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Wednesday, September 28, 2022
Growing Together: Check your boxwood shrubs for infestation by box tree moth
Box Tree Moth Adult
Box Tree Moth Adult Joanne Young

Last fall, I wrote about a new insect that was finding its way into Niagara – the box tree moth (Cydalima perspectalis).

As the name indicates, it is primarily found on all species and cultivars of boxwood plants.  The box tree moth was first reported in Europe in 2007 and made its Canadian debut in 2018.

The moth was first seen in the Niagara-on-the-Lake area in 2020 and this year as I am visiting gardens, I am seeing many cases of the damage this insect is causing.

The box tree moth can have two to three generations per year. The adult moth has a wingspan of about four centimetres and has white wings with thick, dark brown borders around the entire wing.

Its body is also white with a few dark brown specks on it. When the larvae first hatch, they are greenish/yellow in colour with a shiny black head.

As the larvae mature, they become more green in colour, with thick black and thin white stripes along the length of the caterpillar.  It is the larvae that cause the damage on boxwoods.

The adult moth lays eggs on the underside of the boxwood leaves in clusters of 10 to 12 eggs. The eggs hatch in three days and the young larvae feed on the underside of the boxwood leaves and start to form webs between the leaves.

As the larvae mature, they become skeletonizers, leaving just the midribs of the leaf. If there is a heavy infestation it can turn the entire boxwood brown in a short time.

It takes about 14 days for the larvae to mature and pupate. Pupae live about 14 days before becoming adults. The moth overwinters in the larval stage, protecting itself by spinning a silky cocoon among the leaves.

Routinely examine your boxwoods for signs of chewed leaves, caterpillars, webbing and frass (a fancy term for caterpillar poop).  The best times to check for larvae are May 15 to June 15, July 15 to Aug, 15 and Sept. 1 to 20.

By frequently monitoring your plants, you can catch the problem early, making it much easier to take care of and stop it before it becomes a problem.

The best time to spray this pest is when it is in the larvae stage. There is a biological control spray called BTK (Bacillus thuringiensis var. kurstaki) whose active ingredient is safe  for humans and pets. It is sold at most retail garden centres.

If you still see the caterpillars feeding now, spray them. If you seei damage but no caterpillars hold off spraying until July 15 to Aug. 15.

BTK application is most effective when used two or three times a year during the time frames listed above. Recheck your plants five to seven days after spraying to see if the spray has worked.

If caterpillars are still feeding, repeat the application of BTK. You can also hire a professional arborist to spray for you.

Please note that even though high populations of box tree moth caterpillar may defoliate boxwood plants, and in some cases, result in plant dieback, as long as the outer stems are green, there is a possibility that the shrub can push out new leaves.

In severe cases, though, it may be necessary to remove or trim the infested shrub. Effective removal and disposal of infested plants is crucial to slowing the spread of box tree moth.

Place all cutting into a black plastic bag, tie it tightly and place in the full sun for 48 hours.  This should be enough to kill off the caterpillars. If you handpick the caterpillars, drop them into a bucket of soapy water.

All findings of the moth should be reported to officials who are tracking the pest. Contact the Canadian Food Inspection Agency via https://inspection.canada.ca.

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca