July 2020: Now is the Time to Treat for Boxwood Leafminer

Adult females deposit fertilized eggs through the underside of the tender new boxwood leaves.

Although boxwood are known for their low maintenance and tend to have few pests, Boxwood Leafminer (Monarthropalpus buxi) can be a challenge for growers in the Mid-Atlantic region and around the United States. Technically a midge, not a leafminer, this pest causes damage that starts small as discolored leaves but can become severe as populations build over a period of years. Luckily, this tiny orange insect has only one life cycle per year, providing easy and effective ways to treat it.

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The life cycle begins in the early spring as the previous year’s larvae pupate and cause the boxwood leaves to discolor, blister and swell. Those pupae emerge as adults in April/May (in central Virginia), hovering only inches above the boxwood due to their weak flying abilities. Over an approximately 3-week period, adults emerge in waves to mate. Females then complete their life cycle after they lay eggs in the tender underside of the new boxwood leaves. These eggs hatch in early summer (mid-June in central Virginia) and the larvae begin the cycle of growth that will conclude the following spring.

Photo taken July 1: The young Boxwood Leafminer larvae are only visible under a microscope.

These early instars are very small, see them here compared to a pin head.

Chemical controls are the best alternative for complete treatment of Boxwood Leafminer. It is unnecessary to time a chemical application that successfully kills Boxwood Leafminer adults because the adults emerge over several weeks and live only several days, requiring multiple sprays. The larval stage provides a much longer spray window and is likely to be significantly more effective. The eggs hatch around mid-to late June in central Virginia, and the larvae are eating and growing, during the summer and fall. Research has found systemic insecticides to be effective in killing larvae until temperatures turn cold, which in some years is not until late October or early November. This strategy can provide control for up to 2-3 years as a thorough spray totally wipes out the population.

Severe blistering and discoloration are the result of heavy Boxwood Leafminer populations.

Boxwood Leafminer adults are small, orange, mosquito-like midges, that are often found hovering only inches from the foliage because they are weak flyers.

For Boxwood Leafminer control, growers have had excellent success with products in the neonicotinoid group that contain the active ingredient imidacloprid, thiomethoxam, or dinotefuran. There is a great deal of ongoing discussion regarding neonicotinoids and other chemicals and their possible effect on pollinators. Nurseries and gardeners should follow good science and alternative methods to control these pests to further eliminate use of this group of pesticides. All growers should minimize the use of any pesticide by practicing Integrated Pest Management or IPM. The goal of IPM is to effectively control a pest while minimizing negative impacts on pollinators, the environment, and employees.

Examples of insecticides listed in the Virginia Tech Nursery Crops Pest Management Guide are thiamethoxam (Flagship 25WG at a rate of 6 oz./100 gallons of water) or dinotefuran (Safari SG ¼ to ½ lb. / 100 gallons of water). These systemic products are absorbed and dispersed throughout the plant. Only one application a year is necessary to target the larvae feeding within the leaf. In central Virginia, it is recommended that the application takes place late June through mid-October.

To be noted: While Saunders Genetics has worked with nursery growers who have successfully used the pesticides listed above, no guarantees or promises, nor any opinions concerning the effectiveness or safety of these or any other pesticides are made. Always read the labels and other product information from the manufacturer and discuss the proper use and application of products with appropriate company representatives or acknowledged experts.

Although chemical control is an option on susceptible cultivars, there are also many boxwood varieties that have shown genetic resistance to Boxwood Leafminer. This genetic resistance is a defining feature in the NewGen™ Boxwood program where the goal is to provide grower-friendly plants that can thrive when faced with pest and disease pressure. NewGen™ boxwood were chosen for the program because they are more resistant to Boxwood Leafminer than most varieties currently on the market. If you are replacing or planting boxwood, consider NewGen™ varieties so that costly and troublesome sprays will not be necessary.

The Story Continues

The Saunders Family

Growing up a Saunders predisposes you to have a certain affection towards boxwood. In our previous post we told the story of Paul Saunders and how he turned his passion for boxwood cultivation into a successful family business. Saunders Brothers Nursery began growing boxwood over 70 years ago and this was just the beginning. As the business has transitioned to a new generation of leadership, Paul’s sons, Robert, Bennett, Tom, and Jim reflect on their memories from sticking boxwood cuttings to earn an extra penny, to the discovery of Boxwood Blight in the United States and, to the creation of the NewGen™ program.

