Forest Insect and Disease Newsletter

Bittersweet Battles

image: Aerial surveys over great river bluff state park (DOTs represent photo centroids)

The Department of Natural Resources (DNR) executed early detection and rapid response to Oriental bittersweet, Celastrus orbiculatus, a highly destructive invasive vine. Oriental bittersweet strangles and smothers forest stands. It can dominate tree canopies and reduce forest floor light to levels that prevent other plant species from growing. The vine weight compounded with snow and ice or high wind can break trees. We have swaths of forest downed by Oriental bittersweet in the city of Winona and rapidly developing infestations in the city of Red Wing. Laura Van Riper, Terrestrial Invasive Species Coordinator, recognized the vine at Frontenac State Park near Lake City. She notified Shawn Fritcher and other natural resource managers with the Parks and Trails Division. They destroyed the vines and seedlings. Parks and Trails will continue to control seedlings that emerge from the seedbank. Finding and eliminating this small quantity of Oriental bittersweet plants likely prevented an environmentally and economically damaging infestation that could have spread to other areas. The knowledge and concerted effort to achieve this success are commendable.

Oriental bittersweet is native to eastern Asia and was planted in North America for ornamental uses as early as 1736. The vines are easily propagated, grow vigorously in a wide range of environmental conditions, and produce attractive fruit that has been used by the cut-flower and craft industries. American bittersweet, C. scandens, is native to much of the temperate United States. American bittersweet vines are also vigorous but are much better behaved and produce larger and showier fruits. Both species are diecious and produce flowers but only female plants produce fruit. The species are easily distinguished by fruit (female plants) and flower placement (male and female plants) at the leaf axils of Oriental bittersweet versus only at the terminal ends of American bittersweet. Fruit capsule color is also a distinguishing factor. Oriental bittersweet has yellow fruit capsules and American has bright orange capsules.

The Winona and Red Wing infestations are decades old, but were not officially documented until 2010 and 2011 respectively. Minnesota Department of Transportation vegetation managers first alerted the Minnesota Department of Agriculture (MDA) to this issue. MDA and partners, including the green industry, responded promptly. An emergency noxious weed risk assessment and evaluation were performed by the Minnesota Noxious Weed Advisory Committee external link and Oriental bittersweet was designated a prohibited noxious weed on the eradicate list. Noxious weed status prohibits the sale and deliberate movement of Oriental bittersweet, can aid rapid response to new infestations, and facilitates qualification for federal programs that assist landowners with controlling existing infestations. DNR, MDA, and University of Minnesota Extension began training DNR staff, Forest Pest First Detectors and Master Gardeners to identify and report infestations. The green industry took steps to ensure that only American bittersweet plants are sold. These actions greatly reduce human mediated spread of Oriental bittersweet.

Containing and controlling existing infestations remains a dire challenge. Oriental bittersweet continues to spread inadvertently by wildlife and people. Wildlife, especially birds, consume the fruit and move the seed to new locations. People collect the fruiting branches to make seasonal decorations and thereby move seed to new locations. Garden related publications and television programs do not always correctly distinguish bittersweet species and thus confuse the public. This complicates our educational challenge. Although we face these challenges, important first steps were taken that are now having a positive impact. This is evidenced by the DNR's early detection and rapid response to the Oriental bittersweet find at Frontenac State Park.