The population of monarch butterflies east of the Rocky Mountains has noticeably declined over the past two decades. The decline is due, in part, to loss of breeding and forage habitat in the Southern and Midwestern USA. To support a resilient overwintering population of six hectares of occupied forest canopy, approximately 1.6–1.8 billion additional ramets of milkweed are needed in the summer breeding range. Milkweed establishment that facilitates natural behavior of monarchs is necessary for effective conservation restoration. This study explored the effect of milkweed ramet density on larval search behavior, milkweed utilization, and survival without predation, parasitism, or competition. Under our experimental greenhouse conditions, monarch larvae abandoned their natal ramet, and subsequent ramets, prior to the pre-pupal wandering stage and before all available leaf biomass on a ramet was consumed. This is consistent with previous field observations. Larvae consumed biomass from three or four milkweed ramets that totaled the approximate biomass of single 10–35 cm ramet. Movement behavior suggests that isolated ramets may not support development through pupation, even though an isolated ramet could provide enough biomass. Our results suggest milkweed patches containing at least two to four ramets of closely-spaced common milkweed would provide sufficient biomass for development and increase the likelihood that larvae moving in random directions would encounter non-natal ramets to support development. Larval movement behavior and biomass requirements are critical aspects of monarch larval biology that should be considered in habitat restoration and maintenance plans, monitoring survey designs and protocols, and population modeling.

中文翻译：

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君主幼虫（Danaus plexippus）对普通马利筋（Asclepias syriaca）利用的估计和幼虫运动的意义

在过去的二十年里，落基山脉以东的帝王蝶数量明显减少。下降的部分原因是美国南部和中西部的繁殖和饲料栖息地的丧失。为了支持 6 公顷被占用的森林冠层的有弹性的越冬种群，夏季繁殖范围内需要大约 1.6-18 亿株马利筋。促进君主自然行为的乳草建立对于有效的保护恢复是必要的。本研究探讨了马利筋分株密度对幼虫搜索行为、马利筋利用和没有捕食、寄生或竞争的生存的影响。在我们的实验温室条件下，君主幼虫放弃了它们的出生分株，随后的分株，在前蛹游动阶段之前和在分株上所有可用的叶子生物量被消耗之前。这与先前的实地观察一致。幼虫消耗来自三个或四个乳草分株的生物量，总计大约单个 10-35 厘米分株的生物量。运动行为表明，即使分离的分株可以提供足够的生物量，分离的分株也可能不支持通过化蛹发育。我们的结果表明，含有至少两到四个紧密排列的普通马利筋分株的马利筋斑块将为发育提供足够的生物量，并增加向随机方向移动的幼虫遇到非出生分株以支持发育的可能性。