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.

Invertebrate responses to the management of genetically modified herbicide–tolerant and conventional spring crops. II. Within-field epigeal and aerial arthropods

“…In the USA, milkweed (Asclepiadaceae) growing within maize fields has been shown to provide a resource for larvae of the monarch butterfly, Danaus plexippus (Oberhauser et al 2001). In the UK, within-field weeds are considered to be of little benefit to larvae, particularly in comparison with field-margin vegetation (Feber & Smith 1995).…”

Ecological Impacts of Genetically Modified Crops: Ten Years of Field Research and Commercial Cultivation

“…Extensive follow-up studies thus determined where the monarchs occur during their breeding season [65], and what percentage of the population of monarchs is possibly affected be the Bttoxin in areas where Bt-maize is presently grown [66]. The results showed that larval exposure to pollen on a population-wide basis is low, given the proportion of larvae in maize fields during pollen shed, the proportion of Bt-maize fields, and the levels of pollen within and around maize fields [65]. The proportion of monarch butterfly population exposed to Bt-pollen was estimated to be less than 0.8% [66].…”

“…It was concluded that the risk of exposure is low and that it is unlikely that Bt-maize will affect the sustainability of monarch butterfly populations in North America [66,67]. Furthermore, several authors claimed that effects of Bt-maize should be compared to mortality caused by other factors, which is very high in natural monarch butterfly populations, and averages around 80% over the entire larval development period [65,67]. More important factors that may influence monarch butterfly survival include loss of over-wintering habitats in Mexico, use of insecticides to control lepidopteran pests and accidents such as collision with automobiles [57].…”

The release of genetically modified crops into the environment

“…This study led to considerable debate over the environmental impact and relevance for the potential risks from Bt maize. Follow-up studies to investigate the impact of widespread plantings Bt-maize on the Monarch butterfly essentially concluded that the impact of Bt-maize pollen from current commercial hybrids on Monarch butterfly populations is negligible Oberhauser et al, 2001;Pleasants et al, 2001;Sears et al, 2001;Stanley-Horn et al, 2001;Zangerl et al, 2001). This is based on the low expression of Bt toxin genes in pollen for most maize hybrids, lack of acute toxicity at expected field rates, limited overlap of pollen shed and larval activity, and the limited overlap in distribution of Bt-maize and milkweed.…”