Boxwood Leafminer is considered the most destructive insect that damages Boxwoods. This pest occurs across the entire United States and can infect all types of Boxwoods. If left unchecked, plants will decline, look poorly, and eventually die.
The insect itself is a small orange gnat but the damage is caused by the larvae, a small worm-like maggot that feed inside the Boxwood leaves.
How To Recognize Boxwood Leafminer
There are two opportunities to identify that Boxwood Leafminer is an issue on your plants including identifying damage to the leaves and identifying the small gnats.
In mid to late spring, the adults emerge and swarm around Boxwoods for 10-14 days. These small, orange gnats fly around for about a month to lay eggs on the leaves. Each individual adult only lives for about 24 hours. If you see the adults, chances are good your plants have or will have the harmful maggots doing damage to your plants.
The other sign of trouble is the damage the maggot does to the leaves. The eggs that were layed will hatch in about three weeks. The maggots will then feed inside the layers of the leaves until fall. They will overwinter there until spring when they will pupate and emerge as adults, starting the whole process over again. There is only one generation of Boxwood Leafminers each year. The damage caused by the maggots resemble pale blisters on the surface of the leaves. These blisters are very obvious and easy to recognize.
How To Treat Boxwood Leafminer
There are several ways to treat Boxwoods that have been infected with Boxwood Leafminer including choosing resistant varieties, keeping boxwood plants healthy, encouraging natural predators, pruning, and chemical control. Both organic and chemical approaches can be effective if timed correctly.
Choose Resistant Varieties
Slower growing Boxwood varieties, such as those that are English Boxwood cultivars, tend to be more resistant than faster growing varieties, such as those that are American Boxwood varieties.
Keep Boxwood Plants Healthy
Healthy plants can better withstand infestation than unhealthy plants. Maintain plant vigor through proper fertilization, mulching and watering.
Encourage Natural Predators
There are several predatory insects that will help keep Boxwood Leafminers in check. These include Lacewings and Spiders. One important note; if you choose to use chemical controls (pesticides) to control the Boxwood Leafminers, the beneficial predatory insects will also be killed.
Well Timed Pruning
Pruning Boxwoods at the right time can help reduce the Boxwood Leafminer population. This can be done by pruning the foliage right before the adult emerges (mid to late spring) or right after the adult lays their eggs. As always, be sure to do a thorough job cleaning up the cuttings.
Applying Sevin (buy here) or Malathion (buy here) is effective for controlling the adult Boxwood Leafminer. An application of Orthene (buy here) about 3-4 weeks after the adult have emerged will help control the larvae developing in the new leaves. Applying Merit (buy here) around the base of the plant in February through late April will systemically* control maggots before they can pupate and emerge as adults. Make sure to read and follow label instructions anytime pesticides are used.
*Systemic pesticides are taken up by a plant’s root system and distributed throughout the plant, killing the insects that are feeding on it.
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