Wednesday, November 28, 2018

Monarch Successes in 2018

I've been planting native milkweed plants for 5 years here and that while I've seen a few monarch butterflies over the this time, 2018 is the first year that I've actually seen any caterpillars.  While this may have been destined, one thing that was different this year is that I included a couple of pots of tropical milkweed, Asclepias curassavica, into our mix of potted plants.  Since these new plants were on the deck where we could see them easily, it may just have been that the caterpillars were just easier to spot this year.  Or tropical milkweed is a preferred host, so that encouraged the females to lay eggs closer to where we could see them.  These caterpillars showed up around the beginning of August.  This corresponds to the third generation in the monarch migration.  

Monarch butterfly on a tropical milkweed, Asclepias curassavica.
Tropical milkweed is native to South America and has naturalized into tropical areas worldwide, including southern California, southern Texas and Florida.  It is winter hardy in USDA zone 9-11 and can be grown as an annual in colder parts of the U.S.  It has striking red and orange flowers that make it an attractive garden plant and its tender foliage seem to make it a preferred host for monarch caterpillars.  That being said there are a number of reasons to plant more of the native milkweed species rather than planting a lot of this 
tropical species.

Some people have cited concern that this tropical species may interfere with migration patterns, especially in the southern parts of the US.  There, the presence of large tropical milkweed population may fool the butterflies into stopping their migration before reaching their destinations in Mexico.  This has not yet been established by scientific studies.  One negative factor that has been proven is that tropical milkweed can host a parasite that can harm monarch butterfly populations in overwintering butterflies.  I would refer you to this link on parasites on tropical milkweed for a more detailed discussion of this issue.

The authors do not insist that all tropical milkweed be removed, but recommend that all green foliage be removed from over-wintering plants between October and February to prevent any parasites from surviving and infecting any of the northward migrating butterflies along the way the following season.  What would be best is to grow only native species which die back to the ground naturally each year, thus ensuring that there will be no parasites.

The remains of the tropical milkweed after 6
 caterpillars fed on it.

While we were thrilled with seeing the monarch caterpillars, we soon saw that there was a potential crisis developing.  Within a couple of days the caterpillars had totally stripped the potted milkweed plants of all of their foliage.  The caterpillers were approaching maturity (4th or 5th instar) but we still didn't want to risk starvation so close to maturity. 

Fortunately there were the native milkweeds elsewhere on the property.  Since the tropical milkweed was growing in pots, I was able to more the pots to where the native milkweeds were growing and encourage the caterpillars to migrate onto the other plants.  Before moving the caterpillars I tried to do some research into what are the preferred host plants in the Mid-Atlantic region.  Much of the info on the internet is anecdotal but I did find one scientific study that looked at nine North American milkweed species and reported the survival rates for caterpillars reared on each.  Of these nine I knew where I had three of them growing on my property: Common milkweek (A. syriaca), butterfly weed (A. tuberosa), and honeyvine (Cynanchum laeve).  

By August the caterpillars did not seem so interested
 in eating butterfly weed. 
While butterfly weed was among the better hosts based on that study, late in the growing season its leaves are relatively tough compared to my other two species (and much tougher than the tropical milkweed).  By placing the tropical milkweed pots among both common milkweed and honeyvine I was able to coax the caterpillars onto those plants.  

I was able to get the caterpillars to move onto the common milkweed plants.

The honeyvine, with its thin, tender leaves seemed to be a big hit.  This was a bonus for me since the honeyvine, which appeared last year as a weed, is an aggressive grower.  The caterpillars, which are aggressive eaters were a good match for the honeyvine and after a couple of days the vine had been eaten back to an acceptable level. 

Honeyvine is an aggressive vine, similar in habit to bindweed,
except with opposite leaves.  It's native to the
Mid-Atlantic and Mid-Western states.  The small white flowers
have an intense honey-like scent in late summer.


So, based on this year's experience I will try to keep the potted tropical milkweed in the sunroom over the winter so that I will have a head start on growth next spring.  To control potential parasites I will cut the plants down to within 6" of the soil and  remove all leafy sprouts between in late winter.