Are gardeners doing monarchs more harm than good by planting Mexican milkweed?

Mexican milkweed is safe to grow for monarchs in North Texas, experts say, but from San Antonio southward, it could harbor spores dangerous to monarchs over the winter.
Mexican milkweed is safe to grow for monarchs in North Texas, experts say, but from San Antonio southward, it could harbor spores dangerous to monarchs over the winter.


Monarchs have been in the news a lot lately, most of it grim.

Numbers have dropped dramatically over recent years from various causes, including climate change, illegal logging at the insects’ overwintering sites in Mexico and new practices in the agricultural industry. A major factor is the widespread use of GMO crops that are Round-Up resistant, allowing U.S. farmers in the monarch’s summer breeding grounds in the Corn Belt to spray the herbicide on their fields to kill weeds. This relatively new practice eradicates the monarch’s host plants, milkweed.

To counter this trend, butterfly enthusiasts, conservation groups, garden clubs and, just last week, the U.S. Fish & Wildlife Service are urging citizens — and not only gardeners — to plant milkweeds. In 2014, Dallas-area retail nurseries had difficulty keeping Mexican milkweed — far easier to cultivate and grow than native milkweeds — in stock because the urge to help monarchs was so widespread.

But last month, there were headlines about how gardeners are harming monarchs by planting this widely available, non-native tropical milkweed (Asclepias curassavica). A recent study by monarch researchers Dara Satterfield, John C. Maerz and Sonia Altizer showed that where this species of milkweed stays green during the winter, and in locations with winter-breeding monarchs, increased levels of infection by a protozoan parasite called Ophryocystis elektroscirrha, or OE, can occur.

OE spores travel on infected monarchs and are inadvertently transferred onto milkweed plants, usually by females when they are depositing eggs. Caterpillars hatch and ingest the spores when they feed on leaves. The damaging protozoa develop inside the growing larvae. (OE is known only to infect monarch and queen butterflies.)

If the number of spores are high enough on the plants, it can have devastating effects on the butterfly-to-be, weakening it to where it may not be able to emerge from its chrysalis. Even in cases where the OE spore load is low enough for the butterfly to emerge and appear healthy, it will fly off and infect other monarchs and milkweed plants.

Dying back in winter

To clear up some of the initial confusion that resulted from the reporting of the study, Monarch Joint Venture released a statement on some of the key points of the research. There is nothing inherently bad about tropical milkweed, it emphasizes. What seems to have gotten lost in the reporting of the recent study is that this is only an issue where tropical milkweed stays green and where monarchs are present in the winter, allowing for a buildup of spores.

For Texas butterfly gardeners, this applies only to the state’s coastal areas and roughly San Antonio southward, where hard and prolonged freezes do not usually occur to kill back Mexican milkweed. In the Dallas-Fort Worth area and, in fact, most of the country, tropical milkweed dies to the ground after the first hard freeze, and there are no monarchs found here in the winter, so it is not an issue.

Even where it might stay green all winter, Mexican milkweed can be used as a host and nectar plant in spring through autumn as long as its stalks are cut back to the ground and discarded.

If native milkweeds were for sale at retail nurseries and big-box stores like Home Depot and Lowe’s, monarchs would benefit greatly. Natives naturally go dormant by late summer and therefore don’t provide a surface for a fatal buildup of OE, but native milkweeds are not a quick or easy crop for wholesale growers to produce. Mexican milkweed, however, is easy to germinate and quicker to develop, which is why it is the prevalent species for sale locally.

Reason for optimism

There was also some good news in January about monarchs, researchers say. The estimated numbers at the overwintering sites in Mexico have increased. The colonies covered an area of 1.13 hectares (2.79 acres), up 69 percent from the winter of the 2013-14 record low population of only 0.67 hectares (1.65 acres). While this increase is good news, it is still the second lowest number since population estimates have been kept.

What should North Texans do to help maintain the monarch migration? Plant milkweed this spring. Plant native milkweed species if you can find them, but, if not, plant tropical milkweed. It’s perfectly safe for monarchs migrating through the region in spring and fall.

Dale Clark is a butterfly farmer in Dallas County.

Where to buy

The two most common species that grow in the Dallas area are green milkweed (A. viridis) and antelope horns milkweed (A. asperula).

Texas Discovery Gardens at Fair Park plans to sell young, native, pesticide-free milkweeds, raised from seeds in its greenhouse, at its spring plant sale April 17-19. Watch Thursday’s Garden Calendar for details closer to the date.

Monarch Watch, a national organization based at the University of Kansas, started selling mail-order native milkweed plants in 2013. This year it has contracted with native plant nurseries to help fill more orders. On, it cautions Texas buyers that the appropriate native species will not be ready for shipping until late April or early May.

Order now, because inventory is limited. Monarch Watch says it will not cash customers’ checks until the flat of plants ships.

Native American Seeds in Junction sells the seeds of milkweeds that occur in North Texas, plus several other species. Its website also has a link that explains how to germinate milkweed seeds.







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