Participants in this week's Greater Cincinnati BYGLive! Diagnostic Walk-About were treated to an enormous swarm of boxwood leafminer (Monarthropalpus flavus) adult flies flitting around their namesake host. There were so many flies it was difficult not to inhale a few when standing near the infested plants.
The 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. This non-native was accidentally introduced into North America from Europe in the early 1900s and its characteristic leafmining damage is now common throughout Ohio.
The tiny flies emerge from their leafmine abodes at around the same time red horsechestnuts (Aesculus×carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom (440 GDD). Adult emergence is 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 emergence of the fragile adults.
Walk-About participants observed the heavy adult flight at around 9:30 am. However, few adults were on the wing when I went back to photograph them at 1:30 pm. I've observed and photographed adults on heavily infested boxwoods in the past, but never the magnificent numbers we observed in the morning. In reviewing image information, I found that all of my past photographs were taken in the afternoon.
I suspect this may indicate a diurnal rhythm to the adult flight activity focused on the early morning hours although I could find no support for this in the literature. However, adult flight synchrony makes sense for supporting successful mating given that adult emergence is confined to a 10 – 14 day period and individuals only live for 24 hrs.
The flies emerge from last season's leaves then 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 and reduce the detection of 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.
Properly timed topical applications of pyrethroid insecticides targeting the adults before eggs are deposited are effective in reducing first-season leafmining damage. However, plants should be closely inspected to make certain there are no viable 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. Unfortunately, peak adult emergence activity appears to be coming to a close in the southern part of the state, so large numbers of eggs have already been inserted into the leaves.
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. However, applications should be delayed until AFTER boxwoods bloom to protect pollinators.
Plant selection provides a more long term solution to the depredations of boxwood leafminer by removing insecticides from the management equation. 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." You can access this publication by clicking on this hotlink: