Recent research has focused on the effects of GM (genetically modified) crops on the monarch butterfly. The initial focus of research in this area was on Bt corn and whether pollen from Bt corn could be deposited on milkweed leaves and harm monarch larvae feeding there. Along with collaborators, this research, published in PNAS, showed that the naturally occurring density of Bt corn pollen on milkweed leaves was too low to reach a toxic level.

PNAS cover page

As part of that research we found that milkweeds growing in corn and soybean fields had monarch eggs and larvae; in fact there were more eggs and larvae per stem than on milkweeds growing in roadsides or other non-agricultural habitats.

Milkweeds in cornfield

That work was done in the year 2000. Since then there have been some major changes to milkweeds in agricultural fields. Over the last decade Roundup Ready soybeans and Roundup Ready corn have come to dominate in fields. Development of these GM crop plants allows the use of Roundup herbicide (glyphosate) in fields without harming the crops. Roundup herbicide is very effective at killing milkweeds. A survey by Bob Hartzler at ISU found that between 1999 and 2009 there was a 97% reduction in milkweed density in agricultural fields in Iowa. I have made landscape calculations based on these survey results and estimate that from 1999 to 2012 there has been a 64% reduction in milkweeds in Iowa. This reduction has probably occurred throughout the Midwest where the majority of corn and soybeans are grown. This is significant because research by others has shown that prior to 2000 50% of the monarchs overwintering in Mexico came from the Midwest.

Milkweeds in soybean field a few days after Roundup application. Grass beginning to yellow, milkweeds show no signs of damage yet.

Milkweeds in soybean field a few days after Roundup application. Grass beginning to yellow, milkweeds show no signs of damage yet.

Milkweeds a little over 2 weeks after Roundup application

The size of the overwintering population in Mexico has declined 82% since 1999. The size of the 2013 overwintering population was the lowest recorded since counts began in 1994.

Click to enlarge

Could this be due to a reduction in milkweeds in agricultural fields in the Midwest? In collaboration with Karen Oberhauser at the U. of Minnesota we have tackled this question. Karen runs the Monarch Larva Monitoring Project which has used citizen scientists throughout the breeding range of monarchs to monitor monarch activity for over a decade. We used MLMP data on the number of monarch eggs per milkweed stem in each year and data on the number of milkweeds on the landscape in each year to estimate that there has been an 88% decline in monarch production in the Midwest from 1999 to 2012.

Our estimate of monarch production for each year was positively correlated with the size of the subsequent overwintering population showing the importance of Midwest production.

Future research follows 2 threads.

One, I am working on a way that one can analyze a tissue sample from an adult monarch and tell whether it fed as a larva on milkweeds in corn fields, soybean fields, or some non-agricultural habitat. I have found that milkweeds growing in different habitats have different concentrations of the nitrogen isotope 15N. The concentration of 15N in leaf tissues will be incorporated into the tissues of the monarch larvae that feed on them providing a signature for what habitat they came from. When fully developed, this approach could be used to sample a number of individuals from the monarch population and determine what percentage of them came from different habitats. Several isotope studies of monarch have been done over the last decade and a half but the 15N data have not been analyzed for this purpose. This could prove the decline in importance of agricultural habitats for monarch production as agricultural milkweed populations have been decimated.

Two, as milkweeds in agricultural fields have disappeared CRP (Conservation Reserve Program) land has become the habitat that is the most important for monarch production. This is formerly agricultural land that has been set aside from planting crops. It is typically planted with grass and forb species to reduce erosion. We need to have a better estimate of the densities of milkweed on CRP land and how monarchs use the milkweeds found there. We also need to know how the choice of plant species used to provide ground cover in CRP land and how the management of CRP land (mowing for example) affects milkweeds. I am planning a survey of CRP land in Iowa to answer these questions. Roadside milkweed populations have also become a more important habitat for monarchs. Additional future research will explore how the management of roadsides, such as mowing and herbicide spraying, by departments of transportation can affect milkweeds and the monarchs found there.