This is part of a series on Wisconsin’s very impressive Forest Health Team. It first appeared in My Wisconsin Woods, the online newsletter of The Aldo Leopold Foundation. photos courtesy of Linda Williams.

Spruce budworm (Choristoneura fumiferana)  has been causing defoliation in Wisconsin since 2012 according to Linda Williams, Wisconsin DNR forest health specialist. This pest is native to the Great Lakes area, where outbreaks have been occurring every 30-50 years.

“Our last round was in the 1970s. Ten years is about average for an outbreak,” says Williams. “I would suspect this one will be wrapping up in the next several years. The whole population tends to collapse at the same time, but we cannot predict exactly when that might happen.”

Defoliation was first spotted in Marinette County, but is now found across much of northern Wisconsin. “In particular, we are seeing significant defoliation in Northeast and North Central Wisconsin, and the area extends to the UP, Minnesota, and Ontario,” she says.

Spruce budworm will defoliate spruce and balsam fir. “Historically it has been hard on white spruce in plantations. We also have black spruce swamps — low areas where black spruce grows naturally — although black spruce is not defoliated as much by spruce budworm,” Williams says. “Balsam fir, which spruce budworm actually prefers, can be found in plantations, but also grows naturally in understories, growing very densely up to 30 feet tall.”

In spring, spruce budworm caterpillars feed on buds just starting to break, and also on pollen cones. “They finish eating fairly early in the year and pupate right on the needles before people usually even notice the damage,” says Williams. “After a couple weeks in the pupa, they emerge as small, reddish brown moths, mate, lay eggs and are done for the year.”

The new needles are most at risk, “but after the new needles have been eaten on a tree for two or three years, there aren’t many needles left on the tree,” she says. “Unlike our hardwoods — which, if their leaves are eaten early in spring, will send out additional leaves to get them through the rest of the year — our conifers don’t re-foliate after their needles have been eaten. The growth they push out in the spring is all they’ve got. These trees can suffer top die-back or die completely.”

Early in spring, the caterpillars are about a quarter inch long. They grow to about an inch long and tend to be a brownish-green color that blends in well. They tend to web together a couple of needles and hide inside. “They are not easy to spot even when there are thousands of them,” Williams says. “They are messy feeders though. They sometimes clip needles without eating them completely, and the clipped needles get caught in the webbing they create around the tree. Eventually, the clipped-off needles turn a reddish-brown, and the tree will look like it is scorched. That stands out from the air, and it is something we look for in aerial surveys.”

To read more of this article, check out the link to My Wisconsin Woods.