Monarch butterfly (Danaus plexippus L.) migratory populations in North America have declined over the last 20 years. Milkweed (Asclepias, Apocynaceae) is the primary larval host plant for monarchs. The primary conservation strategy for monarchs has been to build habitat, with a focus on milkweed seeds and plugs. However, not all milkweed may offer the same benefits for monarch conservation, as monarchs may prefer some taxa over others, or taxa may vary in the nutrients and protective toxins they provide. Comparisons of milkweed suitability have been made for milkweed taxa in the midwestern and eastern United States (U.S.), but very little is known about milkweeds found in the southwestern U.S. We tested female egg-laying preferences and larval performance in captivity on two species of milkweed commonly available for habitat plantings in the Southwest: Arizona milkweed (Asclepias angustifolia Schweig.) and pineneedle milkweed (A. linaria Cav.). We tested four measures of larval performance: survival, duration, mass and adult forewing length. Monarchs were more likely to lay eggs on Arizona milkweed than pineneedle milkweed. Larvae had a higher survival rate and grew more rapidly on Arizona milkweed, and adults from larvae raised on Arizona milkweed were larger than those raised on pineneedle milkweed. In the field, monarch caterpillars were more likely to be found on Arizona milkweed. We recommend Arizona milkweed for monarch habitat conservation projects in the Southwest within the natural range of Arizona milkweed, but that D. plexippus preference and performance be tested for additional western milkweed species.

中文翻译：

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评估 Danaus plexippus 对两种西方马尾鱼的偏好和存活率

北美帝王蝶 (Danaus plexippus L.) 的迁徙种群在过去 20 年中有所下降。乳草（马利筋属，夹竹桃科）是君主的主要幼虫寄主植物。君主的主要保护策略是建立栖息地，重点是马利筋种子和插头。然而，并不是所有的乳草都可以为君主保护提供相同的好处，因为君主可能更喜欢某些分类群而不是其他分类群，或者分类群提供的营养和保护性毒素可能有所不同。已经对美国中西部和东部 (US) 的乳草类群进行了乳草适宜性的比较，但对美国西南部发现的乳草知之甚少 我们测试了两种常用于西南栖息地种植的马利筋的雌性产卵偏好和幼虫在圈养条件下的表现：亚利桑那马利筋（Asclepias angustifolia Schweig.）和松针马利筋（A. linaria Cav.）。我们测试了四种幼虫表现的衡量标准：存活率、持续时间、质量和成虫前翅长度。与松针乳草相比，君主更有可能在亚利桑那乳草上产卵。亚利桑那乳草上的幼虫存活率更高，生长更快，亚利桑那乳草上饲养的幼虫成虫比松针乳草上的成虫大。在野外，在亚利桑那州的乳草上发现帝王毛虫的可能性更大。我们推荐亚利桑那马利筋用于亚利桑那马利筋自然范围内西南地区的君主栖息地保护项目，但 D.