Monarch Migration Superhighway

Monarch on Seaside Goldenrod

By late September migrating Monarch butterflies east of the Rocky Mountains coming from as far north as Canada have reached the “Texas Funnel” on their way south to wintering grounds in the Oyamel Fir Tree forests in the mountains of central Mexico in the state of Michoacán. Their annual migration is truly one of the wonders of nature. Every Fall, a Monarch “Super Brood”, a special generation that can live up to six months, makes its way south to wintering grounds that they’ve never been to before, but are hard wired to find. Monarchs can travel fifty to a hundred miles in a day and by mid October the thick of migration has reached the LRGV. There are actually two migration flyways that come through our state, the “Central Flyway” and the “Coastal Flyway” making Texas a critical area since a large percentage of the Monarch population in North America comes through it twice a year. While there is overlap, most of the Monarchs we see in the LRGV are coming from the North East through the Coastal Flyway, with numbers becoming concentrated as you near the gulf coast. The Texas coastline becomes a Monarch Superhighway during the Fall migration. The Fall rain showers bring a colorful palette of coastal wildflowers into the bloom just as the migrating Monarchs arrive in our area. It is all perfectly timed and in synch. A variety of Asters (plants in the sunflower family) seem the most favorable. Splashes of yellow Seaside Goldenrods mixed in with purple Palafoxias and blue Betony Mistflowers are important nectar sources to the kaleidoscope of Monarchs migrating over the coastal prairies and dune systems. During the thick of migration, and in good habitat, it is possible to encounter a half dozen or more Monarchs on a single Goldenrod, making the plant droop down from the weight.

Nectar sources are not all the Monarchs are looking for. The first waves are also looking for their host plants (plants that the caterpillars eat) to lay eggs on, which are specific plants in the Milkweed family (Asclepiadaceae). There are a few native milkweed species in the LRGV that serve as hosts to the Monarch. One of them, the Climbing Milkweed (Funastrum cynanchoides) is sadly better known as a nuisance growing on our fences. Closer along the coast, the Prairie Milkweed (Asclepias oenotheroides), also known as Zizotes Milkweed, is an important host that can be quite common. This short herbaceous plant can even be frequently found growing voluntarily in yards and vacant lots on South Padre Island and Port Isabel. Female Monarchs are experts at finding their host plants and can lay many eggs on them as they are encountered. This usually results in several hungry caterpillars that can consume entire milkweed plants before pupating and eclosing as adults. This late generation is born with the looming winter and will likely not breed until next spring. They too are part of the “Super Brood” and will eventually join the others ahead at the wintering grounds.

Prairie Milkweed (Asclepias oenotheroides)

Monarch Caterpillars on Prairie Milkweed

Unfortunately, the natural marvel that is the Monarch migration is under threat. Studies have shown alarming Monarch population declines in recent years. This is mainly due to human caused habitat loss and overuse of pesticides and herbicides. But there’s plenty you can do to help like; planting native milkweeds and nectar sources in your garden, adjusting mowing/trimming practices, restricting the use of pesticides and herbicides, and participating in citizen science. Learn more and enjoy the migration on “Monarch Day” happening Saturday, October 22nd from 9am – 3pm at the South Padre Island Birding & Center. Visit spibirding.com for more info.