Effect of parasitoids,seed‐predators and ant‐mutualists on fruiting success and germination of Acacia drepanolobium in Kenya

Acacia drepanolobium is an obligate ant‐plant that bears dehiscent pods exploited by predispersal seed‐predators and parasitoids. Fruit set and seed germination in relation to ant‐association, bruchid and parasitoid infestation were investigated in a large‐scale multi‐year study in Kenya. Ant‐association had a significant impact on the overall numbers of fruiting trees. 94.8% of Crematogaster mimosae‐occupied trees, 25.6% of C. nigriceps‐occupied trees, and 82.2% of Tetraponera penzigi‐occupied trees set fruit. Within each tree, ant‐association had no significant impact on the amount of seed produced: C. mimosae‐occupied trees produced 47.5 seeds per branch, C. nigriceps‐occupied trees 44.9 seeds per branch and T. penzigi‐occupied trees 38.3 seeds per branch. Ant‐association did not limit seed‐predators or vary significantly by ant. Seeds infested by bruchids germinated in significantly lower proportions (6.2%) compared to uninfested seed (78.6%). Bruchid‐infested seed is also exploited by parasitoid wasps. Parasitoids appear to have a moderate but significant ‘rescue’ effect on bruchid‐infested seed with 18.4% of parasitoid‐infested seed germinating. Stable isotopes (δ15N) revealed the trophic structure of the seed‐associated insects, showing clearly that bruchids are seed predators and the parasitoid wasp Dinarmus magnus exploits the dominant Bruchidius sp.