Black Bark on Maples? Pest Alert

By Michelle Wallace

Do your maple trees have black bark?

If you just happened to notice that the bark on your Maple trees looks different – but you can’t pinpoint what or why here are some things you need to know:

1)      Maple tree bark should be grey not black.  If your tree has black bark it is because it probably is infested with gloomy scale .  Gloomy scale is a tiny insect with an armored covering.  The outer covering is what protects the juvenile insect from the outer elements and predators.  When it is in this stage – it doesn’t look like an insect at all.  It just looks like mildew on the tree’s bark.  While this insect is in this stage of development, it sucks out nutrients from the tree.  Over time, populations of this insect can get so high, that they can damage or even kill the tree. 

2)      Gloomy scale occurs predominately on maple trees that are under stress.  Maples are bottomland trees that prefer moist well drained soils.  Unfortunately, in Durham, they are planted in compacted clay soils low in oxygen.  In addition, weather can stress out a tree.  High heat, low temperatures, high fluctuation in rain from drought conditions to abundance of water, as well as improper planting, pruning, over mulching, and more – all can stress out a tree.  When this tree is under stress, it will increase photosynthesis and the production of sugar which is what attracts the insects.

3)      Unfortunately, if you don’t do anything, the problem will get worse.  At this time of year we recommend scouting for this pest on your maples.  If present, apply horticultural oil to the bark of your tree and repeat application after 7-10 days.  There are systemic pesticides available – but they will kill our pollinators – so we discourage their use if there is an alternative.  Power washing the bark with a high pressure nozzle may also help to remove some of the crusted over insects.

4)      It will not be enough to treat the insects.  You need to improve the health of your tree.  Get a soil test done.  If recommended on the soil test, add lime to raise the pH and fertilizer to improve the nutrients available to the tree.  During times of prolonged drought – make sure your tree gets 1” of water per week.  Finally, if your tree is planted in compacted soils – use an auger to excavate the soil around the roots and fill the augured holes with gravel or engineered soil.