It’s late in the growing season. Most plants have flowered and set seeds while many insects have grown big and hungry. For caterpillars, the last growth stage is the most damaging stage of the life cycle, just before pupating and transforming into harmless adults. Butterflies are important pollinators. While butterfly caterpillars eat plants, such as fennel and passionflower, they are not as troublesome as moth larvae.
Many gardeners have noticed yellow-striped caterpillars on azalea shrubs as early as the first week of August. Red-headed azalea caterpillars, as they’re called, have yellow and black longitudinal stripes, but the legs and head are red. At full maturity, caterpillars can get 2 inches long, but the markings and sizes aren’t the real identifiers. As the name implies, they feed on only one host, azalea. And they don’t just eat it, they mob it.
Azalea caterpillars have one generation, meaning they go from egg to adult moth once a year. An adult will lay as many as 100 eggs on the underside of leaves on the host. According to some sources, moths will return to the same host where they were hatched. If this is the case, you’re likely to see the same problem on the same shrub next year.
If you’ve witnessed these destructive caterpillars, begin monitoring your azalea shrub at the end of July next growing season. While the caterpillar is young and small, it’ll go through the summer unnoticed, feeding on the underside of the leaves. But they grow so fast it’s like they drop out of the sky overnight. By August, they’re hard to ignore. In some severe cases, it even looks like the shrub is moving.
Azalea caterpillars often maul a single branch. If you approach the shrub or touch the branch, they have a peculiar reaction. In unison, all the caterpillars will aggressively rear their heads and abdomens in menacing fashion. Of course, this is a defense mechanism to ward off predators. If you’re squeamish about creepy-crawly things, you’re likely to leave them alone, but these things are about as dangerous as a palmetto bug. But you probably hate those things, too.
A close relative to the azalea caterpillar in the Notodontidae family is the yellow-necked caterpillar that appears on oaks about the same time of the year. They also rear back when approached and tend to all feed on the same branch. They also return to the same host tree year after year. If you’re not monitoring the tree, its leaves seem to disappear overnight.
A similar-looking caterpillar that occasionally shows up on red maples is the green-striped mapleworm. This isn’t as common as the azalea and yellow-necked caterpillar but may be seen defoliating a red maple in late spring. Since the green-striped mapleworm isn’t in the Notodontidae family, it doesn’t congregate to a branch or have the same rearing reaction when approached.
Trees and shrubs seem to tolerate the once-a-year feeding but can be stressed when the host is completely defoliated year after year. Trees and shrubs have to expend carbohydrates to re-establish foliage. Because the azalea and yellow-necked caterpillar congregate on a single branch, small-scale control is simple. Prune the branch with all the resident caterpillars on it and get rid of them. Burning, smashing or disposing of them in a sealed bag will work.
If you’ve got too many infested azaleas, there are pesticides labeled for control. One of the safest is Bacillus thuringiensis, commonly known as Bt. This is a bacteria that, when consumed, will kill caterpillars. The most common Bt product is Dipel. Also, a strong soapy solution drench will control caterpillars. Just be careful to avoid applying it during hot weather because it can burn the foliage.
They key to controlling and eventually ridding your plants of these caterpillars is to catch the infestation early. Turn over leaves in late July. Methods of control are more effective when insects are small and susceptible.
Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, email him at email@example.com.