Boxwood Leafminer Control for Commercial Nurseries
Saunders Genetics has done extensive work trying to determine the best means for control of Boxwood Leafminer.
For minor infestations, some gardeners choose to prune off affected foliage in early spring before the adults emerge. This reduces the population of the pest, but offers limited results in heavy infestations as populations can explode from unpruned foliage.
Chemical Control Timing
Chemical controls are the best protection for complete control of Boxwood Leafminer. The most effective control of Boxwood Leafminer targets the larval stage because it is a long window and control is likely effective. Systemic insecticides applied just after the eggs hatch around mid to late June in central Virginia are the most effective. Since the larvae are alive, eating, and growing during the summer and fall, research has found a systemic insecticide spray can be very effective when applied throughout the summer and into the fall. The efficacy of the spray drops when temperatures turn seasonally cold, which in some years is not until late October or early November. When these chemicals are applied properly and thoroughly to all boxwood in a given area, they eradicate nearly the entire population and it takes several years for a population to return to noticeable levels. This remarkably easy strategy can provide control for up to 2-3 years!
It is unnecessary to time a chemical application that targets Boxwood Leafminer adults. Because the adults only live a few days, an adult-targeted spray would have to take place every 2 or 3 days over a two week or more period. This control strategy is futile.
Commercial Nurseries and Professional Growers
For Boxwood Leafminer control, growers have had excellent success with products in the neonicotinoid group that contain the active ingredient imidacloprid, thiamethoxam, 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 our 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.
The larvae-targeted neonicotinoid spray discussed above should be applied during a time when minimal flowers are open to minimize the effect on bees. Boxwood flower in the early Spring, so if the summer or fall spray has minimal drift to any other plant, there should be minimal impact to pollinators.
Examples of insecticides listed for licensed professionals in the Virginia Tech Nursery Crops Pest Management Guide are active ingredient thiamethoxam (Flagship® 25WG at a rate of 6 oz./100 gallons of water) or active ingredient dinotefuran (Safari® SG 4 to 8 oz. / 100 gallons of water). The applicator should thoroughly wet the entire plant to the point of runoff. There are also soil-drench granular products on the market with the same two active ingredients listed above that have been used very successfully. 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 in mid-to late June but should be effective through the fall of the year. Saunders Brothers, Inc. has used this control strategy for many years and found it to be very effective.