Temporal and spatial overlap between monarch larvae and corn pollen

Abstract

To assess the likelihood that monarch larvae will be exposed to Bacillus thuringiensis (Bt) pollen, we studied milkweed and monarch densities in habitats which comprise much of the land available to breeding monarchs, e.g., cornfields, cornfield edges, other agricultural fields, and nonagricultural areas, in four regions of the monarch breeding range. We found that monarchs use milkweed in cornfields throughout their breeding season, and that per plant densities are as high or higher in agricultural habitats as in nonagricultural habitats. As a result of the prevalence of agricultural land, most of the monarchs produced in the upper Midwest are likely to originate in cornfields or other agricultural habitats. There was a greater temporal overlap between susceptible monarchs and corn anthesis in the northern than the southern part of the summer breeding range, because of earlier pollen shed in the south. The importance of agricultural habitats to monarch production suggests that, regardless of the impact of genetically modified crops, agricultural practices such as weed control and foliar insecticide use could have large impacts on monarch populations.

Figure 1

Average egg densities over time for (a) Maryland, (b) Minnesota/Wisconsin, (c) Ontario, and (d) Iowa. Bars represent standard error. Date is the first day of the week in which data were collected. Note that y axis scale varies between regions.

Figure 2

Average height of milkweed and corn plants for one Minnesota/Wisconsin site. The pattern was similar in all regions, with corn at least twice the height of milkweed by early July.

Figure 3

Survival calculated as the proportion of each stage observed relative to the number of eggs observed in (a) Minnesota/Wisconsin, (b) Ontario, (c) Maryland, and (d) Iowa. Egg numbers are in parentheses. Data on first instars were not used in Iowa calculations because of inconsistencies in identifying this life stage.