Header image of white pine weevil damage on blue spruce courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org.
Around mid-summer, some spruce trees will turn brown and wilt at the top, displaying something commonly referred to as a shepherd’s crook. In most cases, this is a sign that the trees have become infested by white pine weevils.
In this article, we will cover the details of this tree pest, which has become common in the Colorado foothills and mountains west of Denver. Keep reading to learn more about how to spot the white pine weevil, how it impacts Colorado trees, how to prevent white pine weevils and more.
What are White Pine Weevils?
White pine weevil (Pissodes strobi) is a small insect pest that feeds on and lays eggs in spruce trees in the higher elevations and foothills of Colorado.
Adult weevils are rust-colored and range in size from 4 to 6 millimeters. They have wings, so they may occasionally fly. A large white patch on the back is near the front wings.
White pine weevils lay eggs in the tree’s bark, and the larvae live beneath the bark as they grow and feed. The larvae are white with a brown head and approximately 7 millimeters long.
Which Trees are Impacted by White Pine Weevils?
Here in Colorado, the most common hosts of white pine weevils are spruce trees. A similar pest, Pissodes terminalis, can cause similar symptoms in lodgepole pines.
In the Eastern United States, white pine weevil is destroying many of the eastern white pines, which explains the common name.
The most common spruces attacked are Colorado blue, Norway, white, and Serbian. Pines affected include Scotch, red, pitch, white, jack, and Austrian. Douglas firs are sometimes damaged by the white pine weevil as well.
White pine weevils prefer trees that are 20 feet or less in height, though they have been found on trees up to 30 feet tall.
White pine weevils also appear to prefer trees that are exposed to direct sunlight.
What are the Signs of White Pine Weevil Damage?
We already discussed the most obvious sign of white pine weevil damage (the wilting and browning of the top of the tree), but there are subtle ways you can spot this pest throughout the year:
Spring – Look for drops of resin on the terminal leader (main stem of the tree). The tree may release resin after holes are created by white pine weevils for feeding and laying eggs.
Late spring and summer – This is the time when the larvae will be just under the bark of the terminal leader, and their chewing and burrowing can cause the new growth on the tree to wilt, droop, or even die. Check for oval-shaped “chip cocoons” under the bark of the main branch.
Mid-summer (around July) is the time to watch for the tell-tale shepherd’s crook at the top of the tree.
If a tree has been infested for many years, the tree may grow a lateral branch into a leader, resulting in a tree that has two leaders, otherwise known as a forked tree. Occasionally, a tree may grow multiple leaders.
What is the Lifecycle of a White Pine Weevil?
Winter – Adult white pine weevils overwinter in leaf litter under or close to host trees.
Spring – The adult white pine weevils fly or crawl to the tree, usually on a warm spring day in March or April. The adults mate and the female lays eggs in the tree’s bark. A week later, the eggs hatch and the larvae begin feeding on the tree’s inner bark.
Summer – Around July, the white pine weevil larvae enter pupal chambers (called “chip cocoons”) filled with shredded wood. 10 to 15 days later, they emerge as adults through small holes.
Fall – Adult white pine weevils enter leaf litter on the ground for the winter.
How Can You Prevent or Treat White Pine Weevil Damage in Colorado?
LAM Tree Service applies treatments for white pine weevil in the spring and fall (usually around April or May and again in August or September).
Our spray treatments put repellant on the bark of the tree, where the white pine weevil would normally attack.
If you have spruce trees or if you suspect a white pine weevil infestation, contact LAM Tree Service to schedule a consultation.
In some cases, it may be helpful to prune out infected branches and destroy them. A new leader can be trained from a lateral branch.
Cleaning up the area around your trees prevents white pine weevils from overwintering in fallen leaves or needles.
Learn more about insect and disease management and treatment on our Plant Health Care page.