I think my Rhododendron is infested with lace bugs. What do you recommend?
According to The Organic
Gardener’s Handbook of Natural Insect and Disease Control edited by
Barbara Ellis (Rodale Press, 1996), this problem occurs more often with
plants in full sun. The book recommends using insecticidal soap at the
first sign of damage. Spray the undersides of the leaves in particular
(and always test the spray on a small area first). Repeat the spraying in
mid- to late-summer if the lace bugs reappear. If the problem is severe,
use superior oil or pyrethrin as a last resort. There is a recipe for
homemade soap spray in the book: 1 teaspoon up to several tablespoons of
liquid soap (use unscented, nonchemical soap) per gallon of water. Start
with a lower concentration and adjust as needed. You can also add
insecticides such as horticultural oil, pyrethrins and BTK to increase
the spray’s toxicity to pests. Mix 1 tablespoon liquid dish soap and 1
cup of oil (peanut, safflower, corn, soybean, or sunflower), When you are
ready to apply the spray, mix 1 to 2 1/2 teaspoons of this soap/oil blend
with 1 cup of water. (These are just guidelines for proportions.) You can
also use commercial products, examples of which are here:
Safer Insecticidal Soap and Stoller Natur’L Oil.
University of California, Davis’s Integrated Pest Management website has
a page about lace bugs. Below is an excerpt:
MANAGEMENT
Tolerate lace bug damage where possible; in most cases, it does not
seriously harm plants. Provide proper cultural care so plants are
vigorous. No treatment will restore stippled foliage, which remains until
pruned off or replaced by new growth. If damage has previously been
intolerable, monitor plants early during subsequent seasons. Take action
when populations begin to increase and before damage becomes extensive.
Biological Control
Natural enemies of lace bugs include assassin bugs, lacewing larvae, lady
beetles, jumping spiders, pirate bugs, and predaceous mites. These
predators may not appear in sufficient numbers until after lace bugs
become abundant; their preservation, however, is an essential part of a
long-term integrated pest management program. Growing a variety of
species, mulching soil with organic material, and shading plants from
afternoon sun can reduce lace bug damage to shrubs and increase natural
enemy abundance. If applying pesticides, using only short-persistence
materials such as oils and insecticidal soaps will minimize the number of
beneficial predators and parasites that are killed.
Cultural Control
Grow plants well adapted to conditions at that site. Plants in hot, sunny
locations are more likely to be damaged by lace bugs. For example,
azaleas grown under partial shade experience less damage by the azalea
lace bug in comparison to azaleas that are drought stressed and exposed
to bright sun. Provide adequate irrigation and other care to improve
plant vigor. Prune out damaged foliage if the discoloring is intolerable
and relatively localized. Do not remove more than a small percent of a
plant’s branches during one season and use good techniques so that
pruning does not injure plants, such as by exposing inner branches to
sunburn.
Chemical Control
Contact Insecticides. Insecticides will not restore an undamaged
appearance, but can reduce or prevent further damage. Almost any
insecticide will control lace bugs if it is sprayed directly onto the
insects. Azadirachtin (Safer BioNeem), insecticidal soap (Safer),
narrow-range oil (Green Light, Volck), neem oil (Green Light Garden
Safe), or spinosad (Conserve, Monterey Garden Insect Spray, Spinosad Home
and Garden) sprays temporarily control lace bugs if insecticide
thoroughly covers the underside of leaves where adults and nymphs occur.
These IPM-compatible insecticides have very low toxicity to humans and a
less adverse impact on natural enemies than more persistent,
broad-spectrum insecticides.