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.