Hello,
I know I have leaf miner affecting my pyramid boxwood. Would it be wise for me to prune off as much of the “infected” leaves now, (May 17) before the adults emerge in a few weeks and begin mating and laying their eggs? If I see the adults flying around (when the weigela blooms) would it be wise to spray with Malathion? I purchased this years ago when I had this problem. Lastly, after the new eggs hatch, could I spray with BTK for a number of weeks?
Many thanks for your help.
Thank you for contacting Toronto Master Gardeners about Boxwood Leafminer in your pyramidal boxwood. Toronto has experienced a surge in this invasive pest and the Toronto Master Gardener Guide below is an information source for treatment of this moth. In answer to your specific questions, below are excerpts from the Toronto Master Gardening Guide, University of Maryland Extension service and Carex Canada website.
1. Pruning boxwood back by about 1/3rd to remove the stems with infested leaves, will help reduce this pest. Dispose or destroy the infested material and debris by:
(1) deep burial to a depth of 1 meter covered by soil, or
(2) incineration to ash; or
(3) heat treatment – place plant material in a sealed black plastic bag and expose it to 48 hours of direct sunlight. Following heat treatment, dispose of the sealed bag in your garbage (not compost) bin.
Malathion is a pesticide professionally used in agricultural and commercial settings in Canada. On U.S. websites, Malathion is mentioned for use on Boxwood Leafminer. However, Ontario’s Cosmetic Pesticide Ban Act, the most comprehensive restrictions on lawn and garden pesticides in North America, prohibits the cosmetic use of over 250 pesticide products and over 95 pesticide ingredients, including malathion. See Carex Canada link above for specific regulations.
3. Box tree moth larvae can be effectively managed with a safe biological insecticide, BTK ((Dipel 2X DF PCP#26508), that can be legally used in Toronto. This is the same biological insecticide that is applied by air over the City of Toronto to combat Gypsy moth larvae in the spring. Dipel 2X DF contains a naturally occurring bacterium, Bacillus thuringiensis (B.t.)
When the box tree moth larvae consume the spray residue on the leaves, they soon stop feeding and expire within a couple of days. In answer to your question, this bacterial spray will not kill the eggs or pupae stage, so you need to time your spraying for when the larvae have hatched.
Frequent reinfestation is likely given the mobility of the adult moth. Treatment is not a “one and done”. You need to spray your box trees at least three times a year. Use a pressurized sprayer to spray, and focus on the bottom of the leaves. You can keep spraying every 5 – 7 days as long as you see any caterpillars or pupae on the plants. Note: only spray in the presence of larvae. Preventative spraying for long periods of time will affect good moth catepillars. OMAFRA (Ontario Ministry of Agriculture, Food and Rural Affairs) recommends that the first time to treat your plants and spray the larvae (worms) is between early May and mid-June. The second spraying is done in mid-July. The third spraying is done in mid September.
Gardening Guide on Box Tree Moth
We hope this information helps to remedy and save your pyramidal boxwood.