By Silvia Strobl, Master Gardener in Training
My 2023 garden gave me a front row seat for observing the monarch butterfly (Danaus plexippus) lifecycle. Two monarch caterpillars fed on my garden's swamp milkweed (Asclepias incarnata) lower leaves for about 10-12 days. I was very concerned when one started a long journey across the 10-foot-long patio and then climbed 10 feet up the house wall to make its chrysalis just under the eaves! I was relieved to read that monarch caterpillars typically do leave their host plant to make their chrysalis elsewhere just in case there are younger caterpillar instars feeding on the same plant. It would be tragic if their chrysalis came crashing down as another caterpillar feeds on the leaf from which it is suspended!
One of the two caterpillars in my garden last year feasting on swamp milkweed on August 8. By August 13, many more swamp milkweed leaves at been eaten (right)
I expectantly watched the "eaves" chrysalis daily and was rewarded on September 5th when the green and gold flecked chrysalis became more translucent with visible orange and black inside. An adult male, identified by the two dots on its hindwings (pheromone patches), emerged later that day.
One monarch caterpillar travelled across the patio and climbed up the house wall to make its chrysalis just under the eaves on August 17 (left). On September 5, the chrysalis became translucent, and the butterfly emerged. It dried its wings for a few hours (centre). Once it opened its wings, I could identify it as a male by the two pheromone patches on its hindwings (right).
I never saw the second caterpillar leave the swamp milkweed host plant but discovered its chrysalis suspended from the leaves of savannah grass (Sorghastrum nutans) one day.
Swamp milkweed is a common species in Ontario, usually found in wetlands, wet roadsides, floodplains and wet meadows, but looks beautiful in the garden with its upright stature and slender leaves. Its flowers also bloom for a long time and are floral and vanilla scented. It grows quite tall and maintains its structure well into the winter months to provide winter garden interest.
Swamp milkweed is a better choice for the garden than common milkweed (A. syriaca) which is not recommended for small gardens due to its rhizomatous aggressive spreading nature. Although butterfly milkweed (A. tuberosa) is a great nectar plant it is also a less desirable monarch caterpillar food due to its hairier leaves and lower protein content (UNC Charlotte Institute 2022). Both swamp and common milkweed averaged the highest number of eggs laid by female adult monarch butterflies in a study that evaluated the attractiveness of nine milkweed species common to Iowa (Pocius et al. 2018).
Typical habitat for swamp milkweed in the wild, but "it is fairly drought tolerant and will thrive in drier areas once established" (Gray and Booth 2024) (left). A 2-year-old swamp milkweed plant in my dry, sandy garden (right).
Well-intentioned people hope to increase the monarch population by home-rearing monarchs. However, an analysis of over 135,000 monarch observations at 403 annual butterfly count locations across the species' large summer range[1] compared population numbers in 1993 to those from 2018 and found that although some areas had population decreases, overall summer monarch population numbers in North America are relatively stable i.e., reproduction in summer is compensating for losses at wintering grounds in Mexico (Crossley 2022).
A naturally occurring parasite (Ophryocystis electroscirrha) adversely affects how well monarchs fly as well as their migration survival. It has increased in the last 15 years, raising concerns that parasite transmission is decreasing winter colony populations (Majewska et al., 2021). The Xerces Society has published a joint statement against the release of purchased or mass-reared monarchs by backyard and commercial breeders. If you are raising small numbers of wild-collected monarch eggs and larvae for personal enjoyment, education, or as part of citizen science project they encourage you to follow protocols for safe rearing and collect data on your reared monarchs for programs such as the Monarch Larva Monitoring Project and Monarch Health.
Although the research is not definitive, I recommend letting nature take its course and simply providing for the monarch caterpillar's habitat needs by planting host plants, particularly swamp milkweed, in your garden. This spring I planted two more patches of swamp milkweed plants in my garden. I can't wait to observe the monarch caterpillar drama that unfolds this summer!
[1] The NA monarch breeding range spans nearly the entire United States and southern Canada. Butterfly counts by citizen scientists are held between July 1 and August 31. Only data with at least 5 years of Monarch observations over a 10-year period were used in the Crossley (2022) analysis. An average of 25 butterfly counts are held annually in southern Ontario. Each count covers a 24-kilometer diameter circle. Last year I participated in my first count in Haliburton, Ontario.
Tips for growing swamp milkweed in your garden
- If space allows, plant a group of 3 to 5 plants so pollinators can more easily find the flowers. It can grow in sand, loam or clay and prefers full sun.
- You can grow your own seedlings by winter sowing or purchase seedlings from Ontario native plant nurseries.
- Water consistently during the first year so swamp milkweed can build roots to support flowering in year 2.
- Remove seed heads before seed pods open if you want to control the number of volunteer seedlings.
- Swamp milkweed is not palatable to deer.
References
Crossley, M.S., T.D. Meehan, M.D. Moran, J. Glassberg, W.E. Snyder, A.K. Davis. 2022. Opposing global change drivers counterbalance trends in breeding North American monarch butterflies. Global Change Biology https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.16282
Gray, R. and S. Booth. 2024. The Gardener's Guide to Native Plants of the Southern Great Lakes Region. Firefly Books. 352 pp.
Marinelli, J. 2024. Rethinking Monarchs: Does the Beloved Butterfly Need Our Help? Yale Environment 360, Yale School of the Environment. https://e360.yale.edu/features/monarch-butterflies-milkweed-home-breeders
Majewska, A. A., Davis, A. K., Altizer, S., & Roode, J. C. (2021). Parasite dynamics in North American monarchs predicted by host density and seasonal migratory culling. Journal of Animal Ecology, 2022, 1–14. https://doi.org/10.1111/1365-2656.13678
Pocius, V.M., J.M. Pleasants, D.M. Debinski, K.G. Bidne, R.L. Hellmich, S.P. Bradbury, and S.L. Blodgett. 2018. Monarch Butterflies Show Differential Utilization of Nine Midwestern Milkweed Species. Front. Ecol. Evol., 24 October 2018 https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2018.00169/full
UNC Charlotte Institute. 2022. Milkweeds for Monarchs? The Answer isn't so simple. https://ui.charlotte.edu/story/milkweed-monarchs-answer-isne28099t-so-simple/
Related
The Kiss Principle—Winter Sowing 101 https://peterboroughmastergardeners.com/2022/12/26/the-kiss-principle-winter-sowing-101/
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