Box Tree Moth Larvae Are Starting to Feed in Toronto

LOOK FOR these EARLY SIGNS: Overwintered BOX TREE MOTH LARVAE are waking up and starting to feed on leaves in Toronto. These young larvae chew one side of the leaf and leave behind webbing and frass. (photo: J. Llewellyn).

Box tree moth larvae are emerging from their hibernaria, gradually more and more overwintered larvae will emerge each day as the weather starts to warm up again.

The early instar larvae are between 1 and 1.5 cm long, but they can be difficult to find because of their size and the fact that they like to hide between tiny boxwood leaves that they web together. They seem to prefer boxwood plants that receive partial shade but can also be found in full sun gardens.

Their webbing can be very difficult to see, they will often feed in between leaves that it webs together.
Gently pull leaves apart to find larvae inside the webby mass…(this is the same leaf as the photo above). Look for green larvae with black spots and black heads. If you can only find webbing and frass, the larva may have moved to a nearby twig.
The larval webbing can be difficult to detect. I find having low magnification reading glasses really helps!
As you can see, early instar box tree moth larvae and damage can be difficult to detect. Notice the leaves with surface chewing that has dried out, this is feeding damage from young larvae. Notice the webbing and frass (larval excrement).
Look for bleached white leaves that appear chewed on one side when inspected up close. There is a good chance of finding larva on this plant. The leaf damage is from the previous year.
See how the leaf was chewed just on the one side? The result is a window-pane effect. This is early instar larval feeding damage is from the previous year (and NOT winter desiccation). Winter desiccation can result in leaves that are bleached, but the leaf tissue will look intact and not chewed.
Look for mature larva damage from last year: leaves with only the margins left behind. There may be overwintered larvae on this plant, take a closer look.

Watch this video. Inspect your clients boxwood for actively feeding BTM larvae and tree boxwood shrubs to prevent the next generation of larvae and slow the spread of this important pest. Look closely and carefully pull twigs apart to look for webbing, frass and small larvae (1.5 cm long).

These Symptoms are NOT From Box Tree Moth:

Webbing on foliage can also be caused by SPIDERS. Webbing alone is never an indicator of box tree moth. (photo: J. Llewellyn)
This is NOT box tree moth, this is low temperature injury of current season’s new foliage (from frost events during leaf emergence) and winter desiccation on last season’s leaves on boxwood.
These dry leaf edges might catch your eye, they are from pruning cuts that were made during the previous season.

Insecticides for Box Tree Moth (Dipel, Bioprotec, Xentari): wait until mid-late May when all overwintered larvae are actively feeding and when temperatures are consistently warm.

Dipel, Bioprotec and Xentari contain a lethal bacterium such that when BTM larvae consume the spray residue on the leaves, they soon stop feeding and expire within a couple of days after that.

Pheromone Traps for BTM

For those of you wanting to order some BTM pheromone traps, here is what we have been using. Each trap with pheromone is effective at attracting adult males.

Here is the trap we used from Solida.ca Gypsy moth trap (milk carton style, with sticky folding trap liner inside) Item #2050500

UniTrap Standard, Bucket Funel Trap Manufacturers and Suppliers - Pricelist  of Customized Products - Pherobio Technology
Or you can use their re-usable Unitrap
(Item #301Y602), it is very effective at catching adult moths and can be reused from year to year
Each trap comes with a folding sticky card to place in the bottom of the trap to trap the moths.

Inside we used two pheromone lures to be effective for the growing season:

Injecting 3-Month-Longevity Gel Lure inside Trap
  • 1) Box Tree Moth Gel Lure (3 month longevity, Item #40M2001). Inject gel pheromone inside trap to attract BTM from mid-May to mid-August.
Video tutorials on using pheromone traps to monitor corn ear worm in sweet  corn - Vegetables
1 Month Longevity Septa Lure
  • And 2) Box Tree Moth Septa Lure (1 month longevity, item #401PS04). Then in mid-August, we drop in a Box Tree Moth Septa Lure inside the bottom of the trap to attract BTM adults from mid-August to September) Item #40IPS04. We store these lures in the freezer until we need them and buy extra, just in case.

Cut the trap opening on the side of the carton such that the 2 openings become 1 larger opening (see photo attached) to facilitate BTM adults entering trap. Install trap to hang about 1 meter from the ground.

Here are some box tree moth adults on a sticky card for reference

About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
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