Swarms of boxwood leafminer (Monarthropalpus flavus) adult flies are flitting around their namesake host. I visited a boxwood planting this past weekend that had so many flies flitting around it was difficult not to inhale a few when standing near the infested plants.
The tiny flies are a type of wood midge and belong to the same family (Cecidomyiidae) as gall midges. Except for their bright orange abdomens, the adults superficially resemble miniature mosquitoes both in body form and flight behavior.
The tiny flies begin to emerge from their leafmine abodes once the accumulated Growing Degree Days reach 440. This is about the same time red horsechestnuts (Aesculus × carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom.
The flies emerge from last season's leaves with their emergence heralded by peg-like pupal skins protruding from small translucent "windowpanes" created by the larvae in the lower leaf surface. The pupae will wiggle through these weak points to ease the emergence of fragile adults. Adult emergence is confined to a 10 – 14 day period with the adults living for just 24 hrs.
The females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces of this season's new boxwood leaves. Consequently, heavy spring growth can mask previous leaf damage reducing the detection of even a heavy leafminer infestation. However, I've found that adults suspended in spider webbing can aid in detecting a boxwood leafminer problem.
Each leaf may contain multiple oviposition sites with several eggs per site. The eggs hatch in early summer and the resulting larvae spend the remainder of the season consume interior leaf tissue as they develop through the 1st and 2nd instar stages. Most of these first-season mines appear as slightly raised "blister mines."
Winter is spent as 3rd instar larvae inside the leafmines. The larvae resume feeding in the spring and develop through a 4th instar stage. Much of the leaf damage occurs in early spring with the ravenous larvae rapidly expanding their leafmines. Multiple blister mines may coalesce causing the upper and lower leaf surfaces to delaminate over the entire leaf. Individual mines may turn reddish-green with heavily mined leaves turning from yellow to orangish-brown causing the leafmining damage to be mistaken for winter injury.
Boxwood leafminer can be managed through properly timed insecticide applications or through plant selection which provides a more long term solution. A helpful research-based listing of the relative susceptibility of boxwoods to the leafminer was published in 2014 by the American Boxwood Society in their "The Boxwood Bulletin."
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Topical and systemic insecticide applications should be delayed until after boxwood there are no viable boxwood blooms. Boxwood blooms attract a wide range of pollinators; blooming plants can literally buzz with their activity. Fortunately, boxwoods are finished blooming in southwest Ohio.
Properly timed topical applications of pyrethroid insecticides targeting the adults before eggs are deposited are effective in reducing first-season leafmining damage. However, these foliar sprays may also kill beneficial insects.
Systemic neonicotinoids such as imidacloprid (e.g. Merit, Marathon, and generics) or dinotefuran (e.g. Safari or Zylam) can kill early instar leafmining larvae before they produce significant damage. These will have a lower impact on beneficial insects.