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Milkweed Sampling for Monarchs: Pilot Project

Oct 20, 2015


  • Conservation Stories

Monarch butterfly (Danaus plexippus) population modeling is key to informed habitat conservation and recovery efforts. These models require information on the distribution and abundance of monarch host plants, milkweed (Asclepias spp); there is good evidence that the loss of milkweed in key monarch breeding locations has driven declining monarch numbers (Pleasants and Oberhauser 2013). 

One of the important storylines emerging from the Monarch Conservation Science Partnership’s (MCSP) conservation planning efforts is the potential importance of public lands in addressing deficiencies in milkweed availability. The MCSP identified land cover- and land use-specific estimates of milkweed density as a key knowledge gap in assessing the amount of milkweed available to monarchs. Data are lacking for the critical northward migration corridor, from central Texas through Oklahoma, as well as in areas in the critical north-central breeding region.

The lack of understanding of milkweed density among a wide array of land uses, both public and private, means that monarch recovery strategies are imprecise. In partnership with Monarch Joint Venture, USGS, and the Department of the Interior’s 21st Century Conservation Service Corps, a pilot effort was launched in summer 2015 to collect key information on milkweed densities on and near public lands to help inform national strategies for milkweed restoration.

Three Corps members, through the Environmental Stewards program, have been out scouring the central monarch flyway for milkweed plants of all varieties this summer and fall. Kyle in Minnesota, and Abby and Robbie in Oklahoma have gathered information about milkweed density and diversity from hundreds of randomly selected roadside points throughout the central flyway.  A day in the life of a milkweed surveyor involves planning routes, traveling between sample points, surveying for milkweed, recording information and repeating! These surveyors have counted many milkweed plants, from the most common species to rarer ones, and have worked out many techniques that will make ongoing sampling more efficient. This winter will bring time for data entry and preliminary analyses of the findings.  We’re excited to learn what they’ve found!

The National Strategy to Promote the Health of Honey Bees and Other Pollinators announced this spring by the White House calls for an increase to 225 million butterflies by 2020 in the eastern monarch population. Recent research suggests it takes at least 29 milkweed plants to “produce an adult monarch that will be part of the fall migratory generation” (Nail et al., 2015). If you do the math, we need a lot of milkweed! Targeting the milkweed where monarchs need it most and using information about how much milkweed is already on the landscape will make this ambitious goal possible. This surveying work will help us figure out some of the answers to those questions.

Crescent markings on milkweed leaf Corps member Kyle Kasten in Minnesota reports: "I also look for signs of monarch caterpillar activity. Here [left] we see the typical “C” shape made by first instar caterpillars on the leaf of a common milkweed plant (Asclepias syriaca). While the caterpillar is not visible in this picture, signs like this indicate that caterpillars were on the plant at one point. Caterpillar presence is only noted if the actual caterpillar is seen."

Kyle's notes on the header picture of oval milkweed: "Our protocol also dictates that we take note of “rare” milkweed species. Here [above] is a picture of an oval-leaf milkweed plant (Asclepias ovalifolia). We categorized “rare” species as anything other than common (Asclepias syriaca), swamp (Asclepias incarnata), whorled (Asclepias verticillata), and butterfly (Asclepias tuberosa). Other rare species that I have found have been Sullivant’s milkweed (Asclepias sullivanti), and poke milkweed (Ascelpias exaltata)."



Nail, KR, C Stenoien, and KS Oberhauser. 2015. Immature Monarch Survival: Effects of Site Characteristics, Density, and Time. Annals of the Entomological Society of America 108: 680-90.

Pleasants, JM, and KS Oberhauser. 2013. Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity. 6:135–144


The content in this article does not necessarily reflect the position of all Monarch Joint Venture partners.