THE PERILS OF TROPICAL MILKWEED
September 7, 2024
By Catherine Koenig/Victoria County Master Gardener
Zizotes is a common milkweed in the Crossroads (Contributed Photos)
Butterfly Weed is an attractive native that could be a substitute for tropical milkweed.
Monarch butterfly populations have declined to critically low numbers over the past 20 years, and it appears to be related to the increased use of tropical milkweed in our gardens and public spaces. The popularity in planting tropical milkweed began in the late-1990s over the concern of the dramatic declines in native milkweeds across the North American spring and summer breeding grounds.
Milkweed plants (Asclepias and a few non-Asclepias species) are the sole host plants for the monarch caterpillars and are crucial to the survival of the monarchs. However, native milkweed habitats have been lost due to increased urbanization and the widespread use of herbicides on pastures, croplands, and roadsides.
Tropical milkweed (Asclepias curassavica) is attractive and easy to grow, but is not native to our area and can be invasive. It is the most widely available milkweed in commercial nurseries; whereas our native milkweeds are difficult to grow and hard to find in nurseries.
When planted along the Gulf Coast, tropical milkweed can continue to flower and produce new leaves from spring until a hard freeze. This situation is where some unexpected complications have developed.
Parasite infections
Monarchs, along with queen butterflies, are host to a debilitating protozoan parasite called Ophryocystis elektroscirrha, or OE, for short. Infected adult monarchs harbor thousands to millions of microscopic spores on the outside of their bodies, which are scattered like glitter on milkweed leaves as they lay their eggs or fly over for nectar.
These spores are dormant until ingested by the caterpillars, where the parasites are released and reproduce. The parasites now do their damage to the developing butterfly in the chrysalis. It can be lethal, but mostly it just weakens the monarch so it has less chance of successfully migrating.
Because native milkweeds die back in the fall, there is no place for OE spores to accumulate. Now migrating monarchs returning to Texas in the spring encounter fresh emerging milkweeds with no OE spores.
The risks of year-round breeding
Monarch butterflies are unique in the insect world with their long migration from southern Canada to central Mexico. One of the keys to this migration is reproductive diapause, where they are not mating and store fat for the journey.
Monarchs weakened by OE infections are not able to survive this journey, which significantly reduces the prevalence of OE in the monarch population returning in the spring.
The presence of tropical milkweeds in south Texas can “trick” monarchs into halting migration and breaking diapause to mate and lay eggs. As a result, caterpillars feeding on tropical milkweed significantly increase the spread of OE infections as well as increased mortality due to food shortages and freezing temperatures.
The bottom line is monarchs should not be encountering milkweed in the fall, but rather finding an abundance of fall nectar plants to provide energy for their journey south.
So how can we help?
If you have tropical milkweed, cut it back in late September and keep it cut back until February, so the presence of milkweed plants will not trigger the breeding of monarchs during their fall migration and regrowth will be free of the OE spores when they return in the spring.
Plant milkweeds native to our area such as antelopehorn (Asclepsia asperula), green antelopehorn (A. viridis), zizotes (A. oenotheroidae), and butterflyweed (A. tuberosa). Victoria master gardeners will be working to have some of these available at our 2025 spring plant sale.
Plant an abundance of nectar plants especially for the monarchs’ long journey south in the fall. A few of their fall favorites are blue, fragrant, and Gregg’s mistflowers and frostweeds.
This article appeared in The Victoria Advocate
MORE INFORMATION AND REFERENCES
Potential risks of growing exotic (non-native) milkweeds for monarchs.
Tropical Milkweed — A No-Grow, Xerces Society Blog by Justin Wheeler, April 19, 2018
Project Monarch Health — What is OE?
Project Monarch Health — Results/Monarch Health
Annual Results – 2023 Newsletter
Milkweeds of Oklahoma and Texas see South Central Region Milkweed Species
Milkweed (Asclepias curassavica L.) May Be A Population Sink for The Monarch READ HERE ALSO