Damage to a row of boxwoods from boxwood leafminer. Photo: T. Butzler, Penn State
Would you like a plant in the landscape that has foliage year round (especially in the drab winter months)? Or how about a shrub that will grow in sunny to partial areas of the landscape? Can be heavily pruned into shapes? Or serve as a backdrop to an annual flower bed? Many homeowners would answer 'yes' to these questions and that is why boxwoods are one of the most popular shrubs on the market.
But with the many attributes of boxwood comes a negative element. In late March into early April, some boxwoods look pretty ragged before the new growth covers the problem. Not because of winter damage but for the activity of the boxwood leafminer. At this time, adult female leafminers (which look like gnats) fly about boxwoods looking for newly emerged leaves to lay their eggs. Some people have described it as mosquito-like insects swarming their boxwoods. Their specialized egg laying structure allows them to deposit the eggs right underneath the leaf surface. The eggs will hatch within a few weeks and the newly emerged larvae feed between the upper and lower leaf surface over the rest of the summer.
The adult boxwood leaf miner looks mosquito-like. Here, the female is thrusting a curved needle-like ovipositor through the lower surface of the leaf to lay her eggs. Photo: T. Butzler, Penn State
The activity between the two layers causes the leaf to blister and turn an olive-green or brown. This contrasts negatively against the boxwoods healthy bright green foliage. In some cases, the leaves can drop off and expose woody branches (not the look anyone is looking for). This rarely causes plant death. In late winter, the larva pupates and the gnat-like flies will emerge in May to start the whole process over again. There is only one generation a year.
Infested leaves are usually smaller and are an olive-green (as opposed to the normal dark green). Photo: T. Butzler, Penn State
If boxwoods are in a low profile area of the yard, and look terrible, just ignore them. Some boxwoods that are in very visible areas of the landscape and show great damage, may need to be managed for the leafminer. An insecticide is most effective when the adults are in flight, usually May, and before the female lays her eggs. A general rule of thumb is that treatment could be timed when Weigel sp. are in bloom.
The larval stages (orange in color) of the boxwood leafminer. Notice that the bottom portion of the leaf was removed to expose the leafminer (upper surface of leaf is still intact). Photo: T. Butzler, Penn State
Boxwood blight has received a lot of attention of late and one might point to the poor appearance of their boxwoods to this fungal organism rather than the leafminer. Symptoms of boxwood blight appear as dark circular spots on new foliage. These spots grow to the point where they appear over the whole leaf. At this point, leaves begin dropping and eventually only the twigs are left behind. Plant death shortly follows.