Data from: Fitness consequences of indirect plant defence in the annual weed, Sinapis arvensis

1. Plant traits that enhance the attraction of the natural enemies of their herbivores have been postulated to function as an ‘indirect defence’. An important underlying assumption is that this enhanced attraction results in increased plant fitness due to reduced herbivory. This assumption has been rarely tested. 2. We investigated whether there are fitness consequences for the charlock mustard Sinapis arvensis, a short-lived outcrossing annual weedy plant, when exposed to groups of large cabbage white (Pieris brassicae) caterpillars parasitized by either one of two wasp species, Hyposoter ebeninus and Cotesia glomerata, that allow the host to grow during parasitism. Hyposoter ebeninus is solitary and greatly reduces host growth compared with healthy caterpillars, whereas C. glomerata is gregarious and allows the host to grow approximately as large as unparasitized caterpillars. Both healthy and parasitized P. brassicae caterpillars initially feed on the foliage, but later stages preferentially consume the flowers. 3. In a garden experiment, plants damaged by parasitized caterpillars produced more seeds than conspecific plants damaged by unparasitized caterpillars. Reproductive potential (germination success multiplied by total seed number) was similar for plants that were not exposed to herbivory and those that were damaged by parasitized caterpillars and lower for plants that were damaged by healthy unparasitized caterpillars. However, these quantitative seed traits negatively correlated with the qualitative seed traits, individual seed size and germination success, suggesting a trade-off between these two types of traits. 4. We show that parasitism of insect herbivores that feed on reproductive plant tissues may have positive fitness consequences for S. arvensis. The extent to which plant fitness may benefit depends on parasitoid lifestyle (solitary or gregarious), which is correlated with the amount of damage inflicted on these tissues by the parasitized host.1. 能够增强植食性昆虫天敌吸引力的植物性状，被推测可发挥“间接防御（indirect defence）”的功能。其核心隐含假设为：这种增强的吸引力可通过减少植食性昆虫取食，提升植物适合度（plant fitness）。但该假设迄今鲜有验证。 2. 本研究以短命异交一年生杂草植物——野芥子（charlock mustard）田芥菜（Sinapis arvensis）为研究对象，探究其暴露于被两种寄生蜂物种（Hyposoter ebeninus与Cotesia glomerata）寄生的大菜粉蝶（Pieris brassicae）幼虫群体时，是否会产生适合度相关的后果。其中，这两种寄生蜂均可在寄生期间允许宿主幼虫生长：独居型寄生蜂H. ebeninus会大幅抑制宿主幼虫的生长，相较健康幼虫而言；而群居型寄生蜂C. glomerata则可让宿主幼虫生长至与未被寄生的健康幼虫相近的体型。健康与被寄生的大菜粉蝶幼虫初期均取食叶片，但发育至后期则优先取食花部组织。 3. 在田间栽培实验中，被寄生幼虫取食损伤的植株，其结籽量高于被未寄生健康幼虫损伤的同种植株。未暴露于植食性昆虫取食的植株，与被寄生幼虫损伤的植株，二者的生殖潜力（萌发率乘以总结籽数）无显著差异；而被健康未寄生幼虫损伤的植株，其生殖潜力则更低。不过，此类种子数量性状与种子质量性状（单粒种子大小与萌发率）呈负相关，表明两类性状间存在权衡（trade-off）关系。 4. 本研究证实：取食植物生殖组织的植食性昆虫被寄生后，可对田芥菜（Sinapis arvensis）产生正向的适合度效应。植物适合度获益的程度，取决于寄生蜂（parasitoid）的生活型（独居型或群居型），而该生活型与被寄生宿主对植物生殖组织造成的损伤程度密切相关。