Does this Acer tataricum (Tartarian Maple) have spider mites?

Question: Hi,

We have an approximately 25-year-old Acer tataricum in our ornamental community garden (picture 1) that has struggled the last few years. It did worse after the drought this summer. (We are now watering it regularly; kind of like closing the barn door after the horse has left.) We recently noticed webs (circled in red on pictures 2 & 3) on a few of the branches and suspect that they are an indication of spider mites. I tried to get pictures of what might be mites but the pictures were all too blurry to be helpful.  

We have read your article (https://extension.umd.edu/resource/spider-mites-trees-and-shrubs/). We are considering spraying them with a hose to wash them off or using predatory mites for management if in fact what we have are spider mites. A concern we have is that near the tree are five Buxus sinica (Franklin's Gem Boxwood) and if we use water we don't want the mites to end up infesting the boxwood. 

Do you have any recommendation about how we should proceed?

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied September 19, 2024, 4:36 PM EDT
response: The webbing pictured is not from spider mites, and there is no spider mite feeding damage (which would be quite prominent for that much webbing to be visible) on the foliage. Instead, the webbing is probably due to spiders, which are beneficial because they prey on insects, or due to caterpillars, but not a type of caterpillar that is a serious pest in need of treatment. (If it were one of those, their communal webbing is much more extensive, and defoliation around the webbing would be prominent, which it is not.)  The dieback in the first photo might be attributed to either drought stress (directly or indirectly as a result of a secondary infection like Botryosphaeria canker, which can take advantage of stressed plants) or from poor pruning cuts. We can't quite tell from the photo because of the distance and the foliage hiding some of the branch stubs, but since it looks like the tree canopy was previously trimmed (to accommodate the power lines?) in a way that reduced the length of some substantial branches, we think the cuts might not have been made in the right location, allowing dieback to progress down the branch. Proper pruning cuts are diagrammed on our Pruning Trees page, which includes a link to a handy video tutorial by Iowa State University. We mention this because, even though it's unrelated to the webbing, the dieback in the canopy is significant and appears to necessitate more pruning, so we wanted to make sure additional branch removal was done properly to protect the health of the tree. Once wood decay sets in, it can't be treated except to prune it out. If Botryosphaeria is a contributing factor for dieback, affected branches will need to be pruned out since they won't recover.  Catching up on watering is good (even if intervention is a bit late, you're still helping to ensure roots aren't dying), but use caution so it doesn't accidentally turn into over-watering, since that will cause similar symptoms to under-watering (wilting, leaf scorch, pre-autumn leaf shed, etc.). Feel the soil about six inches deep in the root area and water thoroughly once the soil becomes somewhat dry to the touch at that depth. (If damp at that depth when checked, watering is probably not needed yet.)  Boxwoods can get their own type of mite ("boxwood mites"), but while spider mites (like Two-spotted Spider Mites) are sometimes generalists, able to feed on a wide variety of plants, we usually don't see them on boxwood. The same environmental conditions promote outbreaks of both, though, so they can appear around the same time. Conditions tend to center around heat and drought. Some cultivars of boxwood are naturally more pest-resistant than others, and in our experience, 'Franklins' Gem' does not tend to develop heavy mite damage.  Miri

responder: The Question Asker Replied September 19, 2024, 5:11 PM EDT
response: Hi Miri,  Thank you for your prompt and thorough reply. I am so glad I contacted the Extension Service! I am very pleased to know that we don't have spider mites but instead have beneficial insects! Your comments regarding the dieback are very helpful. As a community garden we have volunteers (including me) who often have more enthusiasm than knowledge. Your pruning guidelines are helpful and we will follow them for pruning our other trees. Because of the current condition of the maple we will be paying a registered arborist to do the trimming in February and hope for the best.  Your caution about over watering is also helpful. My concern about a lack of past water could easily cause me to over water. I'll check the soil at 6 inches and proceed as you have suggested. With gratitude,Lowell

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied September 19, 2024, 5:19 PM EDT
response: You're welcome! Hiring an arborist to perform corrective pruning is a great idea. Some time in November-December might be better than pruning in February, if you have the option to reschedule. Virginia Tech's tree pruning calendar does not recommend pruning maples in winter or early spring, as this can lead to heavier sap "bleeding," those they do note that it's more of a cosmetic issue than a serious threat to tree health. We have seen this happen with Japanese Maples pruned that time of year, and it is a notable amount of sap that runs until the wound is able to start closing-over and drying out once active growth resumes. If the arborist has experience pruning maples in February with few issues, then it's probably okay to do so.  Miri

responder: The Question Asker Replied September 20, 2024, 7:50 AM EDT
response: Hi Miri, Thank you for the additional information about the best time to prune. I have looked at Virginia Tech's tree pruning calendar and it will be helpful as a guide for the other trees we have. We have already signed the contract for pruning the maple and received a discount for the February schedule. We will live with the sap flow this time but will know for any future pruning.Regards,LowellOn Thu, Sep 19, 2024 at 5:19 PM Ask Extension <<personal data hidden>> wrote:

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied September 20, 2024, 6:34 PM EDT
response: You're welcome!