Knapweed can be friend or foe to pollinators

In the June 2016 issue of the Monthly Weed Post, a Montana State University Extension bulletin, author Jane Mangold highlights a research study that examines if invasive plants such as knapweed can be a friend or foe to pollinators. The study set out to determine how pollinator communities are affected by an invasive plant common to Montana and elsewhere, spotted knapweed. Researchers at Montana State University studied how density and growth stage of spotted knapweed affected pollinators.  Another study investigated impacts of spotted knapweed on reproduction of the native forb hairy goldenaster, this species shares many pollinators with spotted knapweed and their flowering periods overlap for six weeks.

In the first study, pollinators were observed at nine sites in western Montana where spotted knapweed was present at varying densities (0 to >15 plant / 16m sq ). Observation included pollinator abundance, specie richness, and flower visitation rates. Observations were also categorized into different spotted knapweed growth stages – pre-bloom, early bloom, peak bloom, and late bloom.  The second study took place near Bozeman, at a site dominated by native forbs. Because no spotted knapweed was present at the site, potted spotted knapweed plants were placed next to naturally-growing hairy goldenaster at varying densities- 0 to 4 spotted knapweed plants/ hairy goldenaster plant. Pollinator visitation and reproduction of the native plant was followed through one growing season.

Results showed 37 pollinator taxa visited spotted knapweed flowers, while 26 pollinator taxa exclusively visited native flowers. Most co-flowering species shared at least 50 percent of their pollinators with spotted knapweed. Prior to spotted knapweed blooming, pollinator abundance and richness was higher in plots without spotted knapweed versus plots with high density. However, during peak spotted knapweed bloom, high density spotted knapweed plots had greater pollinator abundance and richness than plots without spotted knapweed. In the second study, both spotted knapweed and hairy goldenaster were visited by a variety of pollinator groups such as: bumble bees, butterflies, beetles, flies, honey bees, wasps and other native bees. There was little evidence of competition for pollinators between spotted knapweed and hairy goldenaster at spotted knapweed densities used here.  

Researchers indicated that keeping spotted knapweed at low densities may be sufficient to prevent negative effects on pollinators-dependent native plant reproduction and may even be beneficial to some pollinators by providing floral resources later into summer. Furthermore competition for pollinators does not appear to be a mechanism by which spotted knapweed invades a native plant community, at least not in the early stages of invasion. In locations where spotted knapweed is too abundant to eradicate, maintaining ecologically tolerable thresholds is a reasonable management goal, which may come as good news to land managers faced with this widespread, persistent species.  

You can read more about this research at http://onlinelibrary.wiley.com/doi/10.1002/ecs2.1233/abstract