Monarch butterflies (Danaus plexippus) are in decline in the western United States and are encountering a range of anthropogenic stressors. Pesticides are among the factors that likely contribute to this decline, although the concentrations of these chemicals in non-crop plants are not well documented, especially in complex landscapes with a diversity of crop types and land uses. In this study, we collected 227 milkweed (Asclepias spp.) leaf samples from 19 sites representing different land use types across the Central Valley of California. We also sampled plants purchased from two stores that sell plants to home gardeners. We found 64 pesticides (25 insecticides, 27 fungicides, and 11 herbicides, as well as 1 adjuvant) out of a possible 262 in our screen. Pesticides were detected in every sample, even at sites with little or no pesticide use based on information from landowners. On average, approximately 9 compounds were detected per plant across all sites, with a range of 1–25 compounds in any one sample. For the vast majority of pesticides detected, we do not know the biological effects on monarch caterpillars that consume these plants; however, we did detect a few compounds for which effects on monarchs have been experimentally investigated. Chlorantraniliprole in particular was identified in 91% of our samples and found to exceed a tested LD50 for monarchs in 58 out of 227 samples. Our primary finding is the ubiquity of pesticides in milkweeds in an early summer window of time that monarch larvae are likely to be present in the area. Thus, these results are consistent with the hypothesis that pesticide exposure could be a contributing factor to monarch declines in the western United States. This highlights the need for a greater understanding of both the lethal and sublethal effects of these compounds (individually, additively, and synergistically) and suggests the urgent need for strategies that reduce pesticide use and movement on the landscape.

中文翻译：

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北加州低海拔的农业、城市和开放空间中乳草的农药污染

帝王蝶（Danaus plexippus）在美国西部正在减少，并面临一系列人为压力。农药是可能导致这种下降的因素之一，尽管这些化学物质在非作物植物中的浓度没有得到很好的记录，特别是在作物类型和土地用途多样化的复杂景观中。在这项研究中，我们从代表加利福尼亚中央谷地不同土地利用类型的 19 个地点收集了 227 份乳草（马利筋属）叶子样本。我们还采样了从两家向家庭园丁出售植物的商店购买的植物。我们在筛选的 262 种农药中发现了 64 种农药（25 种杀虫剂、27 种杀菌剂、11 种除草剂和 1 种佐剂）。每个样品都检测到农药，即使在根据土地所有者提供的信息很少使用或不使用杀虫剂的地点。平均而言，在所有地点的每株植物中检测到大约 9 种化合物，在任何一个样品中检测到 1-25 种化合物。对于检测到的绝大多数杀虫剂，我们不知道对食用这些植物的帝王毛虫的生物学影响；然而，我们确实检测到了一些化合物，它们对君主的影响已经进行了实验研究。特别是在我们 91% 的样本中发现了氯虫苯甲酰胺，并且发现在 227 个样本中的 58 个样本中超过了经测试的帝王蝶 LD50。我们的主要发现是，在该地区可能存在帝王蝶幼虫的初夏时期，乳草中普遍存在杀虫剂。因此，这些结果与农药暴露可能是美国西部帝王蝶数量下降的一个促成因素的假设一致。这突出表明需要更深入地了解这些化合物的致死和亚致死作用（单独、相加和协同作用），并表明迫切需要减少农药使用和在景观中移动的策略。