Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population
Monarch butterflies (Danaus plexippus L. Lepidoptera: Danainae) in the Eastern North American migratory population undergo a multi-generation annual cycle that includes wintering in central Mexico. In the spring, adults that have overwintered migrate north and reproduce in Texas and states to the north and east. Their offspring move farther north into much of the eastern half of the United States and southern Canada, and two to three more generations are produced (Cockrell et al., 1993; Malcolm et al., 1993; Prysby & Oberhauser, 2004). Most adults that emerge after mid-August are in a state of reproductive diapause (Herman, 1985; Goehring & Oberhauser, 2002) and migrate from the summer breeding range to their wintering grounds, where they remain until spring (Solensky, 2004).
Annual counts of the size of the overwintering population in Mexico indicate that themonarch population has been declining over the last decade and a half (Rendo´ n-Salinas et al., 2011; Brower et al., 2011b). One possible explanation for this decline is that monarch production has been decreasing as a result of a reduction in the availability of the larval host plant.
Monarch larvae feed primarily on milkweeds (genus Asclepias- Family Apocynaceae, subfamily Asclepiodeae).On the basis of milkweed cardenolide fingerprints, it has been estimated that 92% of the monarchs wintering in Mexico had fed as larvae on the common milkweed, Asclepias syriaca (Malcolm et al., 1993). Studies in Iowa found a large reduction in A. syriaca in corn (maize, Zea mays) and soybean (soya, Glycine max) fields from 1999 to 2009 (Hartzler & Buhler, 2000; Hartzler, 2010).
It is likely that a similar reduction has occurred throughout the region where corn and soybeans are predominantly grown. Eighty per cent of both corn and soybeans are grown in the Midwest (USDA, National Agricultural Statistics Service, 2011c), which is composed of the states of North and South Dakota, Nebraska, Kansas, Missouri, Iowa, Minnesota,Wisconsin, Illinois, Indiana, Michigan, and Ohio. A study in 2000 (Oberhauser et al., 2001) found that monarchs heavily used milkweeds in corn and soybean fields.
On the basis of stable isotope analysis, Wassenaar and Hobson (1998) estimated that half of the monarchs overwintering in Mexico in 1997 came from the Midwest. Thus, the Midwestern United States is at the epicentre of a reduction in milkweeds in agricultural fields and is also an area that has in recent history contributed a large component of the monarch population. In this study, we estimate the magnitude of this milkweed loss and its consequences for monarch production.