There are many caterpillars in the family Notodontidae that feed on urban trees and shrubs. Several closely related Notodontids are among the most common late season defoliators of urban trees. These include the greenstriped mapleworm, (Dryocampa rubicunda) which feeds primarily on red maples and orangestripped oakworms (Anisota senatoria) that feed primarily on oaks. We have written about these critters on the blog, in Insect Notes, and in a free book so you can get your fill (maybe you already have).
Members of the genus Datana are among the most beautiful caterpillars ever to defoliate your plants. Around here these include azalea caterpillars (Datana major), and yellownecked caterpillars (Datana ministra). These two species are about the same size and have a defensive behavior someone should name a yoga pose after (Datan-asana?). When approached caterpillars arch their front and back ends to form a ‘C’ shape. This must be scary to some predator or they wouldn’t do it (neither would sawfly larvae). Perhaps it looks like a snake about to strike or maybe it is just unusual and visually arresting enough to make predators like birds think twice.
In host range azalea and yellownecked caterpillars are quite different. Azalea caterpillars feed almost exclusively on azaleas and occasionally on blueberry plants. Yellownecked caterpillars feed on a long list of species including oaks, maples, fruit trees, elm, hickory, beech, linden, and many others. Feeding habits and life histories of both species are similar to orangestripped oakworms. They hatch from clusters of eggs laid on the bottom of leaves in late summer. Young larvae feed gregariously defoliating one branch at a time. Larvae leave the plant to pupate underground until the next summer.
I found large yellownecked caterpillars on a willow oak last weekend. Azalea caterpillars come out later than yellownecked caterpillars or orangestripped oakworms. I have not seen any yet and usually find them around the end of August. Last year I found eggs hatching the first week in September.
Start monitoring for azalea caterpillar egg masses now by turning over leaves to look for egg masses. Also scan azalea shrubs and hedges for branch tips with skeletonized or missing leaves. This is the first sign of young caterpillar infestations and can be pruned to prevent more defoliation.