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Friday - November 20, 2015

From: Houston, TX
Region: Southwest
Topic: Non-Natives, Butterfly Gardens, Diseases and Disorders, Problem Plants, Herbs/Forbs
Title: Should Mexican milkweed (Asclepias curassavica) not be used to attract Monarch butterflies?
Answered by: Guy Thompson

QUESTION:

Should I remove Asclepias curassavica (Mexican milkweed) in my garden for threat of OE parasitic protozoan threat to Monarch butterflies? Is this threat as widespread as Chronicle implies? I had great success with this plant in NW Houston before I moved. Is pruning the plant to the ground in Nov-Dec sufficient action? If not, where do I obtain the native variety in Katy, Tx? Also, Milkweed Leaf Beetle infestation is a problem as well as what I believe is assassin bug. I have been picking off and squashing. Good idea or bad? I want to be helpful not hurtful. Any answers will be appreciated.

ANSWER:

The use of Asclepias curassavica as a Monarch attractant is a subject for current debate.  I attach below a recent answer to a similar Mr. Smarty Plants question.

"Mexican milkweed (Asclepias curassavica) is also known as tropical milkweed and is not a native plant. Wikipedia says that it is an introduced species in the US states of California, Florida, Hawaii, Louisiana, Tennessee, and Texas, as well as the US unincorporated territories of Puerto Rico and the United States Virgin Islands. It is native to the American tropics.

There's quite the debate about whether it's wise to plant Asclepias curassavica or not.

The Cockrell Butterfly Center in Texas has put out the following message about Mexican milkweed recently... Biologists studying monarchs have discovered that tropical milkweed may be a factor in the spread of a parasitic infection that attacks monarchs. The infection is called Oe (short for Ophryocystis elektroscirrha) and is transmitted by spores that fall from an infected female’s body onto the hostplant when she lays her eggs. The hatchling caterpillars eat the spores along with the leaves, and become infected themselves. After a generation or two or three, the infection level becomes so high that the butterfly dies (sometimes in the caterpillar stage, sometimes in the pupal stage, and sometimes as the adult).

This could happen with any milkweed – the problem with the tropical species is that it does not senesce (die back) in Houston’s mild winters but is perennial, growing throughout the year. In contrast, native species die back to the ground in the winter, and when they regrow in the spring they are spore free – so the infection cycle is broken.

Also, researchers have found that some monarchs in the southern part of the USA don’t bother to migrate if they have milkweed available. These year-round residents have been found to have very high levels of Oe infection, because they are mostly using the tropical milkweed species generation after generation. While this probably doesn’t greatly impact the migration as a whole, we don’t want to contribute to the local spread of the disease.

If you do already have tropical milkweed, one solution is to cut it back severely a couple of times a year. Even better is to remove the tropical variety and switch to native milkweed species. Unfortunately, so far these are not widely available in the nursery trade and are not as easy to grow as the tropical variety!"

The Wildflower Center is currently trying to develop more efficient methods for propagating native milkweeds.  When available, seed will be shared with plant nurseries so that they can grow them.  Some of your local native plant nurseries may now have native milkweeds, especially Asclepias tuberosa (Butterflyweed).

In Texas milkweeds are often attacked by Swamp milkweed leaf beetles and Milkweed bugs.  It is recommended to pick these bugs off by hand rather than risk harming monarch caterpillars by using insecticides.  Aphids are also likely to be present at times, but thay seem not to cause a serious problem.  They can be washed off with a hose.

Good luck and thank you for your interest in preserving the monarch butterfly.

 

 

 

From the Image Gallery


Butterflyweed
Asclepias tuberosa

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