Abstract Spruce budworm (Choristoneura fumiferana Clem.) defoliation of balsam fir (Abies balsamea (L.) Mill.) has been shown to be less severe in stands with higher hardwood (broadleaved trees) content during both building and declining phases of budworm outbreaks. Reduced defoliation associated with forest composition could be explained by habitat fragmentation and/or natural enemy hypotheses, which posit higher early-instar larval dispersal loss or more intense parasitism in hardwood-softwood mixed stands than in pure balsam fir stands, respectively. We carried out field studies in 27 hardwood, mixedwood, and softwood plots with varied balsam fir-hardwood composition to assess effects of forest composition on: 1) first- and second-instar larval dispersal loss, and 2) stage-specific spruce budworm density and parasitism rates. Results indicated that increasing hardwood content increased second-instar (L2) dispersal losses. Stand type significantly affected dispersal loss of L2 larvae, both measured directly for three years and sampled on balsam fir seedlings. Post-hoc pair-wise comparisons indicated that L2 dispersal loss was less in softwood than in mixedwood or hardwood stand types based on measurements on ground traps and less in softwood and mixedwood than in hardwood stands based on sampling budworm larvae on regeneration. This suggested that high early-instar larval dispersal loss is a plausible explanation for the decreased spruce budworm density and balsam fir defoliation associated with increasing hardwood content. Stand type did not significantly affect parasitism rates.

中文翻译：

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硬木-软木成分影响云杉幼虫爆发期间的早龄幼虫传播死亡率

摘要 云杉 budworm (Choristoneura fumiferana Clem.) 对香脂冷杉 (Abies balsamea (L.) Mill.) 的落叶已被证明在芽虫爆发的建立和衰退阶段在硬木（阔叶树）含量较高的林分中不太严重。与森林组成相关的落叶减少可以通过栖息地破碎和/或天敌假说来解释，这些假说分别表明硬木 - 软木混合林分比纯香脂冷杉林分更高的早龄幼虫扩散损失或更强烈的寄生。我们在 27 个具有不同香脂冷杉-硬木成分的硬木、混交木和软木地块中进行了实地研究，以评估森林成分对以下方面的影响：1) 一龄和二龄幼虫传播损失，以及 2) 特定阶段的云杉幼虫密度和寄生率。结果表明，增加硬木含量会增加二龄 (L2) 扩散损失。林分类型显着影响 L2 幼虫的传播损失，直接测量三年并在香脂冷杉幼苗上取样。事后成对比较表明，软木中的 L2 扩散损失低于基于地面陷阱测量的混合木或硬木林分类型，而软木和混合木中的 L2 扩散损失低于基于再生时对芽虫幼虫进行采样的硬木林分。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。林分类型显着影响 L2 幼虫的传播损失，直接测量三年并在香脂冷杉幼苗上取样。事后成对比较表明，软木中的 L2 扩散损失低于基于地面陷阱测量的混合木或硬木林分类型，而软木和混合木中的 L2 扩散损失低于基于再生时对芽虫幼虫进行采样的硬木林分。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。林分类型显着影响 L2 幼虫的传播损失，直接测量三年并在香脂冷杉幼苗上取样。事后成对比较表明，软木中的 L2 扩散损失低于基于地面陷阱测量的混合木或硬木林分类型，而软木和混合木中的 L2 扩散损失低于基于再生时对芽虫幼虫进行采样的硬木林分。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。事后成对比较表明，软木中的 L2 扩散损失低于基于地面陷阱测量的混合木或硬木林分类型，而软木和混合木中的 L2 扩散损失低于基于再生时对芽虫幼虫进行采样的硬木林分。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。事后成对比较表明，软木中的 L2 扩散损失低于基于地面陷阱测量的混合木或硬木林分类型，而软木和混合木中的 L2 扩散损失低于基于再生时对芽虫幼虫进行采样的硬木林分。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。这表明早龄幼虫的大量传播损失是与增加的硬木含量相关的云杉幼虫密度降低和香脂冷杉落叶的合理解释。立柱类型对寄生率没有显着影响。