Tree diseases and insects of concern

Dutch elm disease

Since the establishment of Dutch elm disease in Minnesota in the 1970s, it’s killed hundreds of thousands of elm trees. Elm bark beetles spread the Dutch elm disease fungal pathogen by feeding on healthy elm trees. When the fungus is introduced into the water conducting system of the elm tree, the leaves wilt, yellow and often drop from the branches. The tree dies because in its attempt to block the flow of the fungus, it blocks its own ability to transport water. The disease can also spread directly from tree to tree where elm trees are growing within 50–100 feet because their root systems often graft, or fuse together. The best management strategy for controlling the spread of Dutch elm disease is to destroy the places where elm bark beetles breed—dead and dying elms with intact bark. Removing diseased elms, de-barking elm firewood, chipping elm brush and branches are good sanitation measures that can help keep the elm bark beetle population in check. For large elms, another option is to inject it with a preventive injection treatment.

Oak wilt disease

Like Dutch elm disease, oak wilt is caused by a fungal pathogen. However, rather than being spread primarily by insect vectors, the fungus is spread most often via connecting underground root systems within 50–100 feet. When the fungus is introduced into the water conducting system of a red oak tree, the leaves wilt, brown and drop from the branches. The disease can kill a red oak quite rapidly. The process is usually a little slower in white and bur oaks. The oak tree dies because in its attempt to block the flow of the fungus, it blocks its own ability to transport water. The best management strategies for controlling the spread of oak wilt are root graft disruption using a vibratory plow, preventative injection treatments and good sanitation.

Emerald ash borer

Emerald ash borer (EAB) is a non-native invasive insect from Asia that kills ash trees. According to the Minnesota Department of Agriculture (MDA), resistance against emerald ash borer has not been found in any native North American ash populations, and in areas where emerald ash borer has become established, ash tree mortality rates approach 100 percent. Once emerald ash borer infests an area, it cannot be eradicated. It has killed over 25 million ash trees in the United States. Minnesota is home to approximately 900 million ash trees. Emerald ash borer was discovered in St. Paul in 2009 and in Minneapolis in 2010. Learn more about emerald ash borer as it pertains to Minnetonka and learn how to prepare for the arrival of emerald ash borer.

Gypsy moth

Gypsy moth is a non-native insect that has been destroying and threatening woodlands in the United States for over a century. The gypsy moth caterpillar does the damage by feeding on the leaves of over 300 species of trees and shrubs. Their feeding takes place from early spring until July and can stress, defoliate and kill trees. Very few natural enemies and a seemingly endless array of hosts make the gypsy moth one of the most dangerous pests in the country.

While gypsy moth is not yet an established pest in Minnesota, it has been found here. State officials monitor the gypsy moth population and take actions to eradicate it when moths are detected. Treatments that target the gypsy moth in its caterpillar stage have been effective in suppressing its further spread and establishment.

Learn more about gypsy moth as it pertains to Minnetonka.