Fly visiting highbush cranberry
by Kornhauser & Mulder
In Alaska’s boreal forests, plants are starting to flower earlier than they used to, and this could affect how they get pollinated — especially plants that bloom early in the season. One of the first plants to flower in the boreal forest understory is highbush cranberry (Viburnum edule). Because it flowers so early, there may not be many pollinators (like bees and flies) around yet. We studied how the timing of flowering affects how much pollen the flowers receive and which insects visit them. This work was done over two years: one year when the plants bloomed earlier than usual, and another year when the timing was close to average. We used branches of flowers set to bloom either at the start or at the peak of flowering for each year.
When we kept insects away from the flowers, fruit production dropped by more than 90%. But even when insects had full access, fewer than 10% of the flowers turned into fruit. Flowers that bloomed later had more insect visitors and were more likely to receive pollen. In the earlier-blooming year, the flowers that bloomed later also received more pollen per flower. Nearly all insect visitors were syrphid flies, solitary bees, and muscid flies, with syrphid flies showing up more often on later-blooming flowers. Even though there seemed to be enough pollen for the earliest flowers (more than 25 grains on average per flower), fruit production was still low. This suggests that either the pollen isn’t being transferred effectively between genetically different plants, or the plants don’t have enough resources to make fruit. Because flies (especially syrphid and muscid flies) are the main pollinators, we need to better understand what causes them to emerge in the spring in order to see whether earlier blooming could lead to a mismatch between plants and their pollinators in Alaska’s boreal forests.
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