Cyclic Changes in the Chemical Defense of the Eastern North American Population of the Monarch Butterfly

帝王蝶毒性的季节性急剧增加似乎对它们在北美春季和夏季的重新繁殖至关重要。帝王的幼虫以马利筋植物为食，并从它们身上隔离有毒的卡地那内酯。这些化学物质保护蝴蝶免受捕食，因为它们很苦，会导致鸟类呕吐。帝王蝶每年秋天迁徙到墨西哥，然后在次年春天再次入侵北美东部。由于卡登内酯在每年的迁徙周期中会发生变化，以不同种类的乳草为食的帝王蜂会以不同的速度丧失它们的威慑能力。秋季候鸟以一种催吐性很弱的乳草为食；随后卡地那内酯的损失导致他们在墨西哥的化学防御系统部分崩溃，在墨西哥，金莺和罗汉嘴每年冬天吃数百万只蝴蝶。然而，存活下来并迁徙回美国的帝王蝶在剧毒的南方马利筋上产卵，产生新一代明显有毒的蝴蝶，完成了春季向北迁徙到加拿大的过程。利用鸟类进行的生物测定和对十字花内酯进行的化学分析将证明春季世代的毒性增加。这些结果应该提供确凿的证据，证明帝王以马利筋为基础的化学防御是一个可预见的动态现象，与其每年的迁徙周期密不可分。墨西哥中部冷杉林的砍伐正在破坏帝王蝶越冬的栖息地。正因为如此，帝王在北美东部的迁徙被指定为濒危生物现象，一种新的生物多样性保护类别。这项研究计划将记录在未来10-15年内预测的帝王种群崩溃之前，春季重新迁徙的生物学特征。这也与保护北美东部的马利筋栖息地，以便在春季和夏季维持帝王种群有关。

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.