Planting a boxwood? Think again, says Hamilton arborist

Deathe said boxwoods are being attacked by two problem pests — the box tree moth and boxwood leafminer — leading to headaches for homeowners.

These creatures have become so pervasive in Hamilton over the last few years that Deathe recommends finding an alternative hedge plant when upgrading your landscaping.

The Canadian Food Inspection Agency notes the box tree moth spreads primarily through the movement of infested boxwood plants and can fly between five and 10 kilometres per year. Signs of this invasive species include chewed leaves, larvae, webbing and sawdust-like debris.

“They strip and defoliate areas and they hide within the plant,” said Deathe. “By the time the homeowner recognizes they have a problem, that section of the plant is defoliated.”

Deathe noted box tree moths are masters of camouflage. The green and black caterpillars blend in with the leaves to avoid detection.

For homeowners with more than 10 linear feet of boxwood, he suggests applying the insecticide Btk. “Anything less than that, unless the plant is of significant importance to the overall look of the landscaping, remove and replace with a species that doesn’t get box tree moth.”

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• Dec 31, 1969
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• Dec 31, 1969
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Deathe added multiple applications of Btk may be necessary in the spring and summer to keep the box tree moth at bay.

If you think you’ve spotted a box tree moth or an infestation, you’re to report it to the Canadian Food Inspection Agency.

The boxwood leafminer is another cause for concern.

The pest attacks several boxwood varieties, weakening the foliage, and can make mature plants susceptible to other diseases. The yellow maggot-like larvae consumes the green tissue inside the leaf. The orange or red adult gnats measure about one quarter of a centimetre.

“They’re chewing up the leaf right now,” said Deathe.

If you break open a leaf on an infested plant, you’ll see the tiny larvae wiggling around, he added.

“Btk will not control boxwood leafminer,” Deathe said. “It’s not exposed and on the outside of the leaf. It’s within the two layers of the leaf. So it is protected and that’s what makes it difficult to control.”

Instead of spraying, Deathe said homeowners can trim the shrub to remove most of the eggs before they hatch.

“If you trim that off — and do a very good cleanup afterwards by removing the caterpillar before it hatches — you’ll notice that previous year’s growth within the boxwood usually doesn’t have the larvae,” said Deathe. “So you’re reducing the population by 80 or 90 per cent by just trimming and removing the affected leaf.”

To knock down the moth and reduce the number of eggs on future growth, homeowners can spray a nursery insecticide containing horticultural soap mixed with a small amount of permethrin.

Overall, given the threats posed by the box tree moth and boxwood leafminer, Deathe said he no longer recommends boxwood shrubs.

“When you couple the two pest disorders together and you don’t have many boxwoods, it’s not probably worthwhile in retaining that plant material,” he said.