I first raised the alarm on boxwood leafminers (Monarthropalpus flavus) in a BYGL Alert posted in late March (see "Blistered Boxwoods and Hissing Hedges", March 30, 2017). That report focused on alerting landscape managers that high localized populations were producing heavy leafmining symptoms that could be mistaken for winter injury.
I began receiving reports this past weekend that boxwood leafminer midge fly adults were emerging en masse in parts of Greater Cincinnati. Indeed, I visited heavily infested boxwoods in a local park yesterday and was treated to clouds of the tiny flies billowing around the foliage. Some leaves still contained pupae while others had pupal skins hanging from lower leaf surfaces signaling that flies had left the building.
Except for their bright orange thorax and abdomens, the delicate-looking midge fly adults superficially resemble miniature mosquitoes. Spider webs can provide a helpful monitoring aid in determining whether or not boxwood leafminer midge flies are on the wing. It's easy to spot agglomerations of hapless midge flies entangled in the spider silk.
The newly emerged females mate and then use their sharp, needle-like ovipositors to insert eggs between the upper and lower leaf surfaces of boxwood leaves. If you look closely, you may see a female appearing to be "stuck" on a leaf as she lays her eggs. Several eggs may be laid per leaf. Eggs will hatch in a few weeks and the resulting yellowish-orange fly larvae (maggots) produce a tiny, blister-like leafmine.
I talked with Dave Shetlar (a.k.a. Bug Doc) yesterday about effective management options for heavily infested boxwoods. He indicated that insecticides targeting the adults may not provide a desired level of suppression. Adults emerge and lay eggs over a relatively short period of time. As Dave noted, a high percentage of the eggs may have already been laid by the time a topical adulticide is applied.
Dave recommended applying a systemic insecticide now to target the 1st instar maggots inside their leafmines. Effective systemic products include dinotefuran (e.g. Safari) or acetamiprid (e.g. TriStar) as foliar or basal bark applications. A foliar application of azadirachtin (e.g. AzaGuard, Azatrol, etc.) would also be effective owing to translaminar translocation of the active ingredient. Dave noted that products applied to the foliage should be mixed with a non-ionic surfactant (i.e. "spreader") to prevent the insecticides from rolling off the waxy boxwood leaf surface. Of course, the additive should not include a "sticker" which would prevent systemics from becoming systemic.