Sector Spotlight: Box Tree Moth

Pests and diseases continually challenge all of us when growing and maintaining ornamental plants, whether we are growing them in our nursery or in our landscapes. Boxwood, or Buxus, species are no exception and when boxwood blight was first reported in Europe in the early 1990s, selection and breeding programs were initiated to identify and develop plants resistant to boxwood blight. Although it took significant time and dedication, four Better Boxwood varieties that exhibit boxwood blight resistance have recently been introduced. With the introduction and spread of box tree moth (BTM) into Europe and North America, it seems natural that breeding programs will shift to focus on developing boxwood varieties that exhibit resistance to box tree moth.

CNLA’s New Plant Development Committee (NPDC) has had multiple discussions on this topic over the past couple of years and contracted Dr. Daryl Somers this past spring to conduct a literature review on the topic of breeding BTM resistant boxwood. The committee was interested in the feasibility of using genetic modification to insert the Bacillus thuringiensis (Bt) toxin gene into the boxwood genome as one method for resistance breeding and asked that the process be investigated and outlined in the report.

Historically, Buxus breeding has relied on chance seedlings and sport mutations to derive new varieties, or relied on open pollination of Buxus collections to generate seedlings. Sheridan Nurseries introduced three varieties in the early to mid 1970s (Green Gem, Green Mountain and Green Velvet) which were interspecific hybrids of B. sempervirens x B. microphylla var. Koreana. B. sempervirens is native to Europe, northwest Africa and southwest Asia while B. microphylla is native to Japan and Korea. The Sheridan varieties are grown extensively here in Canada, as they are some of the few varieties in the market that combine winter hardiness with the valuable aesthetic ornamental traits. 

Currently there are three Buxus breeding programs of note: Herplant in Belgium (Better Buxus), Saunders Genetics in Virginia (NewGen) and USDA-ARS program at the national arboretum in Beltsville, Maryland. 

Dr. Didier Hermans of Herplant began breeding Buxus in 2007. They have bred a large collection of hybrids that are interspecific crosses between B. sempirvirens and different B. microphylla subspecies of Asian origin. 

Although not breeding specifically for BTM resistance, Herplant tested several Buxus genotypes for BTM resistance which included the four boxwood-blight-resistant varieties: Renaissance, Skylight, Heritage, and Babylon Beauty. Once BTM larvae developed into caterpillars on the plants, they were able to determine the percentage of dead versus live caterpillars. The new varieties showed a remarkably high percentage of caterpillar mortality whereas B. sempirvirens remained a good food source for caterpillars. These tests concluded that Asian Buxus species and the four hybrid varieties: 1) showed less pupae forming, 2) showed a high number of dead caterpillars and 3) remained relatively green in field tests under BTM pressure. 

Since they were not directly breeding for BTM resistance, these results were simply a side effect of using Asian species in their hybrid development and selecting hard for blight resistance. Similarly, the use of Asian boxwood parentage in the Saunders program is present with the intent to develop resistance to both BTM and blight. The USDA program was not working on BTM resistance and is focused heavily on blight resistance and other performance features. However, they have recruited a Post Doctoral researcher to begin work on genetic modification for BTM resistance.

The obvious modification would be to add the Bt toxin expression into Buxus that would provide resistance to BTM just as the Bt spray does. Genetic modification involves the insertion of a specific DNA sequence (a gene) into the cells of the receptor plant to introduce the expression of that gene and thus change the traits of the plant. In regard to BTM resistance in Buxus, one could imagine introducing the gene for Bt toxin into Buxus and have Bt toxin expressed to eliminate BTM larvae.

First, the receptor plant should be amenable to tissue culture and in vitro propagation methods. The second step is introducing the gene sequence typically via agrobacterium infection or particle bombardment. Particle bombardment is coating microscopic gold particles with the gene sequence and ‘shooting’ the particles at the tissue to be modified. Finally, the sections of the cultured tissues or single cells that have stably incorporated the gene into their genome would need to be amplified. This involves in vitro growth followed by conversion into plantlets with roots/shoots using various hormone treatments that could then be transplanted to soil and eventually clonally propagated by cuttings. These cuttings would then be grown into plants that could be lab and field tested for BTM resistance.

As can be seen, classical breeding or genetic modification will be a slow process. Is it worth the commitment and effort, and who will invest the resources into this long-term project? As mentioned, CNLA’s NPDC is currently having those discussions based on Dr. Somer’s review and further investigations with other tissue culture labs and breeders. It’s likely others are having similar conversations.

If you are interested in reading Dr. Somer’s full report, please email jamie@cnla-acpp.ca to receive a copy.