Plant infested with boxwood leaminer. Photo: Sandy Feather, Penn State
A quick Internet search revealed that boxwoods with a heavy leafminer infestation often make a crackling noise. When I transitioned from consumer to commercial horticulture, I began doing regular woody ornamental scouting and made a note of boxwood leafminer larval development and adult emergence. I often put an ear to the shrubs I was scouting, but I never heard a peep. Of course, the plants I used for scouting were well managed and had minor or no infestations.
Fast-forward to several years ago, when a group of horticulture geeks met at a friend’s house for brunch, and dividing and sharing perennial plants. I found myself working in an area of the garden that was bordered by a boxwood hedge that had a substantial leafminer infestation. To my amazement, the boxwoods sounded like a bowl of Rice Krispies in milk! Snap! Crackle! Pop! I called my friends over to share in the fun of my discovery, and they were as fascinated as I was.
Boxwood leafminer larvae voraciously feed as they mature in spring, leaving a fragile, translucent “window” of leaf tissue on the lower leaf surface. As they pupate, they work their way through this thin leaf tissue to facilitate the emergence of adults—which is the cause of the crackling sound. If you examine the underside of infested leaves, you can often find the pupal cases sticking out.
Boxwood leafminer pupal cases are sticking out of the underside of the leaf. Photo: Sandy Feather, Penn State
Boxwood leafminer adults are orange-red flies that resemble mosquitos. They emerge between 448 and 700 growing degree-days, roughly about the time that weigela is in bloom.
Management strategies include:
- Selecting varieties that are less susceptible to boxwood leafminer, including Buxus sempervirens ‘Suffruticosa,’ ‘Pyramidalis,’ ‘Argenteo-variagata,’ and ‘Varder Valley.’
- Pruning right before adults emerge or right after adults lay their eggs can reduce the overall population. Be sure to clean up the clippings as thoroughly as possible and remove them from customers’ properties.
- Chemical controls include abamectin, acetamiprid, azadirachtin, bifenthrin, carbaryl, imidacloprid, malathion, permethrin, spinosad, and trichlorfon. Make applications to control adults from mid-May to early June—repeat applications as needed.
- Applications to control larvae should be made early to mid-summer, 1200–2400 growing degree-days. Systemic products such as imidacloprid and acetamiprid or those with translaminar activity such as azadirachtin or spinosad will provide better control of larvae in the leaves.
Be aware that all of the products labeled to control boxwood leafminer are toxic to bees.