From an early age, the sons participated in every part of boxwood production around the farm. Bennett Saunders, General Manager of Saunders Genetics, remembers, “As kids, we would get paid a penny apiece for each boxwood cutting we stripped and stuck. This is how we would earn our extra spending money.” As the boys grew, they continued to play an essential part in the daily farm responsibilities. “I remember spending most weekday evenings at the nursery loading trucks with boxwood from March to May,” recalls Bennett. “At the time, boxwood were about 40% of the plants we grew.”

Boxwood Decline affecting English boxwood in a landscape

One of the first setbacks in boxwood production came about in the 1970s with the advent of Boxwood Decline. Up until this point, most of the boxwood market was made up of English boxwood (Buxus sempervirens ‘Suffruticosa’). As Boxwood Decline became more prevalent, the company began to move towards a wider selection of boxwood cultivars that were more resistant to the disease. Robert Saunders, General Manager of Saunders Brothers remembers, “In the spring of either 1981 or 1982, I was in high school and I got a note from the principal’s office saying I needed to call home. I called and my dad explained that he wanted me to come home to see an experiment he was doing. He was planting in an area just outside the yard at our house, a test garden of new varieties of boxwood. At that time, we grew only English and American boxwood, but my dad thought we should try some new ones.” Robert was dumbfounded for two reasons, “Number one, I hated plants, so I did not want to have anything to do with it. Number two, why did we need new varieties of boxwood? Little did I know I would end up working in the middle of it. From those trials, an evolution of perspective began and, we soon found Green Beauty, Green Velvet, Green Mountain and many other varieties. Most importantly it taught us about life beyond our small boxwood world.”

The exploration into new varieties of boxwood began to shed light on another lurking boxwood pest. “There was a field planting of Buxus sempervirens ‘Elizabeth Inglis’ that had just been eaten up with Boxwood Leafminer,” Bennett remembers. “It had never been sprayed and was just covered in blistered leaves.” Deciding this might be something worth exploring, they planted some other varieties near the host plant to see if different cultivars showed any natural resistance to the pest. Early observations showed that there was some hope for these different cultivars which encouraged Saunders to pursue more formal research.

Boxwood Leafminer larvae causing the boxwood leaves to swell and blister.

Dissecting boxwood leaves to count larvae.

In the early 2000s, Boxwood Leafminer incidences started to become more prevalent, coinciding with a general desire to reduce the use of pesticides, particularly the neonicotinoids. Saunders Brothers recruited retired nematologist Bob Dunn from the University of Florida to help them set up a replicated plot to collect data. They began by planting a test block where host plants infected with Leafminer were planted alongside many different cultivars of boxwood. Since the fall of 2006, thousands of leaf samples from the test area have been collected. Each leaf was dissected, and larvae were counted. The collection of this data over a decade showed a strong variation of cultivars’ natural resistance to Leafminer. This in turn meant a reduction in the use of pesticides and an increase in the value of boxwood in today’s gardens.

Bob Dunn, retired nematologist, working on Boxwood Leafminer research.

Boxwood Blight affecting a field nursery.

Then, in the fall of 2011, Bennett recalls that, “we began to hear horror stories about a disease called Boxwood Blight which was devastating boxwood nurseries in North Carolina and Connecticut. We called a meeting the Monday after Thanksgiving to decide what to do. We decided we needed to go to Europe before the MANTS show to have more information to better answer customers’ questions. We immediately bought tickets for a team from Saunders to go to Europe the week before Christmas. My wife was not happy, but it was something we had to do.”

Researcher in Europe showing examples of Boxwood Blight.

Bennett Saunders loading a truck with sample plants for NCSU trials.

