Ants interfere with the reproduction of oval-leaved milkweed (Asclepias ovalifolia)

Background/Question/Methods

Despite considerable work on ant-plant interactions, our understanding of ants' effects on plant reproduction remains fragmentary. Ants can affect the fitness of plants that they visit indirectly by changing the behavior of other visitors. Specifically, ants are known to steal nectar from flowers and thus indirectly inhibit pollination by flying insects. However, ants rarely affect plant fitness directly by removing or depositing pollen, especially in milkweeds that have pollen packaged into large pollinia. Therefore, a report that large ants, especially the western thatch ant (Formica obscuripes), reduced the male component of fitness of oval-leaved milkweed (Asclepias ovalifolia) by removing – but not depositing – many pollinia was surprising. Following up on this finding, we wanted to determine whether the thatch ants also reduce the female component of fitness of the milkweeds, which could have consequences for population growth in this regionally threatened species. In a population of oval-leaved milkweed at Cedar Creek Ecosystem Science Reserve in Minnesota, we applied treatments to the plants that selectively excluded crawling insects (mostly ants), selectively excluded flying insects, excluded all insects, or excluded no insects. Reproductive success was measured as pollinia removals and depositions, fruit set, and number of viable seeds per fruit.

Results/Conclusions

Plants from which ants were excluded had fewer pollinia removed per flower than plants open to all insects, supporting the earlier finding that ants remove a substantial proportion of pollinia. Plants from which ants were excluded also had fewer pollinia deposited per flower than plants open to all insects. However, the pollinia deposited on plants from which ants were excluded resulted in more fruits, and these fruits contained more viable seeds, compared with plants open to all insects. This is consistent with the pollinia deposited by ants being self-pollinations that not only failed to produce seed but prevented subsequent cross-pollinations from producing seeds due to late-acting self-incompatibility. We conclude that large ants, particularly thatch ants, directly reduce female as well as male components of fitness in oval-leaved milkweed. Preliminary evidence suggests that this also occurs in other North American milkweed species that interact with other large ants; it is a striking and intriguing exception to the efficient cross-pollination that is typical of milkweeds and other groups with aggregated pollen.