Data for: "The potential of roadside verges as insect habitat: road salt has few effects on monarch butterfly performance and migration"

STUDY ABSTRACT: Roadside habitat has been touted as a conservation opportunity for insect pollinators, including the declining monarch butterfly. The spectacular monarch migration is under threat from the loss of habitat and the decline of their milkweed host plants. In the northern part of their range, roadsides could potentially produce millions of monarchs annually due to high densities of milkweed, however roadside milkweed can accumulate chemicals from roads, such as sodium from road salt. Controlled lab studies have shown mixed effects of sodium on monarch development: small increases can be beneficial as sodium is an important micronutrient in brain and muscle development, but large increases can sometimes decrease survival. It is unclear how dietary sodium affects performance in ecologically relevant conditions, and the migration itself. In this experiment, we raised monarchs outdoors, in migration-inducing conditions, on milkweed sprayed with three levels of sodium chloride. We released 2464 tagged monarchs and held an additional 246 for further lab assays. While our recovery rates to the wintering grounds were low (N = 7 individuals), individuals from all three sodium chloride treatments made it to Mexico. Butterflies reared on control milkweed and low salt concentrated sodium in their tissues, while those on high salt diets excreted sodium, suggesting levels were above a physiological optimum. There were no effects of treatment on wing coloration, survival, body size, immunity, or parasite prevalence. Taken together, our results suggest that monarchs are robust to levels of sodium in milkweeds found along roadsides, which is promising with respect to the value of roadside habitat for monarchs and other declining insects.

研究摘要：路边生境被视为包括帝王蝶（monarch butterfly）在内的昆虫传粉者的保护潜力栖息地。壮观的帝王蝶迁徙正受到栖息地丧失及其寄主植物马利筋（milkweed）种群衰减的双重威胁。在其分布范围的北部区域，由于马利筋种群密度较高，路边生境每年或可孕育数百万只帝王蝶，但路边的马利筋会积累道路沿线的化学物质，例如道路融雪盐释放的钠元素。
现有受控实验室研究显示，钠元素对帝王蝶发育的影响存在争议：少量钠元素提升可带来益处，因为钠是大脑与肌肉发育所需的重要微量营养素，但钠元素过量有时会降低个体存活率。目前尚不明确，在生态相关的自然条件下，膳食钠元素如何影响帝王蝶的生存表现，乃至其迁徙过程本身。
本实验中，我们在可诱导迁徙的室外环境中，将帝王蝶饲养于喷洒了三种浓度氯化钠（sodium chloride）的马利筋植株上。共放飞2464只佩戴标记的帝王蝶，另有246只被留存以开展后续实验室检测。尽管本次实验的越冬地回收率较低（样本量N=7），但三种氯化钠浓度处理组的帝王蝶均成功抵达墨西哥越冬地。
取食对照组马利筋与低钠浓度马利筋的帝王蝶，其体内会富集钠元素；而取食高钠日粮的个体则会排出钠元素，这表明高钠组的钠水平已超出其生理最优阈值。不同处理组对帝王蝶的翅色、存活率、体型、免疫能力以及寄生虫感染率均无显著影响。
综合来看，本研究结果表明，帝王蝶对路边马利筋中的钠元素水平具有较强耐受性，这为路边生境作为帝王蝶及其他种群衰减昆虫的保护栖息地的价值提供了积极支撑。