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.