Karan S. Shakya

Threats to Monarch Migration: A Look into Changes in Herbicide Use and Land Cover

This research project was completed under the supervision of Professor Maggie Douglas (Assistant Professor of Environmental Studies, Dickinson College), and collaboration with Wayne Thogmartin, PhD (Research Ecologist. Upper Midwest Environmental Sciences Center, U.S Geological Survey) and Eric Lonsdorf, PhD (Lead Scientist. Natural Capital Project/Institute on the Environment, University of Minnesota)

In 2022, Professor Douglas, along with Dickinson College students Sara (with whom I worked on this project) and Paige and other collaborators, published a paper mapping pesticide application across the United States. This paper is published in Scientific Data.

Introduction

The population of Monarch Butterfly (Danus plexippus) is declining. One of the earlier hypothesis attributes this decline to the loss of milkweed (Semmens et al., 2016). However, there is a newer hypothesis that proposes an alternate explanation: monarch population is declining because these butterflies fail to complete their fall migration (Inamine et al., 2016).

This project focuses on the second hypothesis and explores possible reasons for why monarchs might not be completing their migration. We identify a decline in floral resources to be hindering the migration. We assess how floral resources have changed along states where monarch frequently travel during their fall migration. Floral resources provides these butterflies with essential source of energy which is critical for these butterflies who migrate thousands of miles.

Research Question: Are floral resources declining in states where monarch travel during their migration?

Figure 1. Seven states (Arkansas, Kansas, Louisiana, Missouri, Nebraska, Oklahoma, and Texas) were selected to assess the floral resources.

Methods

Since there were no easily accessible floral resource data, proxy indicators were used to assess floral availability of the seven states. The proxy indicators used in this project are listed below:

Area Treated with Herbicide - USDA, National Agriculture Statistical Service (Data Agriculture Census)
Change in Herbicide Active Ingredient - USGS, Pesticide National Synthesis Report (County Level and State Level pesticide use estimates)
Change in Land Cover - USGS, Land Cover Modeling Data, EROS Center (Cropland, Shrub land, Grassland, and Hay/Pasture land)

Two software: R and ArcGIS was used to analyze these data sets. The results were also compiled and presented through GitHub.

Results

Area Treated with Herbicide

Almost all the area of seven states had increases in the 'percent area treated with herbicide'. Kansas and Nebraska were the two states with the most increase (42% and 36% respectively).

Changes in Herbicide Active Ingredient

We compiled a table of herbicide active ingredient that has increased and decreased from 1997 to 2012. However, the top 10 increases and decreases herbicide active ingredient were broad leaf herbicides. Simply looking at the tabulated list of herbicide active ingredient did not indicate if broad leaf herbicides were affecting floral resources.

We then spatially illustrated the changes of Glyphosate (which increased by 788%) and Dicamba (which increased by 120%). These spatial changes are presented in the figure below. These spatial differences hold key information about how different herbicide active ingredient affects different part of a monarch's life. For example, glyphosate has increased by a lot in the mid-western area - where monarch larvae feed, and dicamba has increased a lot in Texas funnel - where monarch funnel through on their way to Mexico.

Change in Land Cover

Finally, we analyzed percent changes in land cover to see how potential changes in land cover might change floral resources. We hypothesized that grassland and cropland would decrease, and hay and shrub land will increase. We expected these changes to cause the most decline in floral resources.

But all these four land cover changes were very small amount. 1% on the county level, and on a migratory scale - not amount to a lot of amount.

Discussions

It is possible that with the combination of increase in area treated with herbicide and no significant changes to the land cover, floral resources at these states is decreasing. The area occupied by floral resources have not increased significantly while the area treated with herbicide has increased. Floral resources, thus, might be decreasing which can cause fewer and fewer monarch butterfly to complete their fall migration.

It is also important to note, that our results provide no evidence to refute the 'milkweed limitation' hypothesis. Rather, this project opens up to the possibility that both hypothesis might play a role in hindering monarch population. Looking into the maps of 'Changes in Herbicide Active Ingredient' we can see how different herbicides affect different stages of monarch population. Considering the 'milkweed hypothesis' where the focus is on the milkweed producing mid-western states, it is possible that the use of glyphosate is decreasing milkweeds whereby decreasing monarch larvae population. Similarly, considering the 'migration failure' hypothesis, it is possible that herbicides such as dicamba might have adversely affected the floral resources of southern Texas whereby starving the travelling monarch butterfly of their food. This research concludes that paying attention to the spatial pattern of different herbicides in relations to monarch life cycle might be the best way forward in determining the causes of monarch population decline.

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