Red, white, and bur oaks are some of our most-loved trees in Minnesota, and they face a mortal threat: oak wilt. Red oaks form the backbone of the red oak-sugar maple-basswood forests of the west metro. White and red oaks intermix through the Anoka sand plain and the southeast metro. Bur oaks line the uplands along the Mississippi and Minnesota river valleys and ring the Chain of Lakes in Minneapolis. These elder-state trees are often remnants of a time before European settlement and provide critical habitat and food for many critters.
Oak wilt is a vascular wilt disease, caused by a fungus. Similar to Dutch elm disease, oak wilt plugs the vascular system of a tree and causes it to wilt and die from the top down. Oak wilt behaves differently in oaks in the red oak group (red, black, northern, and eastern pin oaks) than in oaks in the white oak group (white, bur, swamp white oaks). Infected red oaks will die within a matter of weeks, while white oaks can often survive infection for many years and, occasionally, even completely recover on their own. See this post for more details on oak wilt and how it affects oak trees.
An Unwitting Vector
Photo by Wisconsin DNR
Picnic beetles are one of two ways oak wilt spreads from tree to tree. These beetles feed on the sap that oozes from wounds in an oak tree’s bark. The sap is full of defensive chemicals that fight off bacterial and fungal infections, a bit like white blood cells. Picnic beetles can “smell” these chemicals from very far away, and are attracted to the source of food. They only have a limited amount of time to feed, however. As soon as the wound occurs, the tree will begin to close off that portion of its vascular system and stop the flow of sap. Because picnic beetles do not have the mouthparts to create wounds from which to feed, they must be constantly on the lookout for their next meal.
In the spring, oak wilt forms spore mats on red oaks that died the previous season. These spore mats are oak wilt’s fruiting bodies, or what we commonly call mushrooms. A spore mat is as attractive to a picnic beetle as a morel is to an avid mushroom hunter. Oak wilt draws beetles to its mats with enticing pheromones and the promise of a free meal. The mat also deposits spores on the picnic beetles as they feed, turning the beetle into a vector. The beetle vectors will carry spores from infected to non-infected trees, with every visit having the chance of creating the next infection.
Why Does Pruning Timing Matter?
Trees constantly sustain small wounds when twigs break in the wind, hail impacts branches, or a particularly ambitious squirrel does their best parkour impression. Major storm damage can also create very large wounds. The size of the wound is one of many factors for oak wilt transmission, as large wounds present more surface area for sap-feeding. The number of wounds is also a major factor; the more wounds, the more attractive sap.
We also wound trees when pruning. Pruning to remove dead tissue does not usually cut into living tissue, and rarely releases sap. When we prune living branches for safety, maintenance, or risk reduction, we will create an opening for picnic beetles. Every wound presents oak wilt an opportunity. The worst time for oak wilt transmission is normally April-June when beetle activity coincides with spore mat formation. It is critical to never intentionally wound oaks during this high-risk period.
When Should I Prune My Oaks?
There is always some risk when picnic beetles are active, because of this we only recommend pruning oaks when beetles are dormant. We use the University of Wisconsin’s model for picnic beetle activity, which monitors Growing Degree Days (GDD). GDD are a measure of how much heat the landscape has been exposed to over the current year and can help us predict when specific insects will hatch. In most years picnic beetle hatch occurs in late March or early April, we also leave an extra buffer to be safe.
Regardless of whether we are able to prune oaks, our ISA Certified Arborists and Board Certified Master Arborists would be more than happy to review your trees with you. Contact us today to set up an appointment.
