Timing is critical for white pine weevil control
Timing for white pine weevil control is very important, yet may be difficult to determine. The optimum control window is after overwintering adults emerge, but before they begin to feed and lay eggs on the terminal.
White pine weevil larvae can kill the top whorl of spruce, pine, Douglas fir and, occasionally, fir trees. White pine weevil becomes active early in the growing season during the first few warm, spring days and will likely be active this week as temperatures warm (as of April 12).
White pine weevil feeding under bark in June.
Photo credit: Jill O’Donnell, MSU Extension
Overwintering adults move from the litter underneath the tree to the treetops to mate and lay eggs. Usually this begins between 7 and 58 GDD50. The female chews a hole to create a place to lay a single egg. Over a few weeks, a single weevil may lay up to 100 eggs on the terminal. The eggs will hatch in a couple of weeks and larvae will bore into the terminal. They feed in the cambium area just under the bark, eventually girdling the top couple of whorls. Controlling this pest involves applying a registered pesticide to control the egg-laying adults.
White pine weevil mating and laying eggs in the leader of a
white pine. Photo credit: Daniel Herms, The Ohio State University, Bugwood.org
Here are several ways Michigan State University Extension recommends scouting for emergence of these adults:
- Take time now to walk through your fields and examine your trees carefully. On sunny days, check the leads for adults or drops of resin. The adult weevil makes a small, round feeding puncture on the terminal leader and you may see a little resin oozing out of the holes.
- You can trap adults using tedder traps. These pyramid-shaped traps are baited with alcohol and turpentine to attract the adults. Place the trap next to a tree that was damaged last year. The key is to make sure to get these out in the field before the adults become active. When we have used the weevil traps, we usually catch our first weevils around 35 GDD50. In southern parts of the Lower Peninsula, you would want to get these traps out as soon as possible.
- A three-year project in Pennsylvania monitored ground temperatures at the 2-inch level. They found that weevils began emerging when ground temperatures are above 50 degrees Fahrenheit. If you want to try this method, use a soil thermometer to monitor temperatures on the south or sunny side of the tree.
If you need to use a pesticide, make sure to thoroughly cover the leader and the upper part of the tree. Then in the growing season when you see leaders beginning to die, clip them out and remove them from the field.