The pruning season for many woody plants has been especially difficult to determine this year as the temperatures have swung wildly between frigid and too warm for pruning certain species. Temperature is critical in determining when it is safe to prune oak trees. This is due to the activity of the beetles that carry oak wilt. These insects are highly attracted to fresh wounds and can arrive within minutes to potentially infect the trees. Pruning should not be done when the beetles are active, but determining when it is safe can be difficult.

I have been getting a lot of questions lately about whether it is still safe to prune oaks and what the ‘cutoff’ date is. Unfortunately, it is not that simple, especially with climate change creating such wide and unpredictable oscillations in temperature.

The general advice from arborists had been to only prune between November 1 to April 1 in southern Wisconsin. Oak wilt is a lethal fungal disease that can sometimes kill oaks in the red-black group within a single season. There are two species of beetle that are typically involved in transmitting it: sap-feeding beetles in the family Nitidulidae, and oak bark beetles. The recommendations on pruning were put in place to avoid pruning when the beetles are active. Being cold-blooded creatures, insects’ developmental stages and activity are based on temperature. Development corresponds to the accrual of heat units above a base temperature, often 50°F but in this case, 41°F for the two beetle species in question. The accrual of heat units is called ‘degree days’, and there are charts that have been developed to match degree days with the development of many of our pest insects, such as the spongy moth. This is helpful, if you know the number of degree days that have accumulated in your area (or close to your area) at any particular time, you can extrapolate whether a particular pest insect is hatching from eggs, feeding as a nymph or larva, pupating, or is in the adult stage. Using this information, you can often get ahead of the damaging stages of development by being prepared to act if needed. Degree days are used by vegetable and fruit growers as well as agricultural producers to help guide management decisions.

So, the idea was that temperatures when pruning should be below 41°F, and that temperatures were pretty reliably below that during the no-prune period arborists recommended. But temperatures have become so unpredictable with longer, warmer autumns and warm periods during winter (especially when there is no snow!) where temperatures go soaring into the 50°F area for several days that it is hard to accurately give a recommendation based on calendar dates at any one time.

But there is good news! There is now a degree-day model based on recent insect  trapping data in Wisconsin that has been developed by UW-Madison Extension AgWeather and the Wisconsin DNR that can help predict what the risk level is for pruning in your area. This model is a really useful tool in predicting when pruning should definitely be avoided to reduce the risk of introduction of oak wilt. The one thing you need to use the tool, is your location latitude and longitude. This is easily found by using the locator on the site and clicking ‘Get my location’ or you can use Google Maps or other map software. Type in your location and right click to get the latitude (the number on the left) and longitude (the number on the right) and enter them into the corresponding boxes on the oak wilt tool site. You can enter the date you want to prune (accuracy is best when it is the current date rather than a date too far out from then). Then click ‘Compute Estimated Oak Wilt Risk Status’ and you will get an estimate of the risk. I did this myself a few weeks ago, and for that date and my location the readout said ‘The risk of above-ground transmission of oak wilt is low’. It added that it is estimated that less than 5% of Colopterus truncatus beetles have emerged as of this date, and that C. truncatus and Carpophilus sayi (the two most important insects that transmit oak wilt in Wisconsin) are not very active yet. Since the tool says that C. truncatus emerges earlier in spring than C. sayi, there is less than a 5% chance of transmission.

It should be noted that this tool provides an estimate of the risk of beetle activity risk only! It can’t account for microclimates, for example. If your oak is near the south side of a white house or in an area surrounded by asphalt, the temperature is likely to be quite a bit higher, and the risk is correspondingly higher as the beetles would be more likely to be active. But it is still a good tool to have.

You can find the oak wilt risk estimator at https://agweather.cals.wisc.edu/thermal-models/oak-wilt or doing a search for “WI DNR oak wilt risk tool”.