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：路边生境被视作包括种群衰退的君主斑蝶（monarch butterfly）在内的昆虫传粉者的保育机遇。君主斑蝶的壮观长途迁徙正受到生境丧失及其寄主植物马利筋（milkweed）种群衰退的威胁。在其分布范围的北部区域，由于马利筋种群密度较高，路边生境每年理论上可繁育出数百万只君主斑蝶，但路边的马利筋会累积道路带来的化学物质，例如道路融雪盐中的钠。 受控实验室研究显示，钠对君主斑蝶的发育存在混合效应：少量钠摄入可带来益处，因为钠是大脑与肌肉发育所需的重要微量营养素，但摄入量过高有时会降低存活率。目前尚不清楚膳食钠在生态相关的自然条件下以及迁徙过程本身会如何影响君主斑蝶的生存表现。 本实验中，我们在可诱导迁徙的室外条件下，将君主斑蝶饲养在喷洒了三种浓度氯化钠（sodium chloride）的马利筋上。我们共放飞2464只带有标记的君主斑蝶，并额外留存246只用于后续实验室检测分析。尽管本次实验中越冬栖息地的回收率较低（N=7只个体），但三种氯化钠浓度处理组的君主斑蝶均成功抵达墨西哥越冬地。 取食对照组马利筋与低盐马利筋的君主斑蝶，其体内会富集钠；而取食高盐马利筋的个体则会排出钠，这表明高盐处理组的钠摄入水平超出了其生理最优阈值。不同处理组对君主斑蝶的翅色、存活率、体型、免疫能力及寄生虫感染率均无显著影响。 综合来看，本实验结果表明，君主斑蝶对路边生境中马利筋的钠含量具有较强的耐受性，这对于路边生境作为君主斑蝶及其他种群衰退昆虫的保育栖息地的价值而言，是一个积极的信号。