City Gardener: The invasion of the boxwood snatchers

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Southern Ontario has been identified as an epicentre of the deadly box tree moth – deadly
to boxwood shrubs, that is

Southern Ontario has been invaded by a monster from Japan. Okay, it’s not quite Godzilla, but it’s actually far more destructive: known as the box tree moth, this miniature monster can wipe out your boxwoods, and since it has no natural enemies in Canada, it’s spreading unchecked across the region.

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It’s believed that the moth first arrived in Canada around 2018 on infested shrubs from outside the country, possibly the U.S. or Europe.

A native of east Asia, it exclusively dines on boxwood, which unfortunately, happens to be one of the most popular hedge and garden shrubs grown here in southern Ontario. The result has been a population explosion; if you grow boxwood hedges and haven’t fallen victim yet, be on your guard.

Box tree moths have brown-edged white wings about 4 cm across. But it’s their larvae, which hatch from eggs laid on the undersides of plant leaves, that are the real scourge: they’re green with black heads, with a double stripe of black dots down their back.

When the caterpillars are ready to pupate and turn into moths, they spin silky pupae that you can spot inside the branches.

If your shrubs aren’t too badly affected, and you can see caterpillars or eggs on the leaves, you can pick them off manually; but generally speaking, once you’ve seen them, there’s no time to dawdle.

These little creeps are so voracious, they’ve been known to strip a boxwood shrub to twigs in a matter of days, then start in on the bark and young stems.

After that, they’ll head over to your neighbour’s boxwoods, which is how you probably ended up with them in the first place. (The moths have been known to travel as much as 5 to 10 kilometres in search of their favourite food.)

Fortunately, one of the most effective insecticides is an organic product called BTK, which you can buy at most garden centres and box stores. BTK is a foliar spray that you apply with a pressure sprayer to the entire plant.

Carefully following the instructions on the box, thoroughly saturate the shrub, especially the interior and the undersides of leaves, where they pupate and lay their eggs. You’ll have to repeat this process every five to seven days, until all evidence of the caterpillars is gone.

It is possible for infested shrubs to recover and grow new leaves, if you’ve caught the infestation in time. The thing to watch for is damaged bark and trunks.

Look for bare spots, especially if the bark has been removed right round the trunk, known as “girdling.” If that’s happened, the shrub is doomed, and will need to be dug up and replaced.

However, do not just dig up the dead shrubs and bundle them for collection. (Especially, don’t put them in your compost pile.) Even if you’ve gone the BTK route, if any eggs or caterpillars survive, they could rise from the dead to terrorize the city again.

The best solution is to burn them down to ash – though that’s not always possible in the city. An old-fashioned but effective method is heat destruction: seal the dead plant inside a black plastic bag and leave it out in direct sunlight for a couple of days. A third solution is to bury them at least a metre deep underground.

Whether you’ve suffered an invasion or not, when you do your regular rounds to inspect your garden, it’s good to have a bottle of Safer’s soap (or whichever everyday pest control you prefer) in hand to catch aphids, lily beetles and other common pests. Apparently it works on box tree caterpillars too, but only if you spot one or two at a time.

Please feel free to write in with questions, to comment or to share your own city gardening adventures
with Martha. Write to her at marthasgarden07@gmail.com.