Data from: Trade-offs in parasitism efficiency and brood size mediate parasitoid coexistence, with implications for biological control of the invasive emerald ash borer

1. Parasitoids often are selected for use as biological control agents because of their high host specificity, yet such host specificity can result in strong interspecific competition. Few studies have examined whether and how various extrinsic factors (such as parasitism efficiency, i.e. the ability to optimize host-finding attack rates) influence the outcome of competition between parasitoids, even though they could have profound effects on the implementation of classical biological control programmes. 2. To determine the potential influence of extrinsic competition and coexistence on host suppression efficacy, we compared parasitism by two larval parasitoids (Tetrastichus planipennisi and Spathius galinae) of the invasive emerald ash borer (EAB) Agrilus planipennis, under different host densities, parasitoid densities, host plant sizes and parasitoid–host ratios. 3. Spathius galinae had significantly higher parasitism efficiency (≈4 times), but significantly lower brood size (>6 times) than that of T. planipennisi. The attack rates of hosts increased significantly with parasitoid density, whereas host density did not significantly affect multiparasitism. The parasitism rate of T. planipennisi on small host logs was significantly higher than that on large logs, while host plant (log) size had no significant impact on S. galinae parasitism. 4. The multiparasitism rate was rather low regardless of host log size and parasitoid/host density, indicating that intrinsic competition between the two species of parasitoids might seldom occur in the field. The two species of parasitoids could therefore coexist in the same habitat, and any adverse effects on the suppression of EAB populations caused by competitive behaviour between the two species of parasitoids would likely be negligible. 5. Synthesis and applications. Our findings suggest that introducing multiple species of parasitic natural enemies could be feasible for management of invasive species, but it is important to examine multiple extrinsic factors simultaneously when evaluating interspecific competition between them. Among these different extrinsic factors, we found that coexistence between parasitoids can be mediated by trade-offs in their parasitism efficiency and brood sizes. Thus, the differences in life-history traits of natural enemies could be used to select among biological control agents being considered for releases.

1. 寄生蜂（Parasitoids）常因其高寄主专一性（host specificity）被选作生物防治天敌（biological control agents），但这类寄主专一性往往会引发强烈的种间竞争（interspecific competition）。目前鲜有研究探讨各类外在因子（extrinsic factors，如寄生效率（parasitism efficiency），即优化寄主搜寻攻击率（host-finding attack rates）的能力）是否以及如何影响寄生蜂间的竞争结局，尽管这些因子可能对经典生物防治项目（classical biological control programmes）的实施产生深远影响。
2. 为明确外在竞争与共存对寄主抑制效能（host suppression efficacy）的潜在影响，本研究在不同寄主密度、寄生蜂密度、寄主植物大小及寄生蜂-寄主比例条件下，比较了两种入侵性害虫——翡翠灰螟（Agrilus planipennis，简称EAB）的幼虫寄生蜂（larval parasitoids）：白蜡花翅跳小蜂（Tetrastichus planipennisi）与伽氏脊柄茧蜂（Spathius galinae）的寄生情况。
3. 相较于白蜡花翅跳小蜂，伽氏脊柄茧蜂的寄生效率显著更高（约为其4倍），但子代数量（brood size）却显著更少（仅约为其1/6）。寄主的攻击率随寄生蜂密度升高显著提升，而寄主密度对多重寄生（multiparasitism）无显著影响。白蜡花翅跳小蜂在小型寄主木段（host logs）上的寄生率显著高于大型木段，而寄主植物（木段）大小对伽氏脊柄茧蜂的寄生率无显著影响。
4. 无论寄主木段大小、寄生蜂/寄主密度如何，多重寄生率均处于较低水平，这表明两种寄生蜂间的内在竞争（intrinsic competition）在野外可能极少发生。因此，这两种寄生蜂可在同一栖息地共存，二者间的竞争行为对翡翠灰螟种群抑制所产生的任何不利影响大概率可忽略不计。
5. 总结与应用。本研究结果表明，引入多种寄生性天敌可用于入侵物种的综合治理，但在评估寄生蜂间的种间竞争时，需同时考量多种外在因子。在这些不同的外在因子中，我们发现寄生蜂间的共存可通过其寄生效率与子代数量间的权衡关系进行调控。因此，可利用天敌的生活史特征（life-history traits）差异，筛选候选生物防治天敌用于田间释放。