Photo: Milkweed, east Texas. © 2022 Amber Leung
The Impacts of Herbicides on Milkweed and Monarch butterflies
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Pleasants & Oberhauser, 2013, examined the relationship between increased use of glyphosate in agriculture in the Midwest, the decline of milkweed populations, and the subsequent decline of monarch butterfly populations. Monarchs migrate from North America, with more than 50% migrating from the Midwest, to overwinter in Mexico. Declining monarch populations in Mexico have been well documented for over a decade. Monarch larvae feed primarily on milkweed plants, particularly Asclepias syriaca. Monarch larvae rely heavily on milkweed plants near corn and soybean fields in the Midwest. Native milkweed plants have been treated as weeds in agricultural fields. With the advent of glyphosate-resistant crops in the mid-1990s, glyphosate has become the most dominant herbicide used in corn and soybeans. In Iowa, studies have documented a decline in milkweed in corn and soybean fields from 1999-2009.
Pleasants & Oberhauser, 2013, collated date from a variety of studies to estimate milkweed numbers and monarch numbers. They used the Iowa studies on milkweed populations, data on milkweed populations throughout the Midwest States, data on monarch populations obtained from a monarch tagging program that was begun 20 years previously, and data from the Monarch Larva Monitoring Project, which recorded the density of monarch eggs and larvae present on milkweed plants over several years, along with data the authors collected on eggs and larvae in both agricultural and non-agricultural locations.
Pleasants & Oberhauser, 2013, report that the Iowa data indicated declines in milkweed populations in both agricultural lands (81% decline) and non-agricultural lands (36% decline) from 1999 to 2010. Overall, there was a milkweed decline of 58%. “In 1999, milkweeds in agricultural fields constituted 53% of total milkweeds, but by 2010 were only 24% of the total. The 58% loss of milkweeds on the landscape actually underestimates the loss of resource for monarchs, because most of the loss was in agricultural fields and each agricultural milkweed represents 3.89 times more monarch eggs than a non-agricultural milkweed.”
Pleasants & Oberhauser, 2013, estimated an 81% decline in monarch egg production in the Midwest, during the same time period. There has not been a long-term study of milkweed density in agricultural fields outside of Iowa.
Milkweeds near or within agricultural fields have provided the largest portion of monarch production in the past. The impact of herbicides on that nutrient source has had a devastating impact on monarch populations. Pleasants & Oberhauser, 2013, recommend that planting milkweeds and other native plants in roadsides could be a valuable means of increasing milkweed populations.
Reference:
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The Impacts of Herbicides on Milkweed and Monarch butterflies
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Pleasants & Oberhauser, 2013, examined the relationship between increased use of glyphosate in agriculture in the Midwest, the decline of milkweed populations, and the subsequent decline of monarch butterfly populations. Monarchs migrate from North America, with more than 50% migrating from the Midwest, to overwinter in Mexico. Declining monarch populations in Mexico have been well documented for over a decade. Monarch larvae feed primarily on milkweed plants, particularly Asclepias syriaca. Monarch larvae rely heavily on milkweed plants near corn and soybean fields in the Midwest. Native milkweed plants have been treated as weeds in agricultural fields. With the advent of glyphosate-resistant crops in the mid-1990s, glyphosate has become the most dominant herbicide used in corn and soybeans. In Iowa, studies have documented a decline in milkweed in corn and soybean fields from 1999-2009.
Pleasants & Oberhauser, 2013, collated date from a variety of studies to estimate milkweed numbers and monarch numbers. They used the Iowa studies on milkweed populations, data on milkweed populations throughout the Midwest States, data on monarch populations obtained from a monarch tagging program that was begun 20 years previously, and data from the Monarch Larva Monitoring Project, which recorded the density of monarch eggs and larvae present on milkweed plants over several years, along with data the authors collected on eggs and larvae in both agricultural and non-agricultural locations.
Pleasants & Oberhauser, 2013, report that the Iowa data indicated declines in milkweed populations in both agricultural lands (81% decline) and non-agricultural lands (36% decline) from 1999 to 2010. Overall, there was a milkweed decline of 58%. “In 1999, milkweeds in agricultural fields constituted 53% of total milkweeds, but by 2010 were only 24% of the total. The 58% loss of milkweeds on the landscape actually underestimates the loss of resource for monarchs, because most of the loss was in agricultural fields and each agricultural milkweed represents 3.89 times more monarch eggs than a non-agricultural milkweed.”
Pleasants & Oberhauser, 2013, estimated an 81% decline in monarch egg production in the Midwest, during the same time period. There has not been a long-term study of milkweed density in agricultural fields outside of Iowa.
Milkweeds near or within agricultural fields have provided the largest portion of monarch production in the past. The impact of herbicides on that nutrient source has had a devastating impact on monarch populations. Pleasants & Oberhauser, 2013, recommend that planting milkweeds and other native plants in roadsides could be a valuable means of increasing milkweed populations.
Reference:
- Pleasants, J.M., & Oberhauser, K.S. (March, 2013). Milkweed loss in agricultural fields because of herbicide use: Effects on the monarch butterfly population. Insect Conservation and Diversity, 16(2): 135-144.
Next Sections on the impacts of pesticides:
- Wildlife: Impacts of Pesticides
- Pesticide Residue in Foods
- Funding for Research on Pesticides
- Commentary on Herbicide Use
To return to previous sections on Herbicides & other Pesticides: