Analysis of Long-term Trends in Monarch Chemical Defenses

Common milkweed, Asclepias syriaca, produces steroid compounds called cardenolides to deter herbivores.  Cardenolides interfere with the function of the sodium potassium pump in cells and can disrupt heart rhythm.  In addition to this, they have an unpleasant, bitter taste.  Monarch butterfly caterpillars exclusively eat milkweed species.  They are able to do this by sequestering the cardenolides they consume.  The cardenolides stored by the monarch provide it with chemical defenses as both a caterpillar and an adult.  Monarch butterfly populations have been declining due to a multitude of causes.  One threat to monarch populations is predation by birds as they overwinter in Mexico.  Cardenolides have been shown to deter birds from eating monarchs.  Asclepias species have been declining in abundance as well, but  A. syriaca has adapted to modern land use practices better than other, more toxic species.  We predict that as the relative abundance of different Asclepias species has changed, a greater proportion of monarch caterpillars has fed on A. syriaca.  A. syriaca produces much lower levels of cardenolides than some other milkweed species.  The amount of cardenolides stored by monarchs is related to the amount of cardenolides in their diets overall.  Monarchs that fed on less toxic milkweed plants as caterpillars will be less chemically defended as adults.  This project will assess whether monarch butterflies have become less toxic over the past century as their diet has shifted towards weakly toxic A. syriaca and away from other, more toxic, milkweed species.  I will use High Performance Liquid Chromatography to analyze the cardenolide levels of monarch butterfly samples from the midwestern breeding population.  I will aim to use 100 butterfly samples that were collected between 1900 and today.  I will also assess A. syriaca samples from the same geographical range and time period to control for a change in cardenolide production by A. syriaca.