Optimizing the Nutrition of Roadside Plants for Pollinators

Project summary:

Pollinators have suffered steep declines over the last two decades. Humans rely on bees for pollination of over 70% of crops; however, some species have declined by 90% over the last two decades. Monarch butterflies, which the American public is willing to spend over $4 billion to save, have declined more than 80% over the last 20 years. There are increasing calls for pollinator conservation including everything from the White House's recent "Pollinator Research Action Plan," to state-specific legislation from Minnesota to California, to 7 million acres of new pollinator habitat. One promising direction is the use of roadside habitat as a source of pollinator larval and adult resources. Within Minnesota alone, there are over a million acres of roadside habitat. This research will result in site-specific recommendations for roadside plantings in Minnesota to maximize the health of bees and butterflies that rely on such habitats as conservation corridors. While roadsides are promising pollinator habitat, there is a concern that they may act as ecological sinks, which attract pollinators but result in declines in pollinator health. For instance, plants along roadsides can accumulate sodium from salt runoff. Sodium is a potent attractant for many animals, and while it is an important micronutrient, it can be fed on to the point of toxicity. Nitrogen also accumulates on roadsides, potentially attracting animals with protein-poor diets to areas that suffer from heavy metal buildup from brake and tire wear. Pesticide spillover from adjacent agricultural fields may additionally limit the quality of roadside habitats. Because different plants accumulate chemicals to different degrees, this research aims to develop recommendations for plantings for various road use intensities that optimize the value of roadside milkweed for monarchs and flower pollen and nectar for bees. The first task of this project is to survey roadside plants and pollinators at 50 sites across Minnesota and to measure the nitrogen, sodium, heavy metal, and insecticide content of leaves, nectar, and pollen from four species of plants favored by pollinators. The second task is to use data from the roadside surveys to rear monarchs and bumblebees in nutritional conditions simulating high, medium, and low use road intensities to determine levels at which nutrients and heavy metals become toxic. The third task is to use controlled field manipulations to determine how plants of different families accumulate nutrients and heavy metals under conditions simulating high, medium, and low road use intensity. Taken together, these data can speak to which species of plants should be prioritized in the restoration of roadsides for pollinators.