The Importance of Shifting Disturbance Regimes in Monarch Butterfly Decline and Recovery

Nathan L Haan,Douglas A Landis

doi:10.3389/fevo.2019.00191

Nathan L Haan, Douglas A Landis

Open Access

https://doi.org/10.3389/fevo.2019.00191

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Abstract

The Eastern migratory monarch butterfly has declined in recent decades, partly because widespread adoption of herbicide-resistant corn and soybean has nearly eliminated common milkweed from crop fields in the US Midwest. We argue that in addition to milkweed loss, monarch declines were likely exacerbated by shifting disturbance regimes within their summer breeding range. Monarchs prefer to lay eggs on younger, vegetative milkweed stems. They also benefit from enemy-free space, as most eggs and early-instar larvae succumb to predators. Historically, ecological disturbances during the growing season could have provided these conditions. During most of the 19th and 20th centuries, milkweed was abundant in crop fields where manual weeding and mechanical cultivation set milkweed back, but rather than killing it would often stimulate regrowth later in the summer. Before European settlement, large mammals and fires (natural and anthropogenic) perturbed grasslands during the summer and could have had similar effects. However, presently most remaining milkweed stems in the Midwest are in perennial grasslands like roadsides, old-fields, parks, and conservation reserves, which often lack growing season disturbance. As a result, monarchs may be left with limited options for oviposition as the summer progresses and could have lower survival in grasslands where predation pressure is high. Our recent work has shown that targeted disturbance during the growing season produces milkweed stems that are attractive to ovipositing monarchs and harbor fewer arthropod predators. Targeted disturbance in perennial grasslands could improve habitat heterogeneity and phenologic diversity of milkweeds, and should be explored as a monarch conservation strategy.

Highlights

The Eastern North American migratory population of monarch butterflies is in a decades-long decline and the migratory phenomenon is considered at risk (Brower et al, 2012; Vidal and Rendón-Salinas, 2014)
The majority of monarchs arriving in Mexico each year for overwintering are born in the Midwest and North Central US, where they lay most of their eggs on common milkweed, Asclepias syriaca L. (Malcolm et al, 1993; Flockhart et al, 2017)
Monarchs laid more eggs on milkweeds that regenerated after being cut back compared to those we left undisturbed, and predators took 2–4 weeks to recolonize the regenerating milkweed stems, potentially providing a window of enemy-free space. We believe these results suggest disturbance is an important process influencing monarch habitat suitability in the Midwest, and that some types of habitat could be enhanced with strategically-timed disturbance during the growing season (Figure 1)

Summary

Introduction

The Eastern North American migratory population of monarch butterflies is in a decades-long decline and the migratory phenomenon is considered at risk (Brower et al, 2012; Vidal and Rendón-Salinas, 2014). Until recently, milkweed stems growing in annual crop fields, mostly corn and soybeans, supported more monarch eggs and larvae than stems growing in other habitat types (Oberhauser et al, 2001).

Results

Conclusion