Insect visitation increases seed abundance and quality in invasive Russian olive ( Elaeagnus angustifolia )

Insect visitation is often assumed to increase the abundance and quality of plant seed production, thereby increasing plant recruitment. However, this assumption is often untested. Many invasive plants present substantial floral resources, but whether insect visitation facilitates invasiveness through effects on seed abundance or quality is often not known. Russian olive ( Elaeagnus angustifolia L.), once planted as a beneficial tree in the western United States, has spread within riparian zones at an alarming rate and impacts native plants and wildlife. It is not well understood how arthropods interact with Russian olive expansion. Pollinating insects have largely been overlooked in previous studies, and what insects visit Russian olive flowers and effects on berry/seed production are unknown.

We conducted flower‐visiting insect surveys on blooming Russian olive in western North Dakota/eastern Montana over a 2‐year period. Additionally, a bagging experiment and germination trials were conducted in 1 year to determine if insect visitation affects berry/seed set or seed quality.

Overall, we found numerous insect species visiting Russian olive flowers. These included 13 polylectic native bee species, but the dominant flower visitors were western honey bees which comprised over 80% of all insect visits. Nearly 35% of flowers that were allowed potential insect visitation produced berries/seeds compared to 5% of flowers in which insects were excluded. In addition to increasing seed abundance, insect visitation increased seed weight and the time period over which seeds germinated.

Synthesis and applications . Insect visitation substantially increases seed output and quality, demonstrating the importance of floral visitors in the Russian olive system. Russian olive dominance may reduce native bee habitat in invaded riparian communities, as oligolectic bees do not seem attracted to Russian olive flowers. Conversely, reducing Russian olive flower abundance or attractiveness is unlikely to limit native pollinators but may indirectly affect the weed. If management tools like biological control reduce cross pollination, reductions in seed quality may negatively impact Russian olive recruitment beyond the direct effects of plant damage. Linking insect visitation to direct and indirect influences on weed reproductive success is important to understanding weed invasion and management interactions.