Blistered Boxwoods and Hissing Hedges

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What exactly do you see?  This question is helpful in separating look-alike symptoms caused by different plant maladies.  It's so useful; we made it Question #6 in our "20 Questions of Plant Problem Diagnostics" OSU Fact Sheet (soon to be updated to "23 Questions …").  The question forces us to take a second, closer look.


Yesterday, I spotted at a distance what appeared to be symptoms of winter injury on boxwoods in southwest Ohio.  Upon closer inspection, the telltale blister-like leaf symptoms caused by the boxwood leafminer (Monarthropalpus flavus) came into sharp focus.


Boxwood Leafminer Symptoms


Boxwood Leafminer Symptoms


Gently separating the upper and lower leaf surfaces (the leafminer had already done most of the work!) revealed that the bright yellow leafmining larvae (maggots) of this midge fly are completing their spring development with pupae being found in some leafmines.  The yellow pupae will eventually become orangish-yellow as this stage nears completion.  Pupation does not usually occur until mid-to-late April in my part of the state.  The accelerated development is no doubt due to our balmy winter temperatures.


Boxwood Leafminer Larvae


Boxwood Leafminer Pupae


This non-native midge fly was accidentally introduced into North America from Europe in the early 1900s and is now common throughout Ohio, particularly in the southern and central parts of the state.  Adults emerge at around the same time red horsechestnuts (Aesculus × carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom (440 GDD).  Except for their bright orange abdomens, the adults superficially resemble miniature mosquitoes.  Females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces of boxwood leaves.  Several eggs may be laid per leaf. 


Boxwood Leafminer Adults


Eggs hatch in early-summer and the resulting larvae spend the remainder of the season developing through the 1st and 2nd instar stages as they consume interior leaf tissue.  Winter is spent as 3rd instar larvae inside the leafmines.  In the spring, the larvae resume feeding and develop through a 4th instar stage.  Much of the leaf damage occurs during this time with the ravenous larvae rapidly expanding their leafmines to produce the characteristic blister-mines.  Heavily mined leaves turn from yellow to orangish-brown causing the leafmining damage to be commonly mistaken for winter injury.


A close examination of the leafmines at this time of the year will also reveal small translucent "windows" created by the larvae in the lower leaf surface.  The pupae wiggle through these weak points to ease emergence of the fragile adults.  This pupal activity is responsible for one of the most unusual features of this midge fly:  reports of hissing, crackling, or rustling sounds coming from heavily infested boxwoods.  I first reported on this strange phenomenon in the BYGL in 2003 (BYGL 2003-06) and again in 2004 (BYGL 2004-02) and 2010 (BYGL 2010-01). So, reports from gardeners or landscapers that they've heard boxwoods going snap, crackle, and pop should be taken seriously as the odd sounds are an indicator of a heavy boxwood leafminer infestation.


Effective control options for boxwood leafminer include applications of neonicotinoids such as imidacloprid (e.g. Merit, Marathon, and generics) and dinotefuran (e.g. Safari or Zylam).  As always, read and follow label directions paying particular attention to the "Protection of Pollinators" restrictions including delaying applications until after bloom.  The delayed applications will kill early instar larvae later in the season before they produce significant injury.  Adults can be targeted with topical applications of a pyrethroid insecticide such as bifenthrin (e.g. Talstar).  However, this requires close monitoring to time applications to the adult flight and there is a risk some females will successfully lay eggs before being killed.  Of course, topical applications should never be made to plants in bloom including off-target drift onto nearby plants.