Answer: Based on your description I think the plants have been bothered by lacebugs rather than spider mites. The Sevin should have controlled them, however there can be several generations of this pest per year and your spray applications need to be timed right or they will be ineffective. At this point it is late in the season to have any effect. You will need to treat the plants in the spring when the first generation emerges for next season, usually about mid-May. You may need to spray two to three times per the label instructions on the product you use. Be sure to spray top and bottom sides of all of the foliage in order to contact the pests. Commercially formulated insecticidal soap is usually effective and would be the least toxic control to use. Here is more information about this pest you may find helpful.
Good luck with your azaleas and rhododendrons!
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