Box tree moth continues its spread

“The only way to have insect- and disease-free boxwoods is to use Buxus plasticus,” Joe Boggs joked. (Only a plastic boxwood – Buxus taxonomically – could truly be problem-free.)

Boggs is an assistant professor with Ohio State University (OSU) Extension specializing in entomology. OSU, the Ohio Green Industry Association and the Horticultural Research Institute recently teamed up on a webinar focused on box tree moth (BTM), an invasive pest that was first detected in 2018 in North America. Since the initial confirmation in Ontario, Canada, the pest has been discovered in multiple eastern states.

BTM presents some challenges, especially to those in the horticulture industry that sell boxwoods, but according to Boggs, these challenges are not insurmountable. He wants nurseries to continue raising boxwoods, which he said were probably one of the first landscape plants brought from Europe to be planted in North America.

In Ohio, boxwoods are an important nursery crop with about $12.5 – $15 million in yearly sales.

It’s worth staying in the industry despite BTM, Boggs argued, because there are few alternatives to this versatile species. On a list of positive attributes, there’s easy to grow, evergreen, a great source of food for pollinators and tolerant to shade, varying pH and poor drainage. They are also deer resistant. Boxwoods can even make goats sick.

Another reason Boggs is so optimistic is that there are no native boxwoods in North America. BTM is a native insect to China, Taiwan, India, Korea and Japan, and in these places, BTM is not a threat because the boxwoods co-evolved and natural selection has built-in resistance with various predators, parasitoids and pathogens that keep BTM in check.

On the other hand, in Europe, the arrival of BTM devastated boxwoods because the native species did not coevolve with the pest. The understory of Europe is full of boxwoods, so BTM easily spreads from one location to another. This is why the pest is called the box tree moth rather than the boxwood moth.

Boxwood growers in America are luckier than those in Europe. “We don’t have native boxwoods to keep it going through our forests. Our boxwoods are more like islands where we can have focused management efforts,” Boggs said.

Still, the pest is here and continuing to spread, so understanding its lifecycle and detection are critical to controlling BTM.

BTM goes through a metamorphosis from an egg through seven caterpillar stages (called instars) to a fullgrown caterpillar. It then becomes a pupa and finally a moth. The nocturnal adult moths are very difficult to find and identify. So are the eggs. The best strategy is to look for the caterpillars, which fortunately look alike during all their stages: black, yellow and white stripes, black dots and a shiny, black head.

In winter, larvae pull leaves over themselves and weave little silk tents for protection. They hang out until spring (probably daylength-related, Boggs hypothesized). There can be multiple generations of BTM each growing season, depending on location and climate. For instance, the Buffalo, NY, region has two generations. In southeastern Ohio, where the pest causes significant damage, there may be three.

The instars feed aggressively on boxwood leaves. Feeding usually starts in the middle of the plant where the caterpillars can avoid predation until eventually, without control, they begin feeding on the outermost foliage, causing the plant to take on a bedraggled, see-through look. Defoliation is a telltale sign that BTM is present. Boggs said, “There are no native caterpillars that specifically defoliate boxwoods in North America. Let that sink in.”

As BTM instars get larger, they start consuming entire leaves, often leaving behind the leaf edges. The remaining tissue curls to produce a characteristic hairlike curlicue symptom that turns brown. There is nothing else that produces this symptom on boxwoods.

BTM does produce silk, but Boggs suggested being careful with using this characteristic for identification. When he first started learning how to identify the pest, people advised to be on the lookout for copious silk. According to Boggs, this is not true. The caterpillars produce silk, but the webbing is usually sparse.

Another sign of BTM is the presence of frass (aka caterpillar poop). BTM frass looks like small, light green to greenish- brown pellets and may collect within the foliage or drop to the ground beneath infested plants.

Another detection tool is to use traps which are baited with synthetic pheromones that mimic what female moths use to attract males. Boggs noted that this only works on the males, and that the traps do not contribute to the spread of BTM.

Once BTM is confirmed, there are many insecticides that can control the caterpillars. Boggs recommended spraying in spring after the caterpillars have emerged from their winter hibernation – what’s officially known as their diapause.

“I’m not suggesting one application in the spring is going to eliminate box tree moths from the landscape. If you target the early instar caterpillars, you’ll stop the lifecycle. You won’t have that first complete generation. But then you do have to monitor,” Boggs said.

Looking ahead, Boggs hopes that research will show that specific predators and parasitoids could be used to control BTM. He’s already seen this in action; he’s watched yellowjackets grind up the caterpillars and take the meat back to feed their young.

“Eradication is not going to be pursued in the U.S. or Canada. It’s just too widespread,” Boggs said.

by Sonja Heyck-Merlin