I have a number of evergreen azaleas, deciduous azaleas and rhododendrons growing in our front and back yards. The deciduous azaleas, especially my Northern Hi-Light, are all troubled by a green caterpillar which eats the leaves and flowers. The damage seems to match that of the azalea sawfly however I have never once observed a sawfly. I suspect the caterpillar larvae come out inside the tips of the new leaf and flower buds and I was thinking of applying a dormant oil. For many years I have manually killed the caterpillars but I can never seem to stop the infestations. I have witnessed wasps harvesting the caterpillars. If left unchecked the caterpillars will strip all the leaves however the plants seem to recover and set new growth. Once the plants have finished blooming the caterpillars disappear until next spring. We live in downtown Toronto, close to the lake. Any suggestions you have would be appreciated.
Thank you for contacting the Toronto Master Gardeners with your inquiry concerning your azaleas. There are a number of pests which are documented to feast on the young emerging leaves of Azaleas and Rhododendrons. The following archived post is to a homeowner with the identical problem:
“Adult sawflies emerge in early spring and lay their eggs on the underside of host plant leaves. Larvae appear several weeks later, feed on soft leaf tissue for about a month, and then drop into the soil to pupate.”
Since the larvae fall to the ground to pupate, cultivating around your shrubs in the early spring and again in the fall can help to reduce the overwintering population. Application of Diatomaceous Earth on the ground around the plant will also help reduce the population. Diatomaceous Earth is made up of tiny fossilized aquatic organisms that act like broken glass. It kills soft bodied insects by scoring the insect’s outer layer as it crawls over the fine powder.
The larvae are so well matched to the color of the leaf edge that they are very difficult to see. Look for tiny dark specks of frass on leaf surfaces below feeding larvae. You are correct in your observations in that many animals feed upon sawfly larvae, including some birds, lizards, frogs, ants, predatory wasps. It is these predators and parasitoids that regulate sawfly populations in natural habitats.
Early detection is the key to keeping these pests under control. Make sure to inspect both the upper and lower surfaces of the leaves. Remove the infested leaves and destroy the larvae. A forceful spray of water out of a garden hose can also provide control by knocking off and killing many of the soft-bodied larvae. Be sure to aim the water at both the upper and undersides of leaves. When the larvae are still very tiny, applying insecticidal soap available from your local garden centre can decrease the population. Make sure to read and follow the directions on the packaging. For larger larvae, removing and hand “squishing” is advised. Continue checking plants throughout the growing season.
Without a photo of the insect it is difficult to know with certainty what is eating your azalea leaves. I also found the following information from one of our earlier posts:
“There are a couple of other pests that are potential culprits. The Azalea Society of America says that leaf damage in late spring can be from the Rhododendron Looper, “a caterpillar that looks exactly like a branch, stem, or even a stamen. It can align itself along the stem to hide.” The caterpillars won’t kill the plant unless there’s a huge population of them. The best way to get rid of them is to pick them off and squish them.
If the leaf damage looks like notching around the outside of the leaf, it could be weevils. Again it is possible to pick them off, but it has to be done at night. You said your azalea leaves were eaten entirely, so this is probably not the problem. But just in case, the American Rhododendron Society says: “Weevils can be picked from the leaves and plant at night if one has the patience and is willing to spend time well after dark with a flashlight. Close examination will reveal the weevils, and they can be picked or scraped into a container for disposal.”
The first line of defence for Rhododendron Looper or weevils is prevention: keeping your plant healthy and free of stress. That means regular watering, a mulch to retain moisture, regular feeding with compost, and pruning of the lower leaves that touch the soil or other plants so that pests will have less access to it.”