Pollen-removing ants interfere with pollination of Asclepias ovalifolia (Apocynaceae)

Background/Question/Methods

While there is growing appreciation of ants as pollinators, they are generally considered unable to pollinate milkweeds. Initial observations of the oval-leaved milkweed Asclepias ovalifolia revealed that a large ant species Formica obscuripes was capable of removing pollinaria. These findings motivated us to investigate the ability of this ant to be an effective pollinator. To do so we characterized the pollination system and analyzed the floral traits of A. ovalifolia. As ants are more likely to produce self- than cross-pollinations we tested for self-compatibility. Pollinator visit rates were estimated by field observations. Insects were captured to count how many pollinia they were carrying. To assess the effects of ants on reproduction, we imposed three insect exclusion treatments (“no insect,” “ant only,” and “all insect”), and recorded subsequent removal and deposition of pollinia, fruit set, and seed set.

Results/Conclusions

We found that of all insects, ants visited flowers at the highest rate and on average each ant carried at least one pollinarium. When compared to all insect visitors, ants alone removed similar numbers of pollinaria but deposited fewer. Ant-only plants also had a lower fruit set than all-insect plants, and fruits on ant-only plants produced no viable seeds. Hand-pollinations indicated self-incompatibility. Ants were found to re-visit the same plant multiple times daily. Based on ant movements and failure of plants to produce viable seeds when visited only by ants, it is likely that ants self-pollinate rather than cross-pollinate A. ovalifolia when they do deposit pollinia. As the milkweed appears to be self-incompatible, ants seem to act as antagonists rather than mutualists. These antagonistic effects are realized through nectar depletion, taking pollen from effective pollinators, preventing cross depositions by blocking stigmatic grooves with self-pollen, and discouraging other insects from landing on ant occupied plants. With these routes of interference, ant visitation may have negative impacts on the reproductive success of A. ovalifolia.