Myers Andrew, Bahlai Christie A, Landis Douglas A
Department of Entomology, Michigan State University, East Lansing, MI.
Program in Ecology, Evolutionary Biology, and Behavior, Michigan State University, East Lansing, MI.
Environ Entomol. 2019 Jun 7;48(3):675-684. doi: 10.1093/ee/nvz046.
As agricultural practices intensify, species once common in agricultural landscapes are declining in abundance. One such species is the monarch butterfly (Danaus plexippus L.), whose eastern North American population has decreased approximately 80% during the past 20 yr. One hypothesis explaining the monarch's decline is reduced breeding habitat via loss of common milkweed (Asclepias syriaca L.) from agricultural landscapes in the north central United States due to the adoption of herbicide-tolerant row crops. Current efforts to enhance monarch breeding habitat primarily involve restoring milkweed in perennial grasslands. However, prior surveys found fewer monarch eggs on common milkweed in grassland versus crop habitats, indicating potential preference for oviposition in row crop habitats, or alternatively, greater egg loss to predation in grasslands. We tested these alternative mechanisms by measuring oviposition and egg predation on potted A. syriaca host plants. Our study revealed that habitat context influences both monarch oviposition preference and egg predation rates and that these patterns vary by year. We found higher monarch egg predation rates during the first 24 h after exposure and that much of the predation occurs at night. Overall, we documented up to 90% egg mortality over 72 h in perennial grasslands, while predation rates in corn were lower (10-30% mortality) and more consistent between years. These findings demonstrate that weekly monarch egg surveys are too infrequent to distinguish oviposition habitat preferences from losses due to egg predation and suggest that monarch restoration efforts need to provide both attractive and safe habitats for monarch reproduction.
随着农业生产方式的集约化，曾经在农业景观中常见的物种数量正在减少。帝王蝶（Danaus plexippus L.）就是这样一种物种，其在北美东部的种群数量在过去20年中减少了约80%。一种解释帝王蝶数量下降的假说是，由于美国中北部农业景观中普通乳草（Asclepias syriaca L.）因种植耐除草剂的条播作物而丧失，导致其繁殖栖息地减少。目前增强帝王蝶繁殖栖息地的努力主要包括在多年生草原上恢复乳草。然而，先前的调查发现，与作物栖息地相比，草原上普通乳草上的帝王蝶卵较少，这表明帝王蝶可能更喜欢在条播作物栖息地产卵，或者相反，草原上的卵更容易被捕食。我们通过测量盆栽普通乳草寄主植物上的产卵和卵被捕食情况来测试这些替代机制。我们的研究表明，栖息地环境会影响帝王蝶的产卵偏好和卵被捕食率，而且这些模式会因年份而异。我们发现，暴露后的头24小时内帝王蝶卵的被捕食率较高，而且大部分捕食发生在夜间。总体而言，我们记录到多年生草原上72小时内的卵死亡率高达90%，而玉米地中的捕食率较低（死亡率为10%-30%），且年份间更为一致。这些发现表明，每周一次的帝王蝶卵调查频率过低，无法区分产卵栖息地偏好和卵被捕食造成的损失，并表明帝王蝶恢复工作需要为帝王蝶繁殖提供既有吸引力又安全的栖息地。
