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 in fields without harming the crops. Roundup herbicide is very effective at killing milkweeds. A study by a colleague at ISU found that milkweed density in agricultural fields in Iowa was reduced 78% over the last decade. I have made landscape calculations and estimate that there has been a 58% reduction in milkweeds in Iowa over the last decade. This reduction has probably occurred throughout the Midwest where 80% of corn and soybeans are grown.
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
A recent paper has documented the decline in the size of the overwintering population of monarchs in Mexico over the last decade. Research by others found that prior to 2000 50% of the monarchs overwintering in Mexico came from the Midwest. So, could the decline in monarchs be due to a reduction in milkweeds in agricultural fields in the Midwest? In collaboration with Karen Oberhauser at the U. of Minnesota we tackled this question. Karen runs the Monarch Larva Monitoring Project MLMP which recruits citizen scientists throughout the breeding range of monarchs to monitor monarch activity. We have used the data from this long-term project to examine this question. Based on MLMP data and other data we estimate that there has been an 81% decline in monarch production in the Midwest from 1999 to 2010.
This is somewhat higher than the 65% percent decline in the size of the overwintering population in Mexico over this same period because there is evidence that the contribution to the population from outside the Midwest has not changed. 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. A paper based on this work was recently published in “Insect Conservation and Diversity” (see Publications for link to pdf). The preliminary results were mentioned in the New York Science Times and the published paper was reported on in the Minneapolis Star Tribune.
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. This could show the importance of agricultural habitats at present and document the declining importance of this habitat in the future as agricultural milkweeds continue to disappear.
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.