Planting milkweed: Are you helping or harming?

Save the monarchs, plant milkweed! You may have seen this slogan at some point, especially if you live on the California coast where we have the wonderful privilege of living in a hotspot where thousands of monarchs come every year. Not only are monarchs an aesthetically beautiful species, their incredible ability to migrate each year from their breeding grounds to overwintering sites has fascinated both scientists and citizens. In order to protect this species we all know and love, it’s important that we educate ourselves and inform those around us on best practices to keep these lovely butterflies alive and well!

The western monarch population migrates from breeding grounds that occur throughout various mountain ranges in California. They typically spend the months of March through October at these breeding sites, where they reproduce and lay eggs on native milkweed plants where caterpillars feed and eventually emerge into the beautiful monarch butterfly. These milkweed plants, which occur in clustered patches, are seasonal plants and die off toward the end of fall and do not grow back until early spring.

Native Milkweed (Asclepias californica)

Once the monarchs leave their breeding grounds in late fall and migrate to their coastal overwintering sites, they enter diapause, a period of suspended development, where they become reproductively inactive (Nylin, 2013). In order to maintain their lipid storage through the entire winter, they must spend very little energy while they cluster in their overwintering groves, which involves minimal flying, avoiding extreme temperatures, and not engaging in reproductive activity (Chaplin & Wells, 1982).

Tropical Milkweed (Asclepias curassavica)

The reproductive cycle of monarch butterflies has been disrupted due to the misconception that planting milkweed in your garden helps the population. People have been told that if they see monarch caterpillars in their gardens, they are benefiting the population by providing more plants for the butterflies to use for breeding. Unfortunately, those who are planting milkweed in their gardens are actually causing more harm than good. 

The milkweed that is commercially sold for gardens is tropical milkweed, which is different from the native milkweed the butterflies normally breed on during the spring and summer. The biggest difference between invasive tropical milkweed and native milkweed in California is that tropical milkweed is perennial, whereas native milkweed is seasonal. This means that native milkweed that naturally occurs in the mountains across California is only up in the spring and summer, and starts to die off during fall. The stems do not grow back until early spring, around March. Native milkweed continues to go through this cycle each year, using photoperiod and temperature as cues for when to die back in the fall and when to grow in the spring. Tropical milkweed, on the other hand, stays alive all year round.

Overwintering monarch cluster in Pacific Grove, CA
Credit: Joanna Gilkeson/USFWS

So what happens if milkweed is available year-round, like the tropical milkweed many plant in their gardens? The outcomes may not seem obvious, but they are very harmful to the monarch population.

Once the monarchs’ milkweed source dies, they migrate to their overwintering sites along the California coast where they become reproductively inactive and conserve energy to last them through the entire winter season (Chaplin & Wells, 1982). During this time, they are not breeding because there is no milkweed source for the caterpillars. The butterflies will occasionally feed on nectar throughout the season, but spend the majority of the time inactive and clustered in the trees of the overwintering grove to use as little energy as possible (Chaplin & Wells, 1982).

If milkweed is present throughout the entire year, the monarch reproductive cycle becomes disrupted and they will be able to breed during the winter when reproduction is normally disabled. These monarchs can then breed throughout the entire year, and there is no need to migrate from their native milkweed patches in the mountains out to their overwintering grove sites along the coast for the winter since they now have a food source for their offspring during the winter season. This results in non-migratory behavior, which has now been found to put the western monarch population at a much higher risk of disease (Satterfield et al., 2016).

Monarch butterflies are often infected by a specialist protozoan parasite called Ophryocystis elektroscirrha, or ‘OE” for short (Bartel et al., 2011). The migration behavior of eastern monarch butterflies, which travel from the eastern United States down to Mexico during winter, is known to reduce OE infection prevalence (Altizer et al., 2000).

OE spores located on a monarch's abdominal scales (Bradley & Altizer, 2005). 

Migratory behavior decreases risk of OE infection through several mechanisms. First, heavily infected monarchs are removed from the population during the strenuous migration, selecting for healthier adult monarchs who will pass on less OE infection to their offspring, a process known as "migratory culling" (Bartel et. al., 2011). Second, the monarchs are able to periodically escape from their milkweed habitats, where they initially become infected with OE (Altizer et al., 2000). 

Grey lines represent healthy, uninfected monarchs. Black lines represent OE infected monarchs. Uninfected monarchs can fly faster and for longer than can diseased monarchs. Data was no longer taken when flying ceased (Bradley & Altizer, 2005).

Migratory monarchs have much lower OE infection rates than non-migratory monarchs (Satterfield et al., 2016).

Non-migratory behavior results in both survival and reproduction of heavily diseased monarchs, as well as continued exposure to contaminated habitats throughout the year. Therefore, the presence of non-native milkweed allows for non-migratory behavior, ultimately resulting in higher rates of OE infection in monarchs, which is a common cause of mortality in the species.

There is hope! You can still plant milkweed in your garden if you'd like, but it's important to cut back your plants during fall and winter to allow the monarch butterflies to complete their migration and overwinter in the groves as they naturally would. We all love the monarchs and many of us want to help, but the first step in helping is getting educated!

Written by: Kiana Saniee

Literature Cited

Altizer, S.M., Oberhauser, K., and L.P. Brower. 2000. Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies. Ecological Entomology. 25: 125-139.

Bartel, R.A., Oberhauser, K.S., de Roode, J.C., and S.M. Altizer. 2011. Monarch butterfly migration and parasite transmission in eastern North America. Ecology. 92: 342-351.

Bradley, C.A., and S.M. Altizer. 2005. Parasites hinder monarch butterfly flight: implications for disease spread in migratory hosts. Ecology Letters. 8: 290-300.

Chaplin, S.B., and P.H. Wells. 1982. Energy reserves and metabolic expenditures of monarch butterflies overwintering in southern California. Ecological Entomology. 7: 249-256.

Nylin, S. 2013. Induction of diapause and seasonal morphs in butterflies and other insects: knowns, unknowns and the challenge of integration. Physiological Entomology. 38: 96-104.

Satterfield, D.A., Villablanca, F.X., Maerz, J.C., and S.M. Altizer. 2016. Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed. Integrative and Comparative Biology. 56(2): 343-352.