Many monarchs that originate in the Midwestern US come from agricultural habitats. Pic milkweed in cornfield Monarch per plant densities in agricultural fields are as high as or higher than densities in nonagricultural habitats, and even though milkweed densities are higher in nonagricultural than in agricultural habitats, more monarchs originate in agricultural habitats because agricultural fields constitute such a large portion of the landscape (Oberhauser et al. 2001).

Fertilization could affect the performance of monarchs that originate in agricultural habitats. Fertilizer increases the supply of nitrogen for plants in the fields and generally increases the plants’ leaf nitrogen content (Mattson 1980). Diet nitrogen content often affects insect size, development time, and/or feeding. As a result, it indirectly alters variables linked to these factors, such as mating success, fecundity, the onset of diapause, and the time exposed to enemies (review in Slansky 1993).

To determine if common milkweed (Asclepias syriaca) from a fertilized agricultural habitat had higher leaf nitrogen content than common milkweed from an unfertilized nonagricultural habitat, I conducted an initial test in early June 2000. I randomly selected three common milkweed plants in each of two fields within 1 km of each other near New Richmond, Wisconsin: a fertilized soybean field and an unfertilized old field separated by a line of trees. I collected, dried, and milled the topmost mature leaf from each plant and determined the leaf nitrogen content (% dry weight) using a Perkin Elmer Series II CHNS/O Analyzer 2400. The milkweed in the soybean field had significantly higher leaf nitrogen content (5.28 ? 0.26%) than the milkweed in the old field (4.20 ? 0.26%) (Mann-Whitney U = 9.0, P = 0.05).

This finding, combined with the findings that many monarchs originate in agricultural fields and that the nitrogen content in insect diets often affects insect performance, led to my questions about the effects of varying nitrogen supply on monarchs. I studied how varying nitrogen supply affects common milkweed leaf nitrogen and monarch consumption rates and performance, using both glasshouse-grown and field-harvested common milkweed fed to laboratory-reared monarch larvae.

My experiments are described at the following links:
The Effects of Nitrogen Supply on Glasshouse-grown Common Milkweed Leaf Nitrogen Content and Condition and on Monarch Consumption Rates and Performance. Here, I report how fertilizers with two different amounts of nitrogen affected the leaf nitrogen and condition of common milkweed grown in a glasshouse and how feeding on plants from these two fertilizer treatments affected monarch consumption rates and performance.

Milkweed Leaf Nitrogen and Condition and Monarch Survivorship and Performance in an Agricultural and a Nonagricultural Habitat. Here, I report the variation in common milkweed leaf nitrogen content within and between ramets (aboveground stems) from a Wisconsin old field, the milkweed leaf nitrogen content and condition in cornfield and nonagricultural roadside habitats, and how feeding on milkweed harvested from these two habitats affected monarch survivoral and performance.