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Coke or Pepsi? Asclepias curassavica or Asclepias viridis

Posted on Saturday, January 1, 2005 at 10:20 am in Monarch Lab Research Updates

Bruce Leventhal

Animals make decision in their environment that can enhance or limit their survival. Like any other animal, monarch butterflies make decisions that influence their capacity to reproduce and survive.

During a summer-long study, I tried to determine if ovipositing (egg laying) monarch butterflies have a preferred milkweed species host. There are over 900 species of plants in the milkweed family Asclepiadaceae, of which more than one hundred can be found in North America. Milkweed (genus Asclepias) can vary in their leaf size and shape, stem length and growth pattern, or by the presence or absence of defensive hairs. Furthermore, plants may differ in their size, flower color and concentration of the toxic cardiac glycosides. Because monarch larvae are milkweed specific herbivores, a female butterfly will only lay her eggs on a milkweed plant. Which plant should she choose?

(Photo: B. Leventhal)

This simple question was the central focus of my study. To assess milkweed preference, I grew more than 200 milkweed plants from seed. Representing five species of milkweed, these plants exhibited a wide range of characteristics (see table). 

(Photo: MITC)

After counting over three thousand eggs laid by thirty butterflies, I found that monarch females prefer A. incarnata (swamp milkweed). Swamp milkweed is a tall growing plant with a low toxicity. Interestingly the plant with the highest toxicity, A. curassavica, received the second most eggs. So if butterflies do not choose their host plant based on toxicity, what characteristics do ovipositing females desire? Maybe if this is a question that you and your students can try to answer. 

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