Do the edges of your Rhododendron leaves look like this?
Chances are, you are being invaded by Root Weevils. Those sneaky little girls (that’s right, no males have been found in North America) come out at night when you are sleeping and hide in the soils during the day. Not only do they do damage to the leaves but in their larvae stage they also will eat the roots of your plants doing far greater damage than the cosmetic damage to the plant’s foliage.
On top of that, there are over 100 host plants so if you are having struggles with Strawberries or Raspberries, these girls may be your problem.
The good news is they are fairly easy to control without resorting to harmful insecticides. There are a number of products that you apply to the trunk of your Rhododendrons that stop them in their tracks — literally — when they come out at night to feed as adults. Tanglefoot is one such product and the Amazon site to view this product is at: http://www.amazon.com/Tanglefoot-Tree-Insect-Barrier-Tub/dp/B000BWY3AA. If you stay up late, no product is required as you can just grab your flashlight and hand-pick them off your plants. Fun activity at your next wine tasting party. We recommend it after the evening’s wine consumption and not before. 🙂
To control them in their larvae stage, try beneficial Nematodes. Arbico Organics can ship fresh Nematodes right when you need to apply them. The specific web page is: http://www.arbico-organics.com/product/nemaseek-beneficial-nematodes-hb-heterorhabditis-bacteriophora/organic-lawn-care.