Since the disease had been prevalent in Europe since the mid-1990s, members of the Saunders team spent time in Belgium and the United Kingdom to learn tactics to combat it. European researchers mentioned that they had seen that genetic tolerance for the disease varied among boxwood species. Saunders realized that trials could be performed, similar to the Leafminer trials, to discern genetic tolerance. In 2012, Saunders Brothers teamed up with Kelly Ivors at North Carolina State University to begin trialing different boxwood species and cultivars. Thanks to years of collection, many different boxwood cultivars existed at the nursery in Virginia, so Saunders sent samples of these varieties to be part of the trials. This research proved to be so insightful that after the research was completed at NCSU in 2015, Saunders continued the trials privately. It was clear this was a challenge that could be overcome.

Kelly Ivors (middle) and team at NCSU.

NewGen Independence® in field trials.

 Since learning about the disease, enormous resources have been dedicated to learning about Boxwood Blight. Saunders Brothers has donated thousands of plants, given input into multiple research projects, and made every attempt to better educate themselves and the growing community about this disease. Teams have traveled domestically and internationally to better understand a disease that many once thought would be the end of boxwood. Working with researchers from state and federal agencies, as well as international groups, the continued message was apparent that through a greater understanding of the disease, the battle with Boxwood Blight will be won with tolerant varieties and best management practices.

NewGen Freedom® in production.

Holding on to that message of hope, and after years of trialing, certain cultivars continued to outperform many of the popular varieties on the market. Two of those cultivars were chosen because they stood out from the rest. “These plants showed up and made us smile and think ‘We’ve really got something here’ that we can share with the industry,” remembers Robert. The discovery of these beautiful plants characterized by their superior resistance to Boxwood Leafminer, high tolerance of Boxwood Blight, and WOW factor in the landscape led to the creation of the NewGen™ brand. You can read more about that story here.

From one 13-year-old boy’s love of a charming little evergreen grew a family business dedicated to innovation and looking for solutions. Saunders Genetics, LLC was created to become a resource for all boxwood enthusiasts for generations to come. With over 70 years’ of experience and a passion for boxwood, they are excited to introduce a new generation of boxwood to the industry.

Paul Saunders and his wife, Tatum, at their home.

Boxwood Leafminer

The arrival of Spring means insect pests are just around the corner. One pest to watch out for is the Boxwood Leafminer (Monarthropalpus flavus).  Although boxwood tend to have few pests, the leafminer can be a headache for growers. Damage starts small with some discolored leaves, but as populations build over a period of years, you will see defoliation, and in severe cases, plant death. Luckily, there are effective options to combat this pest.

Boxwood Leafminers are tiny, orange, gnat-like midges that have only one life cycle per year. The cycle begins each spring as the previous year’s larvae cause the boxwood leaves to blister and swell. Those larvae pupate and emerge as adults in April/May (Central Virginia). After pupating, the adults hover only inches above the boxwood because they are weak flyers and wind will blow them away. Over a 3-week period, adults emerge in waves to mate. Females complete their life cycle after they lay their eggs in the tender leaves. These eggs hatch in early summer (mid-June) and the larvae begin the cycle of growth that will conclude the following spring.

Saunders Genetics has done extensive work trying to determine the best timing and chemicals to control Boxwood Leafminer. In many programs, sprays are targeted towards adults, but because leafminer emerges in waves, you would need to spray every 2-3 days over the 3-week period, making it nearly impossible to eradicate them. Instead, control can be achieved with a single spray of a systemic insecticide after eggs hatch (mid-June). These types of insecticides remain effective until temperatures turn cold in the fall. When a spray is applied properly and thoroughly to all boxwood in a given area, you can receive control for up to 2-3 years. According to research, the most effective products are those that contain the active ingredient imidacloprid, thiomethoxam, or dinotefuran. Please note that Saunders Genetics bases this recommendation on in-house trials. We recommend growers and landscapers contact their local Extension personnel for recommendations and regulations in their area.

Although chemical control is a great option, there are also boxwood varieties available that have shown genetic resistance to Boxwood Leafminer. Saunders Genetics has done thorough research, testing over 129 cultivars. The results are listed in the chart below.

Genetic resistance is a defining feature in the NewGen™ Boxwood program. The goal of NewGen™ is to provide grower-friendly plants that can thrive when faced with pest and disease pressure. NewGen™ boxwood were chosen because they are more resistant to Boxwood Leafminer than many of the varieties currently on the market. If you are replacing or planting boxwood, look to these NewGen™ varieties which will be available for purchase in 2020.