A dramatic seasonal increase in toxicity of monarch butterflies appears crucial to their spring and summer recolonization of North America. Monarchs larvae feed on milkweed plants and sequester poisonous cardenolides from them. These chemicals protect the butterflies from predation because they are bitter and cause birds to vomit. Monarch butterflies migrate each fall to Mexico, then reinvade eastern North America the following spring. Since cardenolides change during the annual migration cycle, monarchs feeding on different milkweed species lose their deterrent qualities at different rates. Fall migrants feed on a weakly emetic milkweed; subsequent cardenolide loss leads to a partial breakdown of their chemical defense in Mexico, where orioles and grosbeaks eat millions of butterflies each winter. However, monarchs that survive and migrate back to the USA lay their eggs on extremely poisonous southern milkweeds, producing a new generation of apparently toxic butterflies that complete the spring migration northward to Canada. Bioassays using birds and chemical assays of the cardenolides will document the postulated increase in toxicity of the spring generation. The results should provide definitive evidence that the milkweed-based chemical defense of the monarch is a predictably dynamic phenomenon inextricably entwined with its annual migration cycle. %%% Logging of the fir forests in central Mexico is destroying the overwintering habitat of the monarch butterfly. Because of this, the monarch's migration in Eastern North America has been designated an endangered biological phenomenon, a new category of biodiversity conservation. This research program will document the biology of the spring remigration before the predicted collapse of the monarch population over the next 10-15 years. It is also relevant to conservation of milkweed habitat in eastern North America for the maintence of monarch population in spring & summer.
