By James C. Trager, Ph.D. / Restoration Biologist / Shaw Nature Reserve
An article from the Wild Ones Journal – lays out the issues regarding planting tropical milkweed for monarch butterflies, and suggests a solution to the possible issue of causing increase protozoan parasite infection, namely, mowing patches of this plant down a couple of times a season, so they are not a constant attracting feature in a butterfly garden. I will note that monarch caterpillars often completely defoliate patches of Asclepias curassavica here in the St. Louis area, thereby rendering them unattractive to breeding females, as I have observed year after year in my garden, so this practice may not even be necessary around here.
Based on my knowledge of insects’ ecological/physiological triggers to reproduction and migration, and the fact that the butterflies naturally encounter this plant as a common garden and roadside plant on their way (passing through Texas and all of Mexico) to the overwintering grounds, I lean with Dr. Chip Taylor of Monarch Watch in not having much concern on this one. The plant itself is winter-hardy through lower Zone 7, and like many subtropical plants and insects, is slowly spreading north as people plant them out and our winters get shorter (now by about three weeks compared to when weather records were first being kept). But overall, habitat loss is more important.
I wish to state and have readers clearly understand that while there is a sizable literature on the use of A. curassavica by monarchs (e.g. Google Scholar search results) at the moment, there is not a single, much less multiple-corroborative, scientific study/ies that I have come across to support either the pro or con-curassavica positions. The whole body of interconnected issues needs study.
However, I can certainly appreciate that people want to exercise caution in this matter.
Since we’re in the stage of hypothesizing and opining on it, I have a few additional thoughts:
- Any large, dense patch of some of our strictly native milkweeds could just as well foment transmission of the protozoan parasite as do people’s garden patches of tropical milkweed. I’m thinking about the sometimes extensive patches of swamp milkweed Asclepias incarnata that one sees this time of year around ponds and in roadside ditches, or late sprouting and blooming patches of orange milkweed A. tuberosa, resulting from mid-season haying or roadside mowing. (Needs study)
- Habitat loss is a primary issue for all declining species. (Doesn’t need study)
- Careless or thoughtless and increased herbicide usage, and excessive mowing frequency on rural roadsides are important sources of this habitat loss, but seem insignificant in comparison to the primary factor – the total destruction of existing grassland vegetation by agriculture, to feed the continuingly burgeoning human population, both directly and to feed our livestock and automobiles.
- The drought of 2012 must have had a huge impact on any migrating species forced to cross the drought-stricken desert that was the 2000 km/1200 mi span from the lower Midwest USA to central Mexico. (Someone must have studied this, but I haven’t read any of the work on it.)
In any case, I hope you all see as many monarchs floating around this September as I have been seeing.
If you have questions or can enlighten me regarding any scientific studies of the matter that I may not have encountered, please contact me.