More milkweed in farmlands containing small, annual crop fields and many hedgerows

Abstract

Milkweed has declined substantially, with over 80% declines in some agricultural regions. This threatens monarch butterfly (Danaus plexippus) persistence, because monarch larvae feed solely on milkweed. Thus conservation actions are needed to enhance the availability of milkweed, particularly in agricultural landscapes. Conservation actions to date have largely focused on reducing intensive agricultural practices, mainly use of herbicides. However, research suggests that landscape-scale alteration of the cropped portion of an agricultural landscape (the "farmland"), for example, to reduce crop field sizes, can benefit herbaceous plants such as milkweed. Here we collected data on milkweed occurrence and cover in agricultural landscapes in Ontario, Canada, capturing variability in milkweed from field edge to interior by sampling in the interior and along the edges of 68 crop fields. We used these data to evaluate the relative effects of farming practices within the sampled field (e.g. herbicide, fertilizer use) on milkweed versus the effects of mean field size, crop diversity, hedgerow cover, and the proportion of farmland in annual crops in the surrounding landscape. Additionally, we evaluated the effects of these variables on the cover of other herbaceous plants, to identify which—if any—could benefit milkweed without increasing overall weed cover. We found more milkweed at sites surrounded by landscapes with smaller crop fields, lower crop diversity, and higher cover of annual crops. Milkweed was more likely to occur at sites surrounded by landscapes with more hedgerows. These landscape-scale effects on milkweed were often larger than those of within-field farming practices. Importantly, we found that most variables had opposite effects on milkweed relative to other plants. Thus, altering the landscape to benefit milkweed does not imply an increase in weed cover.