Spotted knapweed: balancing ecological conservation with economic investment
Spotted knapweed, or star thistle, Centaurea maculosa, is an invasive flowering plant that was first observed in Michigan in 1911 . As an invasive, it is able to out-compete native species, it is environmentally detrimental, and difficult to extirpate. It can out-compete native vegetation by releasing growth-inhibiting chemicals into the soil . It also has a robust root system, grows well in disturbed habitats, and is distasteful to grazers, which allows it to spread rapidly and impair native plant communities [1,2]. Furthermore, it can cause soil erosion and thus diminished water quality . As a result, it can have far-reaching bottom-up effects on Michigan ecosystems.
Because of this, spotted knapweed is seen as a serious problem by the Michigan Department of Natural Resources . As such, several land management techniques have been suggested, including uprooting, mowing, burning, herbiciding, and using biological control such as the knapweed root weevil (Cyphocleonus achates) and the lesser knapweed flower weevil (Larinus minutus) [1,2].
However, Michigan beekeepers are opposed to control of spotted knapweed, as they have come to rely on this flower as an abundant forage resource for their colonies . Spotted knapweed blooms in mid- to late-summer, a time when nectar resources are scarce, and produces quality honey . Quality forage is essential to honey bee health and productivity  and honey bees are economically important through their contributions to crop pollination and honey production .
To balance the ecological conservation goals of the MI DNR, with the economic interests of the honey bee industry, land managers have suggested attempting to control spotted knapweed, and replace it with late-blooming wildflowers [2,3]. This would provide bees with necessary forage, and would restore native habitats. Of course, this plan will take extensive financial support to implement, as native prairie conservation plantings require more site preparation, management and inputs than volunteer weedy species. Additionally, wildflower plantings typically optimized to support native bee populations may not be the best for the honey bee industry; honey bees may be less able to produce a successful honey crop on diverse wildflower plantings than on a monoculture of honey plants.
Figuring out the best solution for spotted knapweed control will require continued communication, collaboration and compromise between beekeepers and land managers. While beekeepers are still reluctant to cede spotted knapweed as a nectar resource, hopefully state land managers can ease the transition by understanding their needs and compromising to achieve the big-picture goals.
For more information:
1. Garcia, Kary Askew. “Uprooting Invasive Knapweed.” Spotted Knapweed. Michigan Nature Association, 2 July 2014.
2. Landis, Doug, “Spotted Knapweed FAQ.” (2012): 1-4. Michigan State University, 22 Aug. 2012.
3. Michigan Bee Keepers’ Association. “Honey Bee Population…The Cornerstone of Agricultural Bounty.” An Open Letter Regarding the Eradication of Star Thistle (Spotted Knapweed). 5 Mar. 2011. MS. Michigan
4. “Michigan’s Plants (An Overview).” Plants & Habitat at Risk, Michigan Department of Natural Resources.