LANSING, Mich. — State officials warned tree owners this month they should expect a small caterpillar to damage a great number of spruce and fir trees this year.

The culprit is spruce budworm (SBW), an insect that lives through the winter as a tiny caterpillar. The creatures spread by dropping from the treetops on webs. As new tree growth expands in the spring, the caterpillars grow and feed on shoots. Budworms produce one generation each year.

Officials say over the past few years white spruce and balsam fir have been defoliated by SBW, describing it as one of the most destructive native insects in northern spruce and fir forests. They say now is the time to protect landscape trees from damage.

"This year, budworm defoliation is widespread and expected to be heavy for the second or third year in many areas," said Bob Heyd, Michigan Department of Natural Resources (DNR) forest health specialist and manager of its forest pest management program.

He said now is the time of year to combat SBW, which primarily affects trees in the Upper Peninsula (U.P.) and northern Lower Peninsula. It’s expected to have its greatest impact in the western U.P.

"Check your trees for signs of budworm activity," he said. "Most of the damage is done between now and late June when the budworms pupate and are done feeding for the year."

According to Heyd, trees should be treated now before caterpillars grow larger. The DNR recommends keeping the following in mind prior to treating:

•Biological sprays with Bt or garden-variety insecticides are effective; follow label directions when selecting and applying a spray

•Be sure to reach the top of the trees where defoliation is heaviest

•Treatment with biological sprays may need to be repeated if heavy rains occur within a few days

•Watering trees during dry periods helps them recover from the stress of early season budworm defoliation

Repeated budworm defoliation can cause top-kill and tree mortality in older and stressed trees, the department says. Balsam fir older than 60 and spruce over 70 years provide prime infestation opportunities. Younger trees infested with SBW lose much of their new growth, but usually survive.

Balsam fir is most severely damaged by SBW; however, white and black spruces are susceptible to infestation as well. SBW can also infest tamarack, pine and hemlock. Spruce trees in stands with balsam fir are more likely to suffer budworm damage than pure spruce stands, although all older, mature to over-mature stands are at higher risk of infestation from SBW.

In February the DNR released its latest forest health update, a report that summarizes disease challenges to Michigan’s forests. Epidemics of SBW periodically cause extensive damage and tree death in spruce and fir forests across the northeastern United States and Canada. Epidemics have occurred on a 30- to 50-year cycle. Experts appear not to be sure yet if another epidemic is starting or not; the last epidemic in Michigan ended in 1982.

According to the report, widespread SBW infestations in spruce in 2010 produced little defoliation in 2011; however, SBW populations that collapsed in 2013 after building in 2012 once again became epidemic in 2014.

"It is still unclear if these cycles of SBW defoliation may point to the onset of the next regional epidemic in the Great Lakes Region," the report stated. "Quebec believes it has already entered the next regional cycle. Ontario has experienced a similar history of SBW buildup and collapse as in Michigan’s U.P."

Periodic outbreaks of SBW are seen as a normal part of the forest ecosystem. In order to minimize losses of valuable trees, however, DNR recommends the harvesting of spruce and fir when they reach rotation age and to "pre-salvage" harvestable stands where SBW infestations have caused top kill or tree death.

For more information on this and other forest health issues, visit www.michigan .gov/foresthealth