The biogeography of irruptive insect herbivores is determined by host availability and climate conditions. As such, outbreak distributions are sensitive to climatic change, especially across large latitudinal gradients. Here, we investigate the outbreak distributions of two understudied defoliators, hemlock sawfly Neodiprion tsugae (Hymenoptera) and western blackheaded budworm Acleris gloverana (Lepidoptera), that have both recently impacted the greatest land area recorded across the Pacific coastal temperate rainforest since the establishment of aerial survey programs. We compiled polygon-based estimates of insect damage collected by aerial observers, forest inventory, and downscaled climatic data to develop gridded estimates of bioclimatic conditions across the extent of the Pacific coastal temperate rainforest, including the continental United States, British Columbia and Alaska. We leveraged these data to develop ensemble machine learning models with the goal of predicting the outbreak distribution of each insect. In this manuscript we: 1) describe the historical patterns of defoliator outbreaks, 2) identify and describe climatic conditions associated with outbreaks in both species and 3) assess whether historic outbreaks have tracked geographic shifts in climate conditions across the region. We demonstrate that outbreaks of hemlock sawfly and western blackheaded budworm have been observed across the Pacific coastal temperature rainforests of North America in each decade since the establishment of the Canadian and United States aerial survey programs. The distribution of outbreaks by both insects were best explained by host availability, a limited range of spring, summer, and winter temperatures, and minimum precipitation. Finally, we demonstrate that outbreaks have tracked the poleward shift in suitable climate over the last century. This study establishes a baseline understanding of the climatic constraints and biogeographic patterns of historic sawfly and budworm outbreaks across the Pacific coastal temperate rainforest and emphasizes the overarching importance of climate in driving the irruptive dynamics of these defoliator species.

中文翻译：

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随着北美太平洋沿岸温带雨林变暖，落叶虫的爆发也随之而来


侵入性昆虫食草动物的生物地理学由宿主的可用性和气候条件决定。因此，疫情分布对气候变化很敏感，尤其是在大纬度梯度上。在这里，我们调查了两种尚未被研究的落叶昆虫——铁杉锯蝇Neodiprion tsugae （膜翅目）和西部黑头芽虫Acleris gloverana （鳞翅目）的爆发分布，这两种昆虫最近都影响了太平洋沿岸温带雨林自建立航空记录以来最大的陆地面积。调查计划。我们对空中观察员收集的昆虫损害、森林清查和缩小规模的气候数据进行了基于多边形的估计，以对整个太平洋沿海温带雨林（包括美国大陆、不列颠哥伦比亚省和阿拉斯加）的生物气候条件进行网格估计。我们利用这些数据开发集成机器学习模型，目标是预测每种昆虫的爆发分布。在这份手稿中，我们：1）描述了落叶虫爆发的历史模式，2）识别并描述了与这两个物种的爆发相关的气候条件，3）评估历史爆发是否跟踪了整个地区气候条件的地理变化。我们证明，自加拿大和美国航空调查计划建立以来，每十年都会在北美太平洋沿岸温雨林中观察到铁杉锯蝇和西部黑头芽虫的爆发。这两种昆虫爆发的分布可以通过宿主的可用性、有限的春季、夏季和冬季温度范围以及最低降水量来解释。 最后，我们证明，上个世纪，疫情的爆发是随着适宜气候的向极地转移而发生的。这项研究建立了对太平洋沿岸温带雨林历史上锯蝇和芽虫爆发的气候限制和生物地理模式的基线了解，并强调了气候在驱动这些落叶物种的侵入动态方面的首要重要性。