecks of gold․ An adult butterfly emerges 10 to 14 days later․
In the Southeastern U․S․‚ the biggest handicap for any gardener is that the Middle and East Tennessee nights by the end of September are 50 °F (10 °C) or less․ It requires two weeks for a caterpillar to form in July‚ but it requires three or four weeks at the end of September․ If found in your garden in early October‚ don’t worry that the caterpillar is ill or sluggish; cool weather has slowed it down․
When Monarchs Pass Through Tennessee
Tennessee’s peak migration period is mid-September to mid-October‚ and the peak is heaviest from the last two weeks of September․ East Tennessee‚ which is located more directly in line with the corridor of the flyway‚ often has heavier movements slightly earlier than in West Tennessee․ Under normal conditions‚ monarchs fly an average of 25 to 30 miles a day‚ usually using thermal updrafts and cold fronts for passive transportation‚ which usually causes a spike in sightings following the passing of a cold front․
They’re not stopping to breed at this point - they’re refueling․ A monarch going to Mexico needs to take in a lot of nectar so that it may build up the energy it needs to survive winter․
What This Means for Your Garden
If you want to help fall monarchs‚ the plants you have blooming for them in September and October are more important than the plants you put in the ground in March․ A lot of well-meaning gardeners will plant milkweed for caterpillars but forget to plant nectar sources for adults migrating south․
Good TN garden plants for the fall bloom include aster‚ goldenrod‚ Joe-Pye weed‚ and sedum․ The blooming season of these plants overlaps with the arrival of monarchs to Tennessee and places them in zone 6b to 7b․ Several native aster and Joe-Pye weed plants are available from TN Nursery․ These plants attract monarchs and other fall pollinators and require little maintenance once established․
A Milkweed Mistake Worth Avoiding
Here’s something a lot of general gardening advice gets wrong‚ or leaves out entirely: not all milkweed is equally good for monarchs in the Southeast․
Unlike native milkweed‚ tropical milkweed (Asclepias curassavica)‚ available at big box garden centers‚ does not die back in milder winters in Tennessee․ However‚ this is not a good thing․ Cultivating tropical milkweed into December and January may disrupt monarch cues that initiate their southward migration․ Tropical milkweed also acts as a host for OE (Ophryocystis elektroscirrha)‚ a protozoan parasite that infects the monarch population at a considerably higher rate than the native milkweed species does․
Other native species carried by TN Nursery include common milkweed (Asclepias syriaca)‚ butterfly weed (Asclepias tuberosa)‚ and swamp milkweed (Asclepias incarnata)․ These and other native milkweeds die back during the fall‚ as the monarchs evolved to expect․ Butterfly weed‚ which grows well in most of Tennessee‚ is more drought tolerant‚ and establishes a deep tap root that allows it to return each spring‚ is popular․ When planting milkweed for monarchs‚ native milkweeds are preferred to tropical milkweeds‚ even if the tropicals are fuller in the nursery pot․
The Overwintering Grounds
The migrating butterflies arrive in central Mexico often as early as the first week of November․ They tend to congregate by the millions in the oyamel fir forests of Michoacn at 10‚000 feet (3‚000 m) elevation‚ where the firs form a microclimate that is just cold enough that the butterflies will go into a low-energy semi-dormant state‚ but not freeze․ They cluster in the tree branches‚ as if the trees had turned to orange themselves‚ to conserve the stores of fat they need to survive until spring‚ when they begin their northerly migration again․ It takes three or four generations of monarchs to complete the round-trip migration; no individual butterfly can make the entire journey to Mexico and back․
