Do Bees Prefer Spotted Knapweed Over Other Co-flowering Plant Species?
Grimmer, Jared P.
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Spotted knapweed (Centaurea stoebe L.) is a biennial herb native to Eurasia that was introduced to North America during the late 1800’s. Although considered a noxious, invasive weed, the plant exists as a floral resource for many insect pollinators, particularly bee species. This study used field and laboratory approaches to investigate the attractiveness to bees of knapweed flowers compared to those of co-occurring plant species. Transect surveys were used to catalog pollinator diversity and abundance on these plant species. From here, knapweed and four focal flowering plant species were selected for use in choice tests of attraction to common pollinators based on floral scent. Bumblebees (Bombus spp.) and honeybees (Apis spp.) were tested in a glass Y-maze apparatus. Each bee was tested for its preference to each flower’s scent against a blank control arm and against spotted knapweed. Inflorescence volatiles were collected from each plant species using Solid Phase Microextraction (SPME) and analyzed using gas chromatography mass-spectrometry (GC-MS). Field observations found the greatest number of bumblebees on knapweed, hyssop hedge nettle (Stachys hyssopifolia L.), and bee balm (Monarda fistulosa L.), but the most honeybees on goldenrod (Solidago speciosa L.) and the least on queen anne’s lace (Daucus carota L.). Y-maze experiments showed that the bumblebees generally preferred knapweed while honeybees seemed to prefer both knapweed and hyssop hedge nettle. Comparisons of volatile profiles and identified compounds add to what is known about knapweed attractiveness. The results of this project demonstrate knapweed’s attractiveness to bees based on floral scent. Further research should focus on the scent characteristics of the benefits that spotted knapweed provides to visiting bees relative to co-flowering species in the community.