Predation of Monarch butterflies in Mexico

Readers of this blog will presumably be familiar with the fact that Monarch caterpillars feed on milkweed plants and sequester the toxic cardenolides in their bodies.  Naive birds that eat a caterpillar or adult butterfly will vomit and learn to avoid the species in the future (the nontoxic Viceroy butterfly has evolved coloration and a pattern similar to the Monarch in order to take advantage of this bird-repellant feature.)

Most of the predation of Monarchs that we see in Wisconsin comes from ants, wasps, and other insects attacking the eggs and young caterpillars.  This week I attended the biannual meeting of the Southern Wisconsin Butterfly Association and learned from a lecture presentation that major predation also occurs in the overwintering sites in the mountains of Mexico.

Overwintering butterflies that fall to the ground in the mountain roosts are eaten by mice:

An individual Peromyscus melanotis [black-eared mouse] consumes an average of 37 Monarchs each night. They feed preferentially on easily accessible butterflies (near the ground) and on "wet" butterflies (dead butterflies that have not dried out). During the winter season, P. melanotis migrate to Monarch roosts where they set up residence and breed intensively... Over the winter season, these mice may consume 4 to 5.7% of the total population of the Monarch colony. This translates into at least one million butterflies each winter in a 2.25 ha roost. 

Up in the trees, the Monarchs are eaten by two species of birds:

Black-headed grosbeaks and black-backed orioles have very different ways of avoiding poisoning when they eat Monarchs. Grosbeaks, which eat the entire Monarch abdomen, are relatively insensitive to cardenolides and can tolerate moderate levels of these chemicals in their digestive tract. Orioles, on the other hand, vomit after consuming much smaller amounts of cardenolides. They avoid poisoning by not eating the cuticle, which is where Monarchs store cardenolides. Orioles slit open the body and strip out the soft insides... Grosbeak and oriole predation causes more than 60% of Monarch mortality at many Mexican roosting sites, killing approximately 7 to 44% of the total population. At one 2.25 hectare colony, for example, birds ate an average of 15,000 butterflies daily and over 2 million for the season, which constituted 9% of the roost's population.

Interesting - especially the adaptive strategy employed by the orioles.

More information at Monarch Watch and Journey North.