Has the top of your Spruce tree ever looked like this?
The insect that causes this damage is the White Pine Weevil. It’s name is deceiving, at least in our area, here it mainly does it’s damage to Spruce trees. This weevil is a small brown beetle with a long ‘snout‘. Although the adult is rarely noticed, the grubs feeding at the tops of spruce is what causes the damage.
The damage to the Spruce is obvious as it loses it’s main leader. an important growth hormone is produced in the terminal growth of the tree. When this hormone is taken out of the picture Spruce can lose their natural cone shape and start to grow very wide very quickly. This is due to the phenomenon of Apical Dominance. Removing the terminal growth and its suppressive hormone allows the lower lateral growth to develop more quickly. New leaders compete to become the new terminal leader. This a more major issue with Spruce than with other species as it can lead to some structural issues if not repaired through pruning, such as multiple co-dominant stems which can split away. Such as in the picture below.
The Spruce in the picture had two leaders starting at about 15 feet high. They were competing to be the terminal growth of the tree which can lead it to be less stable. This because neither leader is well attached to the tree.
The damage to spruce is caused by feeding of the grubs of the weevil inside the developing spruce leader (the top shoot). The female lays eggs in wounds on the leader of spruce in spring, and the grubs spend all summer eating the tissues within the shoot. The leader eventually dies, and then curls over like a shepherd’s crook and turns brown (Usually in August). A close inspection reveals small boreholes at the base of the damage and white sap droplets.
Young spruce, less than 20 feet tall are most commonly attacked. Mature spruce are less likely to be targeted since the beetle does not fly well and usually prefers to climb it’s host.
Treatment and Control
The damage can be removed through pruning. It is important that the tree top be trained back to a single main leader. Remove damaged leaders by cutting it off well below the grub’s tunneling. If available, select a young flexible side shoot and gently bend it vertically and splint it in place to encourage it to become the new leader. Alternatively, in the year following leader removal, select a new shoot that looks like the best new leader and shorten the remainder so they do not compete. There is no chemical or spray control available for this insect.
A Good Time To Prune
Right now is a great time to remove this type of damage from Spruce. It is important to make the cuts before the flush of growth in the spring. This energy in the spring feed the new leader of the tree which will allow it to take over the terminal role on the tree.
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