Reproductive strategy of a rare gynodioecious shrub, Sidalcea hickmannii ssp. novum (Malvaceae), in southwestern Oregon

Background/Question/Methods

Sidalcea hickmannii ssp. novum (Malvaceae) is a rare perennial shrub of southwestern Oregon recently targeted by the Bureau of Land Management (BLM) as a high priority species for conservation efforts. This study sought to assist BLM by determining the reproductive strategy of this rare plant. Sidalcea hickmannii ssp. novum is gynodioecious with either hermaphroditic or female-only plants. This study compared fruit set from hermaphroditic versus female-only plants whose stems were either bagged (pollinator-excluded) or un-bagged (left open to insect pollination). Twenty-five stems were bagged and 25 were un-bagged on hermaphroditic plants and 29 stems were bagged and 29 were left open on female-only plants. The number of flowers per stem and the number of fruits per stem were determined and compared between open and pollinator-excluded stems and between hermaphroditic and female-only plants using standard t-tests on sample means. To determine potential pollinators of S. hickmannii ssp. novum, insects observed visiting the flowers were collected using an aerial net and identified.

Results/Conclusions

Seventy-eight specimens from 6 orders representing 28 species of insects were collected from flowering S. hickmannii ssp. novum plants. Hymenoptera was the most abundant order (41 specimens) with 15 species dominated by Megachilidae. Twenty-three specimens (6 species) of Coleoptera were also collected, where Trichodes was most abundant (13 specimens). Although only 3 specimens of Anthonomus (Curculionidae) were netted in the field, significant numbers of specimens (143) were collected from individual flowers of both the female-only and hermaphroditic plants where they were observed feeding on developing fruit. A comparison of the seed predator Anthonomus revealed a significantly greater number in hermaphroditic plants compared to female-only plants but not between bagged and un-bagged plants. Anthonomus may have contributed to a marginally greater number of fruits per stem in female-only compared to hermaphroditic plants. Seed predation has occurred in other gynodioecious Sidalcea species. A significantly greater number of fruits per stem on un-bagged vs. bagged plants and the abundance of Hoplitis and Osmia sp. as well as Trichodes sp. suggested that these insects may be important in fruit formation. No significant differences were found in the number of flowers per stem between female-only vs. hermaphroditic plants or between bagged vs. un-bagged plants. This study provided background information on the breeding system of this species that will assist future propagation and restoration projects. Future research on the mating systems of these rare plants are planned to determine pollinators’ role in seed set.