Monitor for white pine weevil adult emergence now
White pine weevil larvae can kill the top whorl of spruce, pine, Douglas-fir and fir trees. Overwintering adult weevils will likely be active this week in Michigan. Control should begin as soon as the first weevils are found.
White pine weevils become active early in the growing season during the first few warm, spring days and will likely be active this week as temperatures warm. Overwintering adults move from the litter underneath the tree to the treetops to mate and lay eggs. Usually this begins between 7-58 GDD50. The female chews a hole to create a place to lay a single egg. Over a few weeks’ time, 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.
Over wintering adult white pine weevils on terminal leader.
Photo credit: Daniel Herms, The Ohio State University, Bugwood.org.
White pine weevil larvae feeding under the bark.
There are a few ways you can scout for this emergence of 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 the traps out in the field before the adults become active. When we have used weevil traps, we usually catch our first weevils around 35 GDD50. In southern parts of the Lower Peninsula you may be past that point.
- 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°F. If you want to try this method, use a soil thermometer to monitor temperatures on the south or sunny side of the tree.
Finally, 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.