Non-pollinator fig wasp impact on the reproductive success of an invasive fig tree: why so little?

Classical biological control agents fail to achieve an impact on their hosts for a variety of reasons and an understanding of why they fail can help shape decisions on subsequent releases. Ornamental <i>Ficus microcarpa</i> is a widely planted avenue fig tree that is invasive in countries where its pollinator (<i>Eupristina verticillata</i>) is also introduced. This tree also supports more than 20 species of non-pollinating fig wasps (NPFW) that feed in the figs and have the potential to reduce the plant’s reproduction. <i>Odontofroggatia galili</i>, one of the most widely introduced NPFW, has larvae that develop in galled ovules that might otherwise develop into seeds or support pollinator larvae. We examined the distribution and relative abundance of the pollinator and <i>O. galili</i> on <i>F. microcarpa</i> in China, towards the northern limit of the tree’s natural range, and in Italy where the two species have been introduced. Where they co-existed, we also recorded the impact of varying densities of <i>O. galili</i> on <i>F. microcarpa</i> seed and pollinator production. <i>O. galili</i> and <i>E. verticillata</i> displayed contrasting habitat preferences in China, with <i>O. galili</i> almost absent from warmer sites. <i>O. galili</i> abundance and sex ratios varied between the natural and introduced ranges. Figs with more <i>O. galili</i> contained fewer seeds and pollinator offspring, but reproduction was rarely inhibited totally. Additional species with a greater impact in the figs they occupy are needed if biocontrol of <i>F. microcarpa</i> is to be effective.

经典生物防治因子（classical biological control agents）往往因多种原因无法对寄主产生预期防控效果，明确其失效机制可为后续释放决策提供科学指导。观赏细叶榕（Ficus microcarpa）是广泛种植的行道榕树，在其传粉榕小蜂（Eupristina verticillata）一同传入的国家已成为入侵物种。该树同时支撑着20余种非传粉榕小蜂（non-pollinating fig wasps, NPFW），这些小蜂在榕果内取食，有可能降低宿主植物的繁殖能力。加拉利齿榕小蜂（Odontofroggatia galili）作为引入范围最广的NPFW之一，其幼虫寄生于虫瘿化的胚珠中——这类胚珠若未被寄生，本可发育为种子或为传粉榕小蜂幼虫提供营养。本研究针对细叶榕自然分布北缘的中国区域，以及已引入该树与两种共生生物的意大利，调查了传粉榕小蜂与加拉利齿榕小蜂的分布及相对多度。在二者共存的样地中，本研究同时记录了加拉利齿榕小蜂不同密度对细叶榕种子产量及传粉榕小蜂繁殖的影响。在中国区域内，加拉利齿榕小蜂与传粉榕小蜂的生境偏好呈现显著差异：加拉利齿榕小蜂几乎不会出现在温度较高的样地。加拉利齿榕小蜂的种群多度与性比在其自然分布区与引入区之间存在显著差异。榕果内加拉利齿榕小蜂数量越多，其内的种子与传粉榕小蜂后代数量越少，但细叶榕的繁殖几乎从未被完全抑制。若要实现对细叶榕的有效生物防治，尚需筛选出在榕果内具有更强防控效果的其他非传粉榕小蜂物种。