Improved performance of the eastern spruce budworm on black spruce as warming temperatures disrupt phenological defenses

Phenological shifts, induced by global warming, can lead to mismatch between closely interacting species. The eastern spruce budworm, Choristoneura fumiferana, an important outbreaking insect defoliator in North America, mainly feeds on balsam fir, Abies balsamea, which has historically been well synchronized with the insect. But as climate change pushes the northern range limit of the budworm further north towards the boreal forest, the highly valuable black spruce, Picea mariana, historically protected against the budworm by its late budburst phenology, is suffering increased defoliation during the current outbreak. We tested the hypothesis that rising temperatures can lead, not to a mismatch, but to an improved match between the budworm and black spruce through differential phenological advancement. For three years, eastern spruce budworm larvae were reared from instar 2 to pupae, on both black spruce and balsam fir, in a temperature free-air enhancement experiment (T-FACE) consisting in 24 field plots, half of which were heated at +2°C from March to October. Our results show that every year, larval development was faster on heated trees and pupation was earlier than on unheated trees. Bud development was also accelerated in heated trees of both species. However, there was no difference in mass between pupae that developed at +2°C and controls at the end of the season. Finally, we found no difference either in development rate or pupal mass between larvae reared on black spruce and those reared on balsam fir. This suggests that under higher temperature eastern spruce budworm will be as successful on black spruce as on balsam fir, as black spruce budburst becomes better synchronized with the insect’s emergence from diapause. This could lead to critical changes in outbreak dynamics and severity with important ecological state shifts at the landscape level.

全球变暖引发的物候变迁可能导致密切相互作用的物种之间出现不匹配。北美重要的爆发性害虫——东部云杉芽蛾（Choristoneura fumiferana），其历史上与云杉（Abies balsamea）的同步性良好，主要取食云杉。然而，随着气候变化将芽蛾的北部分布极限向北推移至北方针叶林，具有极高经济价值的黑云杉（Picea mariana）由于其晚期的芽爆发物候而历史上受到芽蛾的保护，在当前爆发期间遭受了严重的落叶。我们测试了温度升高可能导致芽蛾与黑云杉之间从不适配到通过差异性的物候提前而实现更佳匹配的假说。在为期三年的实验中，我们将东部云杉芽蛾从第二龄幼虫饲养至蛹，饲养的树木包括黑云杉和云杉，实验采用温度自由空气增强实验（T-FACE），共24个田间地块，其中一半地块从三月到十月加热至+2°C。我们的结果表明，每年在加热树木上幼虫的生长速度更快，羽化时间也比未加热树木更早。两种物种的芽发育也得到加速。然而，在季节结束时，+2°C下发育的蛹与对照蛹的重量没有差异。最后，我们发现，在黑云杉上饲养的幼虫与在云杉上饲养的幼虫在发育速度或蛹重量上也没有差异。这表明，在较高温度下，东部云杉芽蛾在黑云杉上的成功程度将与在云杉上相当，因为黑云杉的芽爆发将与昆虫从休眠状态中出现的物候更好地同步。这可能导致爆发动态和严重性的关键变化，并在景观层面上引发重要的生态状态转变。