This is the time of year to watch for certain caterpillar pests feeding on trees and shrubs in landscapes. One of these is the yellownecked caterpillar, Datana ministra. Yellownecked caterpillars feed on a broad range of plants, including azalea, beech, birch, crab-apple, elm, linden, maple, oak, and walnut.
Adult female moths are present in June and July. They deposit white eggs in masses of 25 to 100 on the undersides of leaves. Eggs hatch in July into caterpillars (larvae) that are yellowish with black stripes and are covered with fine hairs. They soon change into red caterpillars with yellow or white stripes. In addition, they have a jet-black head. The caterpillar gets its name from the bright orange-yellow segments behind the head. Full-grown larvae are 2 inches (50 millimeters) long and black with yellow or white stripes. When disturbed, larvae lift their heads and tails to form a distinctive U-shape, which is a defense response. The larvae are gregarious (feed in clusters), generally feeding for 4 to 6 weeks. Young larvae skeletonize the lower leaf surface, whereas older larvae may consume the entire leaf except the petiole. Late-season defoliation may not significantly harm tree health, but the injury can be unsightly. In August, larvae crawl down the trunk and burrow 2 to 4 inches (5 to 10 millimeters) into the soil to pupate. The yellownecked caterpillar overwinters as a pupa. There is one generation per year.
Pruning out small colonies or using pest-control materials are ways to manage yellownecked caterpillars. Pest-control materials recommended for control of yellownecked caterpillar include Bacillus thuringiensis ‘Kurstaki’ (Dipel, Thuricide, Javelin), chlor-pyrifos (Dursban), cyfluthrin (Tempo), and spinosad (Conserve). Make spray applications when caterpillars are small by timing applications when hills-of-snow hydrangea blossoms are changing from white to green. Yellownecked caterpillars are susceptible to attack by various natural enemies, such as birds (for example, robins and bluejays), predaceous bugs, and parasitic flies.