Monarchs and Milkweed: A Migrating Butterfly, A Poisonous Plant, and Their Remarkable Story of Coevolution. Anurag Agrawal. Princeton University Press. 2017. 296 p.

The book covers the relationship between the monarch and the milkweed. This is a quality published book with abundant references.

While I found the subject interesting, I would caution the reader that this is a very in-depth book. I would not recommend it for someone interested in a basic overview of the subject, but if you like more scientific-directed information, this book has you covered.

One topic I found interesting is the ability of a female monarch to determine the amount of cardenolides in a particular milkweed plant. Too little, and the monarch larva is susceptible to predators; too high, and the plant will kill the monarch larva. There is an optimal level for caterpillar diet, and while science has not determined how, the monarch can determine the level before laying an egg.

Another trait monarch caterpillars exhibit is the what is called phenotypically plastic behavior. What this means is that the monarch larva can determine the type of milkweed species it is on, in terms of the plant’s defensive mechanism, and adapt its feeding accordingly.

An example given is the butterfly weed, which has little latex. A caterpillar will simply feed on this plant. If the egg is laid on a sandhill milkweed, which has a very slippery leaf surface, the caterpillar will fall off, so before feeding it will put down a silk mat to hold onto during feeding. This flexibility allows the larva to adapt to whichever species of milkweed plant the egg is laid on. I find this to be an amazing trait.