Parasitoid development and superparasitism in invasive versus native widow spider host egg sacs

The brown widow spider, Latrodectus geometricus, is a hazardous and highly invasive species globally. One of the suggested mechanisms enhancing this spider's invasiveness is lower susceptibility to natural enemies compared to other widow species. The parasitoid wasp, Philolema latrodecti, is known to attack egg sacs of L. geometricus, as well as those of other species of Latrodectus. Despite its potential importance as a natural enemy of L. geometricus, little is known about the development of this wasp and factors mediating its parasitism success. We investigated the development of this parasitoid in two widow spider host species, one native to Israel, the white widow, L. pallidus, and the invasive brown widow, L. geometricus. We compared wasp developmental success in egg sacs exposed to increasing wasp densities. We found increased wasp emergence and brood size, and smaller emerging wasp body size, with increasing wasp density. Across all wasp densities, more and larger parasitoids developed in the egg sacs of the native host species, L. pallidus, compared to the invasive host species, L. geometricus. Parasitoid sex ratio was highly female-biased and the proportion of males increased with wasp density, suggesting local mate competition. Overall, our results suggest that L. pallidus is a better host for the egg sac parasitoid P. latrodecti, which could give a competitive advantage to the less-affected invasive brown widow spider.

棕色寡妇蛛（Latrodectus geometricus）是一种兼具危害性与全球入侵性的物种。目前学界提出，相较于其他寇蛛物种，该蜘蛛对天敌的易感性更低，这一特性或是增强其入侵能力的关键机制之一。寄生蜂（Philolema latrodecti）已知可攻击几何寇蛛（Latrodectus geometricus）及其他寇蛛属物种的卵囊。尽管该寄生蜂作为几何寇蛛的天敌具有潜在的生态重要性，但目前学界对其发育过程以及调控其寄生成功率的相关因素仍知之甚少。本研究选取两种寇蛛作为宿主，开展该寄生蜂的发育相关探究：其一为原产于以色列的白寇蛛（Latrodectus pallidus），其二为入侵性的棕色寡妇蛛（Latrodectus geometricus）。研究团队比较了不同寄生蜂密度梯度下，卵囊中寄生蜂的发育成功率。结果显示，随着寄生蜂密度升高，寄生蜂的羽化率及后代种群规模均显著提升，而羽化个体的体型则随之减小。在所有密度梯度下，本土宿主白寇蛛的卵囊中发育出的寄生蜂个体数量更多、体型更大，整体表现优于入侵宿主几何寇蛛的卵囊。寄生蜂的性比呈现显著的雌性偏倚，且雄性比例随寄生蜂密度升高而上升，这一现象暗示存在局部配偶竞争。综合来看，本研究结果表明白寇蛛（Latrodectus pallidus）对于卵囊寄生蜂Philolema latrodecti而言是更适宜的宿主，这一发现或可为受寄生影响更小的入侵性棕色寡妇蛛（Latrodectus geometricus）带来竞争优势。