What to do about white pine weevil

Our blue spruce is about 7 feet tall, well-established and was very healthy until about a week ago.

It shot up about a foot this year and has several new branches all over. The top branches drooped about a week ago followed by the branches directly below it drooping a few days later. The branches under those are starting to droop now and a clear sappy substance is coming out of the top area.

I can’t see any beetles, worms or any kind of bug on it anywhere. Please HELP!

­— Cheryl, from Durango

I think what’s going on is an insect called a white pine weevil. In Colorado, this is a problem mostly limited to higher elevations such as yours. White pine weevil isn’t a serious threat to the life of your tree. Its damage is mostly aesthetic, limited to the terminal shoots of the plant. Their damage can stunt growth and give your spruce a bushier, shrubbier appearance.

The adult weevils are quite small and spend the winter around the base of the plant under leaf litter. They come out early in the spring and crawl up to the top of the tree, mate and the female lays eggs at the base of the current season’s dormant buds.

The newly hatched larva feed on the wood under the bark. Their feeding damage can extend around the shoot, girdling it and causing the current season’s new growth to wilt in late spring or early summer. The larva also can tunnel downwards, sometimes causing the shoots directly beneath the terminal to wilt as well.

The larva pupate in early summer, emerging as adults in mid- to late summer where they do some minor feeding on the stems of the plant. In the fall, they crawl down to overwinter in debris on the ground to repeat the cycle next year.

Obviously, it’s too late to do anything about this year’s infestation. Cut the wilted portions of your tree off right away to remove as many of these from the equation as you can. Don’t leave those prunings around the yard—the weevil will continue to survive in them. Bag them up and throw them in the trash or burn them.

There are a couple of options to treat this with insecticides for next year. The first is to apply a systemic insecticide around the base of the plant later this fall. It needs to be applied early enough to be absorbed and distributed throughout the tree before things shut down for the winter. The best material to use is an insecticide called Imidacloprid.

Your second option is to spray next spring with a contact insecticide to kill the adult weevils before they have a chance to lay eggs. The best insecticides to use are Bifenthrin or Permethrin. You want to apply these at the higher rates recommended on the label for borers.

Timing of this spray is extremely important since if you get it on too late, you’ll miss the opportunity to prevent egg laying, and once the eggs hatch, the larva would be protected from insecticide sprays by the bark. The rule of thumb is to apply the spray when forsythia are blooming, which usually is in late March to early April in the Grand Valley but later for you. Most sprays are applied in April sometime. It’s helpful to plan on repeating the spray a couple weeks later to ensure good coverage is maintained.

In the next year or two, you might consider selecting the strongest of the side shoots to train as the new terminal leader of the tree. You can tie a stake to the trunk with stretchy green plant tape. Place the stake extending up above where you cut off the infested leader. Carefully tie the side shoot to the stake so that it’s directed vertically.

You will have a “dogleg” in the leader where the side shoot bends upward, but that will disappear over the years and the tree will continue on its normal growth. You can also trim back the other side shoots a bit to bring them into better proportion to the new leader.

Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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