Milkweed for Monarchs

On the move

It’s peak migration season in Texas.  Millions of monarchs have made their way along the Eastern United States, funneling into Texas, on the way to their winter site in central Mexico.  This migratory generation will be making stops all across the Houston area in search of food and shelter over the next few weeks.  Guided by an internal compass, monarchs are on a mission to reach their small mountain side retreat by early November.

In early October, monarchs made national news as they rode a cold front through Oklahoma City in numbers large enough to be captured by the National Weather Service Doppler radar.

Good news!

Researchers tracking the monarch population in Mexico recorded a 144% increase in numbers during the 2018-2019 wintering season. Many reports this year are already boasting record numbers, as well. Encouraging news, as monarchs have seen an overwhelming decline in the past two decades. Most information is recorded through citizen science efforts like Monarch Watch. Increased efforts to protect and restore habitats have played a large role in the population growth and offer simple ways for everyone to help.

Data collected by personnel of the Monarch Butterfly Biosphere Reserve of the National Commission of Natural Protected Areas in Mexico.

For more information on the fall migration, be sure to read The Environmental Services Blog: ‘A Wonderous Journey’.

How to help

Cut back tropical milkweed

Tropical milkweed is a popular choice for many gardeners. With a long bloom season, adaptability to our region and availability in nurseries around town, it is an easy-to-maintain, monarch-friendly plant. This Mexican native can be a much-needed resource for monarchs to lay their eggs. However, there are also concerns with this plant.

Given the ability to grow year-round, tropical milkweed may interfere with the monarch migration causing more butterflies to linger later in the season when they should be on the way to their wintering sites. If butterflies delay travel plans for too long, they will run into temperatures too cold for them to survive. In addition to delaying monarchs late in the season, tropical milkweed may increase the risk of infecting monarchs with the OE protozoan pathogen. OE, ophryocystis elektroscirrha, is a protozoan parasite that is ingested by caterpillars.  The disease affects the development of a caterpillar during its transformation into an adult butterfly.  When adults emerge, they are often too weak to fly.  A mild winter without a freeze, results in tropical milkweed that is unlikely to die back. This allows infected plants to continue to grow and be available year-round for caterpillars to ingest; continuing the spread the OE parasite. The best thing for a strong migration and disease-free plants is to cut back your tropical milkweed and watch healthy monarchs continue south.

Cut back tropical milkweed plants to within four to six inches off the ground each October

Plant for spring. Plant natives.

If you don’t have milkweed in your garden, tropical or native, now is the time to plant for spring. Milkweed is the only plant that monarchs will lay eggs on and the only food source for their caterpillars. Monarch caterpillars can eat an entire plant in one day so be sure to plant at least 4-5 milkweed plants to provide adequate food next spring and summer. The Native Plant Society of Texas has a great article on native milkweed varieties for reference.

Not sure where to find native milkweed?  The Woodlands Township has milkweed seedlings to give away this fall.  In partnership with Nature’s Way Resources and the Heartwood Chapter of Master Naturalists, more than 13,000 milkweed seedlings were propagated over the summer.  Many residents, churches, schools and community organizations have already picked up milkweed to plant in gardens across the community.  If you are interested in receiving free milkweed, you are in luck.  4” pots of orange, common and green milkweed are still available at Nature’s Way Resources. These seedlings need to be planted by mid-November so be sure to pick some up before it’s too late!

In addition to planting milkweed, native nectar-rich plants are a much-needed food source for adult monarchs. Monarchs will travel North, through Texas from March to May looking for nutrients along the way. Planting in the fall allows native plants to establish a strong root system over winter, creating a more drought tolerant and robust plant for monarchs and other pollinators to visit the following year. 

Whatever plants you decide to add to your garden, be aware that insecticides and herbicides can be harmful to monarchs.  Plants can absorb chemicals providing leaves and nectar that is toxic to pollinators. Help protect monarchs and other pollinators by avoiding the use of chemicals in and around your garden.

For more information on Plant for Pollinators and the Milkweed for Monarchs programs, email enviro@thewoodlandstownship-tx.gov

OE and tropical milkweed

The relative virtues and problems associated with tropical milkweed, Asclepias curassavica, continue to be a hot topic within the monarch conservation community, but the disparity between the two is becoming more and more clear. Scientific research suggests that its problems, namely its link to the spread of the Ophryocystis elektroscirrha (OE) disease, far out-weigh its virtues. In fact, it’s those very virtues—availability, adaptability, and long bloom season—that multiply its negative effects relative to the health and sustainability of the monarch butterfly species.

What is OE?

OE is a protozoan parasite that infects butterflies that host on milkweed. Its life cycle starts as a microscopic spore that breaks open when ingested by a caterpillar. Within the caterpillar, it grows and multiplies. Because a parasite depends on its host for its own life, OE rarely kills the caterpillar.

scales and spores

OE spores are only visible under a microscope.

