Boxwood trees are popular in Michigan. Gardeners like them because their evergreen foliage brings much-needed greenery in the winter months. Plus, deer don't like to eat boxwoods.
Boxwoods were imported from Europe in the 17th century. Without any native predators, they thrived in the U.S. That was until 2021, when the box tree moth was discovered in New York.
Soon after, this pest made its second U.S. appearance in a homeowner’s yard in Michigan. This prompted the Michigan Department of Agriculture and Rural Development (MDARD) to issue a quarantine on the movement of infested boxwood tree plants to prevent harm to other plants.
“The purpose of our quarantine is to slow the spread,” Andria McCubbin, a Nursery, Seed & Apiary Specialist at MDARD, said. ”We say slow the spread because eradication is not really an option. This is a very prolific pest. It flies really well. But one of the major ways it spreads is through humans.”
In the past two years, the quarantine area has grown from 11 to 13 counties in southeast Michigan. This is in addition to a federal quarantine imposed by the United States Department of Agriculture. The counties include: Clinton, Lapeer, St. Clair, Eaton, Ingham, Livingston, Oakland, Macomb, Jackson, Washtenaw, Wayne, Lenawee and Monroe.
The quarantine means that every boxwood tree plant coming to the nurseries in these counties must undergo several regulations and protocols before it can be sold to the public. These include insecticide treatments and inspections from MDARD.
MSU Extension has been closely tracking the spread of the box tree moth.
“What homeowners should be doing is making sure that if they are pulling out plants that have been heavily infested, they're double bagging those plants and putting them in the municipal waste, not the yard waste,” said Caitlin Splawski, a Greenhouse and Floriculture Educator. She added that the most important thing for homeowners to do is report any infestations to MSU Extension if they appear outside of the quarantine zone.
June through September are the most common months for infestations. Splawski said people should be on the lookout for brown leaves or a loss of leaves.
“Most often, people are going to notice the damage right around now when the caterpillars are feeding pretty aggressively,” she said. "They actually eat the leaves, so what you'll end up seeing is defoliation of the plant.”
She said caterpillar droppings and webbings are other good ways to tell if there is an infestation.
She said although the boxwood tree is not native to Michigan, it still plays an important role in Michigan’s landscape.
“There's not quite an exact replacement for it,” Splawski said.
She explained that there are some alternatives, but none exactly match the purpose of this plant. “It's going to change the landscape quite a bit. And I think it's going to impact folks the most where there are high deer populations.”
McCubbin added that if the infestation is caught early on, sometimes just cutting back the plant and letting it regrow can do the trick.
“The smaller the caterpillar, the easier it is to control,” she said. ”We're still learning, and it's only our third year into this.”
McCubbin said preserving the boxwood is important to MDARD.
“We want to protect the green industry. It's a big plant for our nurseries, and no homeowner wants to see their shrub eaten up.”
