Monarchs, Milkweed, and Plant Defenses

jhansen49
Mar 29
5 min read

There’s a crucial connection between monarch butterflies (Danaus plexippus) and milkweed plants (genus Asclepias). You may know that monarchs only lay their eggs on milkweed, and that this plant is the only one on which the larvae (caterpillars) can feed. The milkweed plant, however, is a challenge to eat. It’s covered in fine hairs (trichomes, a defense mechanism that helps deter herbivores), exudes sticky latex when bitten that can glue a small insect’s mouthparts together, and it generates cardenolide toxins in its leafy tissues that can poison most plant-eating insects and larger herbivores.

Happily, monarch caterpillars have developed a metabolism that enables them to safely ingest milkweed toxins and sequester the chemical in their bodies, which can help protect them from predators as both caterpillars and adults. Their bright colors have evolved as a warning mechanism that advertises their toxic taste. When a bird or other predator tastes a monarch, it learns to associate its color pattern with the very bad taste and avoids monarchs in the future.

This connection between monarchs and milkweed is an example of co-evolution and mutualism, wherein both species benefit. As the monarch’s sole host plant, milkweed provides a vital food source on which the female will lay her eggs. Adult monarchs also feed on milkweed nectar and the last Eastern generation that will fly to Mexico for the winter uses that nectar as a critical fuel source for their south-bound migration.

The benefit to the milkweed plant, in turn, is that monarchs act as pollinators for the plant population. Additionally, when caterpillars graze on the leaves, the plant responds by spreading more actively through its root system, allowing for denser, healthier, and more widespread colonies. The damage caused by the feeding also induces the plant to increase its cardenolide production, which aids the plant in defending itself against other more harmful herbivores.

Bees on Common Milkweed by Courtney Celley/ USFWS

Monarchs are not the only insect that can feed on milkweed nectar. Several bee species, flies, beetles, and other adult butterflies can drink it, and a few can feed on the leaves as well. The large, brightly colored milkweed bug can feed on the plant parts, including the seeds, as can the young of the queen butterfly (Danaus gilippus), which is closely related to the monarch.

In plants, an elegant defense method can be ongoing, or can be an immunological response in which the plant recognizes the signals from damaged cells and then activates the system. The plant may become unpalatable or poisonous to the herbivore, or it may release chemicals that attract the natural enemies of the herbivore such as drawing parasitic wasps that will attack the predating insect. Another strategy is to provide food or housing sites for use by ants, other arthropods, or fungi in exchange for their defensive help.

Chemical production is a protection that works on larger herbivores, such as deer and plant-eating mammals, as well as on insects. Yet many plants also rely first, or primarily, on mechanical defenses such as thicker or indigestible leaves, thorns or the above-cited trichomes, the fine hair or spine-like growths that can emit oils, resins or sticky substances that repel predators or make it difficult for them to feed. Milkweed does this by emitting its sticky, mouth-clogging latex from the leaf tissues being fed upon. The latex also contains the cardenolide toxins.

Monarch caterpillars sabotage this defense mechanism by cutting the veins in the leaf to reduce the latex while they feed. Interestingly, while young larvae usually do this, sometimes older caterpillars feed on the latex directly after cutting a main vein. They can clear their mouthparts better, and this feeding increases the cardenolide load in their bodies too.

Many species of milkweed grow in the U.S. They bloom at slightly different times, aligning with monarch migration north for eastern monarchs. Spider milkweed matures by March in Texas, where monarchs lay the eggs of the first northbound generation. During this timeframe milkweed is still dormant in colder regions.

It’s not until May that milkweed begins to show itself in the northern states, which corresponds with the second monarch generation eggs being lain in those areas. Finally, milkweed varieties develop in Canada just in time for the arrival of that segment of the second generation that goes that far and that will lay its eggs for generation three.

Monarch butterflies once numbered in the hundreds of millions, but the Eastern population has declined by more than 80% since the 1990s, and the California population has dropped by an even more alarming 95%.

Loss of milkweed habitat — via genetically modified crops and overuse of herbicides and insecticides, mowing and spraying to control roadside vegetation, agricultural intensification, urban, suburban and agricultural development, disease, climate change, and legal and illegal logging in the Mexican forests where Eastern monarchs overwinter — has caused the decline in monarch populations. In the case of climate change, increasingly erratic and extreme weather can delay the emergence of milkweed in spring and change the bloom cycle of these flowering plants that normally provide resources to monarchs migrating north.

Because monarchs lay their eggs exclusively on milkweed and lay only one egg per plant, hundreds of millions of individual milkweed plants are needed to sustain the current population of hundreds of millions of monarchs. Gardeners can help by planting regionally indigenous milkweed in their yards, which will also support a host of additional insect species, including other butterflies and moths, bees, hoverflies, ants, beetles, and true bugs.

Milkweed – For More than Monarch Butterflies US Fish & Wildlife Service — **Milkweed – For More than Monarch Butterflies** US Fish & Wildlife Service

To help monarchs on their journey it’s also critical to ban pesticides in your garden, to grow nectar-rich plants on which adult monarchs can feed in late summer and fall, such as goldenrod, asters, and cardinal flowers, to subscribe to the “No-Mow May” principle, to convert part or all of your monoculture lawn into a diverse, low-maintenance “meadow,” and to support organizations that work to sustain natural habitats such as the South Jersey Land & Water Trust.

We’re planting 1,000 milkweed plants at our Oldmans Creek Preserve headquarters on Saturday, April 25, and will be grateful for your support! If you can help plant, please contact Program Director Jody Carrara at (JCarrara@sjlandwater.org). Also, kindly donate today to help us with this spring campaign that will increase the beautiful monarch butterfly’s chance of survival as a species. Your contribution will enable the purchase of milkweed plants, youth education about monarchs and milkweed, and the investigation of other sites that can be planted.

Sources:

“The Connection Between Milkweed and Monarch Butterflies.” Written by caesweb October 2, 2023. Coastal Georgia Botanical Gardens

https://coastalbg.uga.edu/2023/10/the-connection-between-milkweed-and-monarch-butterflies/

“Defences [sic] Ecology of.” ScienceDirect

https://www.sciencedirect.com/topics/immunology-and-microbiology/physical-defense

“Late-instar monarch caterpillars sabotage milkweed to acquire toxins, not to disarm plant defence [sic].” Anja Betz, Robert Bischoff, & Georg Petschenka. The Royal Society Publishing Proceeding B Volume 291, Issue 2017, February 2024. https://royalsocietypublishing.org/rspb/article/291/2017/20232721/116433/Late-instar-monarch-caterpillars-sabotage-milkweed
“Mechanisms of plant defense against insect herbivores.” Abdul Rashid War, et al. Plant Signal Behav. 2012 Oct 1;7(10):1306–1320. National Center for Biotechnology Information. https://pmc.ncbi.nlm.nih.gov/articles/PMC3493419/
“Milkweed for Monarchs” National Wildlife Federation