Treatment for lace bugs on azaleas and rhododendrons
I had a severe infestation a few years ago and tried a variety of treatments. The most effective by far was a soil drench containing imidacloprid, a neonicotinoid. It's persistent in the shrubs, and I've noticed dead bumblebees even after the passage of 18 months since application. Is there a more ecologically sound systemic insecticide or other measure that I could use. I tried the recommended horticultural oils before turning to imidacloprid.
Washington County Oregon
Thank you for your question.
As yet, there is no systemic insecticide for the use with Azalea Lace Bug that does not contain neonicotinoids. I suggest a multi-pronged approach that supports the healthy growth of the bushes, horticultural oils and employs predator species, timed properly, to eat nymphs as they emerge from their eggs and deter future hatches.
Stress in plants tends to signal a safe place for lace bugs and other sucking insects to lay their eggs and thrive on rhododendrons. To relieve stress:
- Prune plants to increase air circulation through the branches and ease spray application.
- Lightly irrigate every two weeks through our dry months, concentrating on the roots, not the leaves. If you have sprinklers trained on the bushes, it may be a good idea to decommission them and run a drip hose under the drip line of the bush. Timing is important, too, in the cool early morning, you are likely to see a better result from the watering you do, and it may even lower your water bill.
- Decide if the plants are in excess sun – if so, consider rigging temporary shade or moving the shrub to a more dappled sun/shade area.
- Be careful with high Nitrogen fertilizers and Lime. Rhododendrons do not need substantial Nitrogen for healthy growth. Lime raises the pH of soil, bringing it into a range that is inhospitable to the roots of Rhododendrons. If your shrub is adjacent to the lawn, cease using high nitrogen fertilizers in the area. If you use a lawn service, make sure they know about your choice. Organic, slow-release fertilizers will promote strong growth without stressing the rhododendron and making it susceptible to Azalea Lace Bugs.
To review the above information covered more thoroughly, download, free of charge, Azalea and Rhododendron Care and Culture here: https://catalog.extension.oregonstate.edu/sites/catalog.extension.oregonstate.edu/files/project/pdf/...
With the cultural support, you may find, in the long run, that you do not need to spray as often. To knock back a substantial infestation, though, spraying dormant oils or neem oil will help. Start spraying the undersides of the leaves when the first generation hatches, anytime from mid-March to mid-May. To determine the need, check the undersides of the leaves every several days as you look for the small, dark colored, spiny nymphs (youngsters). The most effective time to spray is right after the hatch, when the nymphs are in a tight cluster.
Remember, insecticidal sprays must thoroughly cover the undersides of the leaves. (See the list below.)
Repeat sprays of these materials are required during the growing season because the lace bugs have multiple generations. You have the best chance to decrease the population – and damage – when new generations hatch in mid-May, June, July, August, with perhaps another in September.
Don’t spray if the temperature is, or will be, above 80F. Follow package instructions for best results.
The following contact insecticides will temporarily control lace bugs if the product thoroughly covers the underside of leaves where lace bugs live and feed. Repeat the spray according to label directions:
- - Azadirachtin (water soluble Neem oil)
- - Insecticidal soap
- - Neem oil
- - Spinosad
Also, consider applying a narrow-range horticultural oil. Apply in fall to coat the undersides of the leaves where the eggs are laid in the leaf tissue alongside the midrib.
Another prong of attack is choosing to introduce insect species that predate the Azalea Lace Bug. There are several insects commercially available, each has its advantages and drawbacks:
- Ladybugs, if you can get them to lay eggs on the site, can be very helpful. The adults eat many soft-bodied sucking bugs, but the larvae eat much more. Adults without an appropriate environment to lay eggs tend to fly away in warm weather, though.
- Green Lacewings (Chrysopa rufilabris) can be ordered just as the nymphs are hatching so that they can be timed with the hatches of the Lace Bugs. They also predate aphids and whiteflies, a bonus, if you have them. Adults will fly away if habitat does not support egg laying, as well.
If you decide to replace your azaleas, Encore azaleas have proved themselves to be among those tolerant of lace bugs in other regions but their performance in the northwest remains to be seen.
For additional information on Azalea Lace Bugs, their life cycle and prevention, download Azalea Lace Bug: Biology and Management in commercial Nurseries and Landscapes, here: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/40424/em9066.pdf
I do hope this information helps.
Thank you again for using Ask an Expert.