By David Coyle, Ph.D.
Boxwood, while not native to North America, is a common evergreen landscape plant on this continent. It has shown no signs of becoming invasive despite being planted in landscapes as early as 1653! This plant is extremely common throughout the US and many boxwood (Buxus) species and cultivars are used to provide a touch of green in urban, suburban, and rural managed landscapes. Unfortunately, boxwood is susceptible to many different pests, and management of these pests is key to maintaining a healthy landscape.
A new article published last week in the open-access Journal of Integrated Pest Management outlines several different arthropod pests and pathogens that can impact boxwood in landscape settings. I spoke with two of the authors—Karla Addesso, Ph.D., and Fulya Baysal-Gurel, Ph.D., of Tennessee State University—about what these pests mean for boxwood cultivation in the US
Coyle: It seems to me boxwood is so ubiquitous and commonly planted, especially in new developments, that I’m guessing many folks likely don’t even know they have boxwood on their property. Is boxwood one of those “invisible but everywhere” plants, or do you think most people have a better idea of what it is and where it is on the landscape?
addesso: I also lean toward seeing boxwood as one of those “background” species that are so common they become invisible. They are also easily confused with certain holly shrubs (eg, Ilex crenata), which are commonly substituted for boxwood in the landscape. Because this plant is so ubiquitous, it is a major revenue generator for the nursery industry.
Baysal-Gurel: Boxwood is the no. 1 evergreen shrub sold in the US, valued at $126 million wholesale, and is an iconic species in the American landscape, specifically in boxwood gardens.
Coyle: This is likely why they’re so often unnoticed—they’re everywhere! Of the pests you highlight in your paper, which do you think has the most potential to cause damage, and which one actually does cause the most damage?
addesso: From the insect side, the box tree moth Cydalima perspectalis is the most devastating pest, particularly if populations completely defoliate the plant. After all the leaves are consumed, caterpillars will feed on bark. Repeated damage to the stem tissue can kill plants, as observed in wild stands of boxwood in Europe. In terms of pathogens, boxwood blight is easily spread and will also wipe out entire stands of boxwood. In a contest between these two pests, boxwood blight is the most challenging management issue, because once it is in a planting it can be difficult to eradicate. The caterpillars, in contrast, can be managed with insecticides if detected before too much damage occurs.
The box tree moth Cydalima perspectalis is the most devastating insect pest of boxwood, particularly if populations completely defoliate the plant. (Significant defoliation shown here.) After all the leaves are consumed, caterpillars will feed on bark. Repeated damage to the stem tissue can kill plants. (Photo by Ferenc Lakatos, University of Sopron, Bugwood.org)
Coyle: Are some pests are more common (or more of an issue) in certain regions of the US?
addesso: Box tree moth is new to North America, so while it is likely to become more widespread in time, its impact is currently limited to the northern border of New York state and Ontario, Canada. The other boxwood pests are widespread wherever boxwood is found.
Coyle: For folks that don’t want to use pesticides, what are some of the best ways to manage insect or fungal pests on boxwood?
addesso: As with most issues, prevention can be key. Homeowners and landscapers can reduce the risk of disease introduction into the landscape by carefully inspecting plants prior to transplant. Providing boxwood with optimal growing conditions can also reduce pest and disease pressure. This includes proper pruning, fertilization, and irrigation when needed. If traditional chemical controls are not an option, some boxwood issues can be managed by pruning out infested sections or using horticultural soaps, oils, or microbial products.
Coyle: What are some scientific or management advances that would really help the horticultural industry in terms of boxwood pest management?
addesso: The most effective tool for boxwood pest management may be the breeding of resistant cultivars. Breeding programs for boxwood blight resistance should also screen for resistance to other key diseases and arthropod pests. A more complete picture of boxwood cultivar resistance and susceptibility would allow growers and consumers to make informed decisions about which plants will perform best in their region.
Coyle: Do you foresee an increase in boxwood pests as a result of climate change?
addesso: If changes in climate alter the optimal range for boxwood plants, it can increase plant stress in those marginal regions, making them more susceptible to pests and disease. Similarly, if some pests, such as the boxwood mite Eurytetranychus buxican complete additional generations due to a warmer climate, this can also lead to greater damage in areas where the mite was previously unproblematic.
So, yes, there are challenges to having that perfect boxwood plant or hedge or topiary—but all is not lost! Effective management strategies exist for most boxwood pests. In cases where we don’t yet have good recommendations or where new pests are on the horizon, threatening to impact US boxwoods, rest assured there are pest management specialists and researchers working hard to come up with effective solutions.