by Sally Colby
The versatile boxwood, or box tree, is one of the most popular shrubs in America. With dense, dark foliage, boxwoods are ideal for hedges, topiary and accent plantings. But boxwood is threatened by yet another pest: the box tree moth, an invasive pest that has decimated boxwood species in Europe.
Craig Reggelbrugge, senior vice president of industry advocacy and research for AmericanHort, recently moderated a panel of experts dealing with the box tree moth. “We’ve been watching this pest, Cydalima perspectalis, coming at us in slow motion for a while,” said Reggelbrugge. “We watched as it established in western Europe and proved to be a very high consequence pest of boxwoods. About two years ago, it was discovered in Toronto, Canada.”
Following the arrival of box tree moth from a nursery in Canada, Samantha Simon, USDA-APHIS, said the agency has been working to prepare for the potential spread in the U.S. “In May, we received notice from Canada that they had a confirmed detection in a nursery in St. Catherines, Ontario. That nursery had sent Buxus plants to 25 U.S. locations in six states and a distribution center in Tennessee.”
Simon said detection efforts uncovered seven confirmed samples in Michigan, Ohio, Connecticut and South Carolina. After confirming detections, the USDA implemented emergency response with the assumption that everything was potentially infested. Simon said there were 2,000 potentially infested plants with 4,000 additional potentials in Tennessee.
The pest has also been confirmed as present in western New York near the Canadian border. “New York has identified 152 nurseries and 436 plant dealers within 15 miles of the Canadian border,” said Simon. “We’ll be talking about what steps to take to prevent movement of this pest outside of this area.” The response is currently active and changing daily. “We’ll do what we can to respond to this as quickly as possible,” she said. “Our goal is eradication.”
Dr. Steve Frank, professor and Extension specialist, North Carolina State University, explained box tree moth biology and host range. “The box tree moth chews on leaves initially, especially during smaller stages,” he said. “Then it moves onto soft stems and bark of the plant. Feeding on the bark is even more destructive because branches become girdled.”
Smaller larvae chew extensively on leaves, avoiding larger veins and tougher leaves. Entire sections of boxwood turn brown due to larvae feeding. Other aesthetic damage includes unsightly webbing where frass and other debris accumulate.
Frank said there’s a lot of work to be done to understand the biology in regions where the moth could become established. “Most of what we know is from Europe and from growth chamber studies, so it would be great to get data from other climates,” he said. “Larvae overwinter on plants. In spring, as it warms up, larvae continue feeding to complete development and pupate. Adults become active in early summer.” Existing data from Canada and Europe indicate that adults are active in June and July, but this stage could occur earlier in more southern areas.
Yellow, translucent eggs are deposited singly or in clusters of five to 20 on leaves. Newly hatched larvae have black heads, yellow/green bodies and develop characteristic dark stripes. “They get up to about two centimeters long,” said Frank. “Caterpillars pupate on the leaves and that’s what’s detected in the field. Adults have two color marks.”
Dan Gilrein, Extension entomologist with Cornell Cooperative Extension, presented information on eradicating box tree moths based on what is known in Europe and Canada. Gilrein and his cohorts have compiled a list of products, including Bt, that will likely help manage the moth. Because box tree moths feed and overwinter between leaves during their youngest stages, it may be difficult to ensure spray treatment is reaching the most critical areas of the plant. “You might want to include a wetting agent to reduce surface tension and enable better penetration between leaves,” he said. “Boxwoods have glossy, dense foliage so getting penetration in the canopy can be a challenge.” Gilrein said boxwoods that have been pruned to create a denser plant, and that dead foliage in the interior of the plant, may make treatment especially challenging.
Bart Brusse of Sheridan Nurseries in Canada said the country learned in late 2018 that the box tree moth was present there. Officials were prepared to pounce on it as soon as the snow melted in 2019. The initial plan was to eradicate it, so pheromone traps were placed in southern Ontario and personnel were hired for scouting. The initial plan was to remove and replace boxwoods in home settings.
“As we went street by street out from the epicenter, we realized there was a bigger infestation,” said Brusse. “There was year-old feeding on some of the plants. We offered a free spray program and tried to convince homeowners to treat as many plants as quickly as we could. In Ontario, we re-scouted and retreated. Because we were dealing with homeowners it was a real challenge. Not everyone agrees or participates, but radio campaigns to get the word out helped.”
Bennett Saunders, a boxwood grower in Virginia, spent time in Europe to observe the box tree moth there. He visited European growers and several research stations and learned that boxwood sales had suffered greatly. “The box tree moth came on very suddenly in Europe,” he said. “I don’t think they knew how to react. A lot of the research was done after the fact, and as a result, growers suffered greatly.” He added that Bt was used effectively in Europe.
Box tree moth populations exploded in the natural boxwood forests in Europe, causing widespread damage due to the absence of natural predators. “The U.S. is more spread out,” said Saunders. “The adult moth may travel 10 to 15 kilometers during their lifecycle. If only the moth travels, then the problem only travels that fast. But if we nursery people put it on a truck, we can cover 3,000 miles in two days. I think a lot of this is going to be a challenge for the nursery people – we need to not spread it.”
Saunders believes nurseries should have zero tolerance for box tree moth. ‘’It’s clear that it travels with nursery plants, and what works for nurseries will be different for the homeowner,” he said. “If we can get the population as close to zero as possible, that has to be our goal.”
As is the case with most novel pest issues, research money will be key to solving the problem. Saunders said European researchers know a lot about the problem, so looking at box tree moth management there will be critical. “We knew this was coming,” he said. “We aren’t going to stop it, but if we can slow it down enough to allow researchers in the U.S. and abroad to give us answers, we can keep the population in check until we learn how to control it.”