Pest Alert: White Pine Weevil

Some garden pests and diseases show up every year without fail, like powdery mildew, grubs, and Japanese beetles. Others wax and wane during different years as weather conditions and other factors vary. White Pine Weevil is a pest that you can find here and there most years, but this year I've been seeing it a lot more than usual. Maybe I'm just hyper vigilant after noticing it on a few of my clients' properties and now I'm seeing it everywhere, or maybe there really is a spike in activity this year. Either way, this insect can do serious damage to pine and spruce trees, so it's important to spot it early.

What do white pine weevils do?

White Pine Weevil Photo courtesy of the USDA Forest Service

White Pine Weevil
Photo courtesy of the USDA Forest Service

The white pine weevil is a small, brown insect about a quarter of an inch long. The adults have long snouts that they use to bore holes into branches. They attack the central leader at the top of the tree in early to mid spring, eating holes into the tip and creating wounds to lay their eggs. The eggs hatch in May and June and bore into the branches at the very top of the tree. They continue feeding into July when they pupate. Adults emerge from the twigs in late summer, feed for a little longer, and then climb back down the trunk to the ground where they hibernate for the winter before the cycle starts over.

How do I know if my tree has them?

The upper tips of this Serbian spruce are starting to twist, a sign of weevil damage. Photo by Maria Gulley

The upper tips of this Serbian spruce are starting to twist, a sign of weevil damage.
Photo by Maria Gulley

The easiest way to spot white pine weevil damage is to watch the tips of your pine or spruce for crooked growth. They typically attack younger trees under 20' tall since they have to move from the ground all the way up to the tip of the tree each spring in order to feed. After the tips have twisted and wilted, they will die and by that point they will be very obvious. White pine weevils typically don't move very far down the tree, so this insect will not kill your tree. But for a spruce or pine all it takes it killing the tip to ruin the shape of the tree for the rest of its life. A new lead branch will not naturally take over, and without intervention your tree can develop multiple tops which creates an undesirable look and can set the tree up for structural failure in the future. Fortunately your tree isn't totally doomed. There are some things you can do to help.

What can I do once the damage has been done?

Damaged tips often curve downward and later die. Photo by Maria Gulley

Damaged tips often curve downward and later die.
Photo by Maria Gulley

The key to recovery for your tree is training it to develop a new central leader. Step one is to remove the dead or dying tip before the adults emerge at the end of June (with a good pruning cut, of course!). This way you take out the larvae or pupae so they can't attack the tree again next year. Next you need to select a new leader. Take a look at the next ring of branches below the damaged portion of your tree. One of these will become the new point. Pick the straightest, healthiest branch from that top ring and remove the rest or cut them shorter than your new tip. Then take a stake or other stiff stick kind of thing, tie it to the trunk of the tree below your cuts, and then tie your new tip upright to the stake. It's best to leave a little wiggle room for the new leader, and be sure to tie both the trunk and the tip with a soft material that won't cut into the bark. The next year, remove the stake and ties so they don't interfere with the tree's growth. While you're up there, check for other branches that may be trying to grow to compete with your designated leader. If a new branch is just angling upwards, it's probably not a risk. but if it is growing straight up you should cut it back a bit or remove it. When a tree develops two leaders it may look okay for a while, but eventually the tree will split down the middle as the two central stems push each other apart.

How can I keep this from happening?

Your best bet for preventing an infestation is to do a preventative insecticide spray. If you have spruce or pine trees less than twenty feet tall (not all evergreens are spruces or pines - check out this handy guide to see what your tree is), in mid April you should consider spraying the top foot or so of the tree with an insecticide. You do not need to spray the whole tree. It is best for the mini ecosystem of your yard to only use insecticides when and where you need them to allow good bugs to thrive and help fight off pests naturally. For more details on what and when to spray, check out this information from Michigan State University.
 

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