But the disease affects the development of the adult butterfly while pupating, and adults emerge weak and often with crippled wings. While many monarchs may carry OE as spores attached to its wings and thorax, the size of the spore-carrying population and the heavy level of spores within that population in the Gulf Coast region—especially Texas and Florida—is cause for alarm. Visit Project Monarch Health for more about OE.

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An adult butterfly with OE has no chance of survival when wings are malformed.

Recent studies corroborate earlier studies and tighten the link between tropical milkweed and an increase of OE. Gardening to help conserve monarchs requires an understanding of the risks associated with tropical milkweed as well as the steps to take to minimize its ill effects.

The introduction of tropical milkweed in the U.S.

Monarch enthusiasts with the best intentions were thrilled when local nurseries began to offer tropical milkweed for sale and embraced the Mexican native with gusto. It didn’t take long to discover that aside from being very easy to grow, monarch butterflies love this variety of milkweed. It seemed that a solution was in hand to help restore milkweed habitat for the Eastern migratory monarch population. As a result, tropical milkweed has been well established in parts the southern states—especially southeast Texas and southern Florida.

Then research began to emerge that showed an increase in monarch disease caused by OE was linked to tropical milkweed grown in the southern states.

What the research shows is particularly troubling for the monarch migration that passes through Texas gardens to feed and breed.

The effects of tropical milkweed

Research by Karen Oberhauser, Dara Satterfield, and others has and continues to demonstrate that OE in monarchs increases where tropical milkweed flourishes. (See links to studies at the end of this blog.)

What’s been determined is that the proliferation of tropical milkweed (in the southeastern parts of Texas and south Florida in particular), coupled with its near year-round foliage and flower production does two things:

It interferes with the monarch’s migratory cycle. Tropical milkweed encourages them to linger in the southern states and continue breeding and laying eggs, “trapping” them here where they cannot survive temperatures that drop toward the freezing mark. Possibly more important is the effect of milder winters. Given a non-stop supply of milkweed, interference with normal migratory behavior produces populations of monarchs that overwinter in Texas and Florida instead of completing their migration to the oyamel fir tree forests of central Mexico.

Monarchs who stay in the southern states for the winter are five to nine times more likely to be infected with OE than migrating butterflies.

It significantly increases the rate monarchs are infected by the debilitating OE protozoan pathogen. If mild winters don’t produce a freeze, infected milkweed continues to thrive, not dying back like native milkweed species. This means infected plants persist. Infected plants in Texas are especially harmful because they sit in the gateway for the spring and fall monarch migrations.

Migrant butterflies at sites with overwintering residents were 13 times more likely to have infections compared to migrant populations that don’t come in contact with residents.

Adult monarchs migrating from Mexico in the spring that visit infected plants pick up hundreds of OE spores and carry them to other plants—increasing the number of infected plants and as a result butterflies, exponentially.

What to do?

If there’s any good news in this it could be that originally, most of the tropical milkweed planted was done so in gardens. By definition, gardens are tended. Gardeners should consider taking one of two actions.

Replace tropical milkweed with native species. While native varieties are more challenging to start, the effort would help minimize the spread of OE. Try these native species:

  • Asclepias incarnate, Swamp milkweed
  • Asclepias perennis, Aquatic milkweed
  • Asclepias tuberosa, Butterflyweed
  • Asclepias verticillata, Whorled Milkweed
  • Asclepias viridi, Green milkweed

Or, be diligent about cutting it back every winter. Cut tropical milkweed plants to within four to six inches of the ground each October.

If you have tropical milkweed in your garden and didn’t cut it back in October, do it now.

Milkweed for habitats

Milkweed used for non-gardening purposes poses a more clear guideline. Dara Satterfield  recommends, “that habitat restoration for monarchs focus on native species of milkweed, which are synchronized with the monarch’s natural migratory cycle and do not enable the year-round breeding that can lead to high parasitism rates.”

The spring migration approaches

Tracking the spring monarch migration starts on February 14. Visit Journey North to learn how you can enter your own monarch sightings and track the migration real time.

Delve in and learn more about tropical milkweed and its effect on the health of monarchs with these recent studies:

Patterns of parasitism in monarch butterflies during the breeding season in eastern North America, Ecological Entomology, 2018

Migratory monarchs that encounter resident monarchs how life-history differences and higher rates of parasite infection, Ecology Letters, 2018

Monarch butterfly migration and parasite transmission in eastern North America, Ecological Society of America, 2011

Loss of migratory behaviour increases infection risk for a butterfly host, The Royal Society Publishing, 2015

Learn more about native milkweed species at these resources:

Native Plant Society of Texas

Ladybird Johnson Wildflower Center Plant Database

PDF of Identification of Milkweed in Texas, by Texas Parks & Wildlife