Issue 13, August 9, 2013

Yellownecked Caterpillar

Various caterpillars attack trees in late summer and early fall. These include fall webworm, yellownecked caterpillar, walnut caterpillar, sumac caterpillar, and whitemarked tussock moth. The damage caused by these caterpillars to attacked trees is relatively slight because the leaves have already produced most of the food for the tree that they will produce. The loss of leaves at this time of year is not very important to the tree. The main tree health concern about late-season defoliation is that the tree will break lateral buds and replace the lost leaves.

Yellownecked caterpillars feed on the leaves of a wide range of trees, including crabapple, maple, oak, and walnut. The adult moth lays her white eggs on a host leaf in a cluster about one inch in diameter. Usually the cluster is about one-half longer than wide. Generally, a newly hatched caterpillar's first meal is its egg shell, but yellownecked caterpillars usually leave behind much of the egg shells and start feeding on tree leaves.

Initially caterpillars window-feed from the leaf underside. The lower epidermis and mesophyll is eaten, leaving the upper epidermis which is clear or whitish. It turns brown as the exposed cells die. This first feeding usually damages two or three leaves. During window-feeding, the larvae grow to about one-quarter inch long and are yellowish with black heads.

Older, larger larvae skeletonize the leaves, eating away the edges down to the midvein. These larger larvae are one to one and one-half inches long and reddish-brown with indistinct longitudinal white lines. Because yellownecked caterpillars feed in groups through their lives, feeding damage becomes more obvious with small branches being stripped of foliage.

As the caterpillars grow older and larger, they molt to being black with obvious yellow stripes and the dorsal area behind the head is a reddish-yellow, giving them the name, yellownecked caterpillar. Black caterpillars are two to two and one-half inches long and eat large amounts of foliage, resulting in these colonial caterpillars stripping large branches and portions of trees. This, and their one-eighth inch diameter black cubes of feces dropping all over underlying patios and sidewalks, makes them obvious to homeowners. At this time, the caterpillars are about to descend from the trees and crawl across the ground looking for pupation sites, so their feeding is coming to an end.

Yellownecked caterpillar mature larvae on oak.

They form naked, reddish-brown pupae in the debris and soil below the tree, emerging as tan moths with white bands to mate and lay eggs for a second generation. It is the second generation that we are seeing now. These current larvae will overwinter as pupae to emerge as adult moths next year in late spring.

Walnut caterpillars are similar in appearance to yellownecked caterpillars, and they cause similar damage to walnut, pecan, and hickories. However, older caterpillars are black with long white hairs (setae). They do not have obvious stripes when older nor do they have the "yellow neck." Otherwise, they are similar in life cycle and in the feeding damage that they cause as they are in the same genus as yellownecked caterpillar.

We have seen and heard about infestations of yellownecked caterpillar and walnut caterpillar within the past couple of weeks, which is early enough to cause many trees to refoliate if heavily damaged. Even so, treatment is usually not needed for late-season caterpillars. If control is needed, Bacillus thuringiensis 'kurstaki' (Dipel, Thuricide), spinosad (Conserve), carbaryl (Sevin), and various pyrethroid insecticides are effective. (Phil Nixon)

Phil Nixon

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