Ecol Appl. 2014 Jun;24(4):759-69. doi: 10.1890/13-0157.1.
The impact of plant defenses on insect herbivores is widely accepted, but their relative effects on oviposition choice, survival, and larval growth in preventing pest damage, especially for invasive insects, is not fully understood. Here, we examined the potential for plant defenses to reduce the economic and environmental impacts of an invasive herbivore, the viburnum leaf beetle, VLB (Pyrrhalta viburni), on Viburnum species in North America. We used a common garden with 15 host Viburnum species of North American, European, and Asian origin and evaluated oviposition preferences, twig defense against oviposition (a reaction that crushes VLB eggs), larval performance in the lab and field, and foliar damage to mature shrubs in two consecutive years. VLB oviposition preference was the strongest predictor of plant damage, with twig defense and larval performance explaining little of the defoliation patterns. In particular, we showed that VLB females evade key defenses by choosing poorly defended twigs for oviposition; assays on the 15 Viburnum species revealed that adults laid over four times more eggs on dead (undefended) twigs than on living twigs. We additionally tested the hypothesis that shrubs with a higher proportion of dead twigs are preferentially chosen for oviposition, leading to more defoliation by larvae and increased dieback in the following year. We term this the infestation feedback hypothesis. Indeed, we report consistent positive correlations between percentage dieback, oviposition, and percentage defoliation across Viburnum species, and among individuals within two species tested separately (V. dentatum and V. opulus). Our results demonstrate that oviposition preference plays a major role in the susceptibility of Viburnum shrubs to the invasive VLB through adults choosing high-quality species for their larvae (a strong preference-performance correlation) and avoiding well-defended twigs among preferred species. More generally, where invasive insects can avoid plant defenses and when preference and performance are positively correlated, an infestation feedback loop can lead to persistent pest problems. Because dieback weakens Viburnum defenses by providing optimal oviposition sites, we recommend that Viburnum growers mechanically remove dead twigs from susceptible shrubs at the end of the growing season, especially in the early stages of VLB colonization.
植物防御对昆虫食草动物的影响已被广泛认可，但它们对防止害虫损害的产卵选择、生存和幼虫生长的相对影响，特别是对于入侵昆虫，还不完全清楚。在这里，我们研究了植物防御的潜力，以减少北美 Viburnum 物种上的入侵食草动物，即 Viburnum 叶甲（Pyrrhalta viburni）的经济和环境影响。我们使用了一个有 15 种北美、欧洲和亚洲原产的宿主 Viburnum 物种的普通花园，并评估了产卵偏好、嫩枝对产卵的防御（一种压碎 Viburnum 卵的反应）、实验室和野外幼虫性能，以及两年内成熟灌木的叶片损伤。VLB 的产卵偏好是植物损伤的最强预测因子，嫩枝防御和幼虫性能对落叶模式的解释很少。特别是，我们表明，VLB 雌性通过选择产卵防御较弱的嫩枝来规避关键防御；对 15 种 Viburnum 物种的测定表明，成虫在死亡（无防御）嫩枝上产卵的次数是活嫩枝上的四倍多。我们还测试了这样一个假设，即死枝比例较高的灌木更优先选择产卵，导致幼虫更多的落叶和次年更多的枯萎。我们将其称为侵染反馈假说。事实上，我们报告了在 Viburnum 物种之间，以及在我们分别测试的两个物种（V. dentatum 和 V. opulus）的个体之间，死亡率、产卵和落叶百分比之间存在一致的正相关关系。我们的研究结果表明，产卵偏好通过成虫为幼虫选择高质量的物种（强烈的偏好-性能相关性），以及在首选物种中避免防御良好的嫩枝，在 Viburnum 灌木对入侵 VLB 的易感性中起着主要作用。更一般地说，当入侵昆虫能够避免植物防御，并且偏好和性能呈正相关时，侵染反馈循环可能导致持续的害虫问题。由于枯萎通过提供最佳产卵场所削弱了 Viburnum 的防御能力，我们建议 Viburnum 种植者在生长季节结束时从易受感染的灌木上机械去除死枝，尤其是在 VLB 定植的早期阶段。
