Boxwood leafminer (Monarthropalpusi flavus) is a common and destructive pest that causes significant damage to boxwoods here in the Dayton area, although the symptoms are often mistaken for winter injury rather than insect infestation.
Since boxwoods are such a popular shrub, it’s important to control leafminers so they don’t spread to neighboring plants and properties.
What is a Boxwood Leafminer?
The boxwood leafminer is a small bright orange fly (a midge) that looks a lot like a mosquito. It lays its eggs between the upper and lower surface of boxwood leaves.
When the eggs hatch, the yellowish-orange larvae (maggots) feed on the inside of the leaf, creating “mines” throughout the leaf that look like blisters on the leaf surface. In spring, they pupate and then emerge from the leaves as adults to start the cycle all over again.
Recognizing the Signs of Boxwood Leafminer Damage
There are several symptoms of boxwood leafminer infestation to look for.
First, make sure it’s not winter burn damage (boxwoods are susceptible to this). Winter burn creates a brown, discolored margin around the border of the leaf, making it look like the edges have dried out. In contrast, boxwood leafminer damage appears all over the leaf surface.
If you look closely at the underside of the leaves, you’ll see small blisters caused by the larvae inside. And if you peel a leaf apart, you’ll see the maggots – they’re hard to miss.
Color is another feature to look for. Infested leaves turn yellowish, may appear spotted and generally look unhealthy. Over time, the shrub will decline and can eventually die.
As the adults emerge, you may see small, rice-shaped things hanging under the leaves. These are the pupal skins left behind by the emerging midges. Look for these in April or May, depending on weather (a good rule of thumb is to look when the weigela bloom).
In cases of heavy infestation, you’ll often see swarms of orange, mosquito-like midges flying around boxwoods, especially if you shake the shrubs. You may even hear a hissing or popping sound.
Controlling Boxwood Leafminer
If you’re only just thinking about planting boxwood, consider using cultivars that are more resistant to the leafminer. All varieties are susceptible but cultivars of English boxwood are more resistant than American (or littleleaf) boxwood (Buxus microphylla) cultivars. Some to try are Buxus sempervirens ‘Pendula,’ ‘Suffruticosa,’ ‘Handworthiensis,’ ‘Pyramidalis,’ ‘Argenteo-varigata’ and ‘Varder Valley.’
Another option is to prune the shrub before adults emerge in spring, or right after eggs are laid in May. This can reduce the population overall.
If only a few parts of a shrub are infested, try squeezing the affected leaves hard. This will kill the maggots inside. However, it’s not a practical approach when large parts of the shrub, or multiple shrubs, are infested with leafminers.
Professionals control boxwood leafminer by applying a systemic insecticide in spring (usually in March to early April). This kills the larvae inside the leaves before they can emerge as adults to lay new eggs. It’s generally more effective than targeting the adults later in the season.