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Sunday, June 26, 2016

Life after pesticides

Two years ago I resolved to make a break with pesticides. Well, not a total break, because my hemlocks are still sprayed with horticultural oil for nonnative woolly adelgid, and I still spray for winter moth with spinosad, a natural product derived from bacteria. That was supposed to be all.

    Since I made that decision, I’ve reminded Lueders, the company that does the spraying, that I don’t want horticultural oil sprayed on anything but the hemlocks, and I want them sprayed only in early spring, when insects other than the adelgids are dormant and not in danger of being suffocated by the oil.

White adelgid egg sacs along hemlock twigs. These insects will kill a tree within 5 years

    I convinced myself that I wasn’t going to notice any difference when they stopped spraying other shrubs and trees with the horticultural oil. My reading told me that native insects I’d been spraying for were not going to kill the shrubs and trees. I imagined that the damage they’d do if untrammeled would hardly be noticeable. 

    It’s not turning out to be quite that simple. I’ve already noticed discolored and dying leaves on boxwood. I pruned out some of the worst-looking branches. A few weeks later, more leaves show dots and discoloration. 

Boxwood foliage is yellower than normal, and leaves are stippled with bites

I’d imagined that box (Buxus sempervirens) grew healthily in my garden. More accurately, it looked healthy as long as it got annual spraying.

    With a sinking heart, I noticed last week that scale insects are back on the undersides of the leaves of a variegated kiwi vine (Actinidia kolomikta). I knew they were there last year and hoped I’d eliminated them by swabbing them off the leaves with rubbing alcohol.

White cottony covering protects scale insect on kiwi leaves

    Lueders inspected in March and warned that scale was also affecting two magnolias. I’d noticed black staining on the branches, probably from mold that grew on the insects’ honeydew secretions. 

    So now what? Lueders recommends spraying more horticultural oil. They’d resume treating the magnolias and boxwood in early spring as they used to do, and they’d add a second oil application to the boxwood shrubs in fall.

    The problem is that I’m trying to enable a full population of insects to live in my yard, finding their own balance between leaf-eaters such as the mites and scale and their insect predators. If I keep interfering by applying insecticides, beneficial insect predators won’t be attracted to the garden. 

Dragonflies help maintain balance by eating other insects

Spraying in fall when lots of insects are active would also kill bystanders that are needed as part of the full food web.

    For a start, I think I’ll remove the kiwi vine and replace it with something else that can grow on a north-facing wall. I could try replacing boxwood shrubs with native inkberry (Ilex glabra), which has similar small, shiny evergreen leaves. I’ll hold off on making a decision about the magnolias. 

     I’m thinking of this period as analogous to the transition from conventional to organic farming. It could take a few seasons for a natural balance to be established.

    Here’s hoping that won’t mean losing those lovely magnolias whose flowers raise our spirits in early spring.

Star magnolia (Magnolia stellata) in late April

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