Azalea bark scale infestation
We have about a dozen azaleas which were probably planted many decades ago. A few years ago, some developed azalea bark scale infestations. (This was diagnosed from a photo I sent to your service a couple of years ago.) In the past 2 years I pruned some of the worst infested branches and applied a 2% horticultural spray once or twice per year, I think in the summer and late fall, but I don't remember exactly when. Unfortunately, it now looks like the infestations have spread to all of our azaleas and many are showing a lot of dead infested branches cover with scale and severe thinning out of leaves. This year I sprayed all of the azaleas thoroughly with 2% horticultural spray around the 3rd week in March, and then again 3 weeks later. Most of the azaleas are now coming into bloom. The scale doesn't look any better. When I check around on the internet, I saw inconsistent advice on how often and when to spray. I'm reluctant of course to use strong pesticides, such as those containing neonics, but I'm concerned that the horticultural oil is insufficient and that all of our azaleas will die. Should I be more patient with the horticultural oil? Any recommendations?
The dilution rate of the horticultural oil is critical. As is the timing.
A dormant season (winter, or very early spring), rate should be 4%. Summer (or warm weather) weight is 2%. (Be sure to read the label.) Your dormant sprays may not have been strong enough to be effective.
1. Use a dormant oil spray to kill overwintering nymphs. Coverage of the plant must be completely thorough for any and all types of sprays.
2. Use a summer oil--or insecticidal soap or a contact insecticide such as pyrethrin--in July after all crawlers are out of their egg sac. Crawlers are reddish and appear in June and July. Apply more than once if you're not sure all the crawlers have hatched.
Here's the azalea bark scale info on our website: http://extension.umd.edu/learn/ipm-series-azaleas-and-rhododendrons-hg51
Since the scale is usually controlled by parasites and predators, be sure to encourage these by not spraying pesticides elsewhere in your landscape. It's also possible that your azaleas are stressed by age or their environment has changed, making them more attractive and susceptible to pests. Water them in droughts. Keep all mulch off the trunks and no more than 2-3" deep.