Boxwood leafminers are back in Johnson County — here’s how to prevent infestations, treat shrubs

Boxwood leafminers

Spring is here and warming up, so boxwood leafminers are back. These bugs infest boxwood shrubs and can damage them, making them unsightly. Photo credit Michigan State University ANR Communications. Used under a Creative Commons license.

Some homeowners in Johnson County, particularly in older neighborhoods concentrated in the northeast, may have noticed swarms of insects gathering around shrubs in their front lawns.

These bugs are boxwood leafminers, and they’re not exactly good for the health of your decorative plants.

Dennis Patton at the K-State Research and Extension Office in Olathe, who serves as the county’s horticulturist, said it’s that time of year when these insects are active. But depending on how much damage the bugs have done to boxwood plants, the shrubs could still survive.

Patton said the extension office has received several calls in recent weeks about the insect and its damage to plants.

“The description kind of goes, ‘I was out in the garden around the boxwood and noticed a whole bunch of little bat-like insects flying around,’” Patton said, noting that all signs and descriptions from gardeners seem to point to the conclusion that this is the boxwood leafminer.

What to look for

The boxwood leafminer is a bat-like insect that lays eggs on the boxwood leaf. The larvae then bore into the leaf, eating the fleshy insides. Photo credit Matt Claghorn. Used under a Creative Commons license.

Adult leafminers lay eggs on boxwood leaves. Then, larvae bore into the leaf and eat the soft, fleshy insides. That causes leaves to get blistered or puckered, and then eventually turns the leaf brown and defoliates.

“It doesn’t, probably, in most cases, kill the plant, but it causes enough leaf drop that it becomes unsightly,” Patton said. “In severe infestations going on long enough, it can kill the boxwood, but normally it just goes from healthy to unsightly, and that’s where people get concerned because their beautiful, green boxwood.”

Patton says two factors are at play in the sudden emergence of the boxwood leafminer: timing and location.

Timing

The life cycle of the boxwood leafminer syncs up with the flowering of the weigela shrub.

In general, many insects emerge as temperatures warm up and they instinctively know when to come out in the springtime.

So, in the case of the boxwood leafminers, these insects tend to emerge at the same time that the weigela bush starts to flower. This is critical information for shrub caretakers to know because they can time the preventive treatment of boxwoods with the blooming of other shrubs.

Location

The location also seems to play a factor in the emergence of the boxwood leafminer.

Patton said these bugs tend to emerge in older, established neighborhoods, which are generally concentrated in the cities in the northeast corner of Johnson County.

How to stop it

Homeowners with boxwood shrubs can use insecticide treatments on their shrubs to prevent a leafminer infestation.

Patton said to go ahead and spray this time of year, which prevents the leafminer eggs from hatching and boring into the leaf.

As always when using insecticide, read all instructions carefully before applying.

Another solution is placing insecticide drenches at the base of the plant. Patton said this option allows the shrub to uptake the treatment systemically, killing the leafminer as it feeds on the plant.

Patton recommends folks give the extension office a call at (913) 715-7000, or connect with a local lawn and tree service company to get help.

“When you purchase a new plant, make sure it’s insect- and pest-free, which it should be,” Patton added. “And scout your boxwood from time to time to see if they’re showing any symptoms.”