Lack of Milkweed Is Not Harming Monarch Butterfly Populations
People who attended the 2013 Annual Meeting of the Entomological Society of America are likely to remember Dr. Anurag Agrawal, who talked about Dame Miriam Rothschild when he presented the Founders’ Memorial Award lecture. The ESA Founders’ Memorial Award is one of the highest honors that can be bestowed upon an entomologist.
Now Dr. Agrawal is talking again, but this time it’s about monarch butterflies.
Some groups have blamed herbicides and genetically modified crops for the decline of monarch populations, but new research by Dr. Agrawal and colleagues suggests that lack of milkweed is unlikely to be driving the monarch’s population decline.
Using years of data collected by the World Wildlife Fund and citizen scientists across North America, the researchers looked at monarch populations throughout North America during different times of the year. They found that monarch numbers begin declining at the end of the summer, when the butterflies begin their long migration to Mexico, and the numbers continue to decline as they travel. During this southern migration, adult monarchs do not feed on milkweed, so lack of milkweed is unlikely to be the problem, according to the researchers.
“Milkweed is probably not the limiting thing for monarch populations,” Dr. Agrawal said. “At the end of the summer in August, when they begin that epic journey down to Mexico, they no longer are feeding on milkweed, they’re not nectaring on their flowers. They are simply making that journey. The only thing they eat then is water and flower nectar from other species like golden rods.”
The research also suggests that planting milkweed is not likely to help increase monarch populations.
“By the time they get to Mexico their numbers are plummeting, but at the end of the summer when they start their migration, their numbers are not down,” Dr. Agrawal said. “That presents an important mystery, but it also presents an important finding — that perhaps planting milkweed will not improve or increase the population as much as we thought in the past. It would be a mistake to think that planting milkweed alone is going to solve this problem. Planting milkweed probably isn’t a bad thing to do, but it’s not going to increase their populations or save them from some demise.”
The research also seems to vindicate genetically modified crops that are herbicide resistant.
“Our findings suggest that monarch populations are not limited by milkweed, which in a way suggests that herbicides are not likely to be the problem, and genetically modified crops that are herbicide resistant are not likely to be the problem for the monarch,” Dr. Agrawal said. “Given the intense interest in monarch conservation, the blame being put on herbicide use, and the national dialog about potentially listing monarchs under the endangered species act, we have to get the science right.”
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