Gardeners have encountered a defoliating insects in their azaleas this spring. Although they appear to be small green caterpillars, they are actually a type of sawfly. Sawflies are generally smaller than caterpillars, possess 6 - 10 sets of prolegs (false legs located on the abdomen) and develop into non-stinging wasps. Caterpillars have 2 - 5 pairs of prolegs and turn into moths and butterflies.
In Garden Insects of North America (2004), Whitney Cranshaw lists two species of sawflies collectively called azalea sawfly, Amauronematus azalae and Nematus lipvskyi. These two species are very similar in appearance and habits and are difficult to distinguish. They apparently are easier to identify as adults.
The larvae hatch sometime during May. They are green, smooth skinned, and very closely match the color of the azalea leaves. Although you can find several sawflies on a given leaf, they are not very gregarious compared to other sawflies They feed along the outer leaf margin until eventually only the midvein remains. Azalea sawflies will feed until sometime in June. There is only one generation of this sawfly each year.
Treatment of azalea sawflies will depend on early detection. If most larvae are 3/4 to one inch long, they have already eaten their fill. However if most of them are smaller and especially if chewing damage to the plants is not significant yet, is worthwhile to manage them. You have several options for dealing with these sawflies.
If you are dealing with a small number of sawflies, an easy solution is to handpick them. Just throw them into a bucket of soapy water to be sure they die. Look carefully for them as they blend in really well with the leaves. Watch for partially chewed leaves and droppings on the leaves to help signal their location.
If physical removal isn't practical, consider using an insecticide. If you wish to use something less toxic, try insecticidal soap. However, you need to hit the larvae directly with the spray to be effective. Also, there isn't any residual activity so you won't kill sawflies that walk onto treated leaves later. Another effective, low impact product is spinosad (e.g. Conserve).
If you prefer to apply a residual insecticide, there are a variety of garden insecticides available that would work against sawflies such as esfenvalerate, bifenthrin, permethrin, and carbaryl (Sevin). Products containing these active ingredients are commonly available.
Published in Yard & Garden Line News, Volume 8, Number 8, June 1, 2006