Oaks are some of our most iconic shade trees, playing a central role in many ecosystems. Soils, drainage, and fire created the red oak-sugar maple-basswood forest around Lake Minnetonka to the bur and pin oak savannas that once flanked the Mississippi. Bur oaks still line the bluffs, preserved in city parks and boulevards. They also shade many older neighborhoods, relics before settlers developed the Twin Cities.
Potential Issues
All trees face many potential opponents. Diseases, insect attacks, and weather are set upon our native oaks. They are also quite resilient, having evolved many means of defense.
These issues pose different risks to each oak species. Oak wilt, for example, will kill red oaks outright in weeks, while white oaks will keep it at bay for years. Bur oak blight, a fungal disease, will only infect bur oaks. These issues can have devastating effects on oak trees across Minnesota.
Bur oak is beginning to stabilize after treatment and pruning. Some further decline has occurred, but sprouting will fill in over time.
The Decline Spiral
Historically, TLCB has acted as a secondary pathogen. Adults lay their eggs on trees already stressed by a different primary pathogen. Oak wilt, bur oak blight, and construction damage are the most common primary factors. Oak wilt can disrupt the tree’s vascular system, causing stress. Bur oak blight can kill leaves and reduce the tree’s ability to feed itself. Soil compaction caused by construction damages roots and reduces their ability to absorb nutrients. These issues alone may not kill a tree but can cause significant stress.
TLCB will begin to attack an oak once it’s stressed. As the beetles do more and more damage, stress will increase, and the tree will start to die back and decline. This will attract more beetles, resulting in further damage and decline. We call this series of events the Decline Spiral. A common misconception is that the final step in the spiral is what killed the tree, in this case, TLCB. But, we must reflect on what tipped the tree into the spiral.
What’s Changing?
As climate change continues, we see several shifts that will have lasting repercussions. The most dramatic is the droughts we have had every summer since 2020. While the dry, sunny weather has made for great beach days, our trees are feeling the lack of water.
The stress caused by these droughts is beginning to change how TLCB and our native oaks interact. TLCB is acting more like a primary pathogen, like EAB.
As oaks become stressed, they can’t kill the larvae growing under their bark. This means more larvae survive, mate, and lay eggs. This has caused an increase in the borer population. Sometimes, the sheer number of larvae hatching in a single season overwhelms trees. We see this issue most in our bur oak population, but this also affects other oak species.
Because this change is very new, we still don’t know as much as we would like. We don’t know whether the droughts will continue, and we also don’t know if something else is at play. Research is being done, but we have not yet received much guidance. As a result, we, as arborists, must develop recommendations for our clients.
A boulevard bur oak in Saint Paul at 45 Snelling Ave N, showing signs of two-lined chestnut borer damage in September 2015. St. Paul Forestry treated and pruned this tree after this photo was taken. Google StreetView has a photographic record of this tree from today to 2007.
What Can We Do?
We also recommend that clients mulch around the trunk of their tree, out to the edge of the drip line. These practices will help the tree help itself and will have the best long-term benefit for the tree.
The same bur oak in May 2023, prior to leafing out. Note the substantial sprouting and the beginning development of new branches.
Our technicians can treat trees that are already showing tip dieback. If caught early, the treatment can arrest the damage and provide time for the tree to stabilize. But, treatment is not a guarantee. We have seen trees sustain significant damage in one season, resulting in trees dying. By the time we observed dieback, it was too late, even with treatment.
