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Sunday, January 3, 2016

Return of the repressed

Happy New Year! My resolution for 2016 is to learn more ways to keep unwanted insects in check. In 2010 I told my pest control company to stop spraying my shrubs with pyrethroid pesticides for leaf damage that was only cosmetic. Now the results of this decision are coming to light, and I need some tools other than pesticides to deal with resurgent pests.

    It’s not something I’m proud of, but over the years between 1996 and 2010 I accumulated a schedule of pesticide applications to combat such insects as lace bugs and black vine weevils. True, the insecticides used, cyfluthrin and deltamethrin, are synthetic analogs of pyrethrum, an extract of chrysanthemum flowers. They’re relatively non-toxic and quickly broken down. Nonetheless I decided that to consider my gardening sustainable enough, I’d have to lose these pesticides.

    At first I congratulated myself on having stopped unnecessary spraying. I didn’t notice any new damage. It looked as if there would be no problem.

Lace bug damage on andromeda leaves
     This fall, after the busy gardening season subsided, I started to see telltale signs. The leaves of a prized Japanese andromeda (Pieris japonica) that’s very visible at the front of the house showed speckles betraying the return of lace bugs. 
Corythucha pruni
Lace bugs are named for their lacy markings

The adult lace bugs and the immature form, the nymphs, both suck juices from leaves of their chosen plant hosts.  

    Next, when I was raking leaves, I saw something I’d never observed before: notches chewed out of the leaves of a large Catawba rhododendron (Rhododendron catawbiense).
Notches chewed by black vine weevils
This is the calling card of the black vine weevil. The grubs that are the weevils' larvae are more damaging than the adults, because they feed on roots and lower stems. 

Adult Otiorhynchus sulcatus
The culprit

This shrub too plays an important role in my garden design. It’s about the size of a VW beetle, and it’s right in the foreground of the view from the windows at the back of the house.

This rhododendron was here when we moved in 30 years ago

    I took a deep breath and reminded myself that none of these insects was likely to kill the shrubs they were chewing on. And going back to spraying is not my only alternative. Because I used to rely on pesticides, I’ve never explored other methods of control. 

    I’m starting to collect some approaches used by organic gardeners. Next spring I could spray plain water to knock lace bug nymphs off my andromeda. I could surround rhododendron trunks with burlap or hang sticky traps to collect adult black vine weevils at night. I hope to add more non-chemical methods to this list.

    In 2016 I’m also not going to try to eliminate problematic insects completely. If I want a balanced insect population, I’ll need to have insect prey around for predators such as parasitic wasps to eat. Otherwise they won’t visit my yard. The goal will be to set acceptable levels of plant damage and let each insect contribute its part in the food web.

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