Be on the lookout for a couple of caterpillars often seen in late summer into fall. These insects can be potential problems on newly transplanted trees but are not usually serious on old, well established trees. They have some interesting defensive behaviors.
The walnut caterpillar, Datana integerrima, attacks black walnut, butternut, pecan, hickory, and on occasion, oak, willow, birch, honey locust, and apple trees. Newly hatched larvae have black heads and are generally light green, gradually changing as they grow to become reddish brown or purple with white stripes. As worms approach maturity, they darken, becoming almost black and at that time are covered with long, fuzzy, white hairs. The worms eventually reach a length of up to 2 inches. Larvae behave oddly when threatened, arching their fore- and hind-legs in a defensive posture (shown below in the USDA Forest Service photo). Worms are gregarious, feeding in large groups, but they do not form webs to surround themselves as do the tent caterpillars.
Young worms skeletonize leaves, feeding on the more tender leaf tissue. Larger larvae consume entire leaves, including the midribs and leaf petioles. The later larval instars account for most of the feeding damage. Leaf damage is often to parts of individual branches and is not generally throughout a tree unless it is a small one that was recently planted. Isolated trees seem more subject to attack.
When the developmental time of each instar is complete, they move as a group in a procession to the underside of the limb or to the tree trunk and form a "pile" in which they molt their skins. When finished with the molt, the group again spreads through the leaves of a branch. They leave behind some silk with molted caterpillar skins clinging to it at the group molting site. Mature larvae are about 1.5 inches long. When development is complete the worms drop to the ground and enter the soil where they each form a pupa case and remain in this stage for the winter. Pupae are 3/4 inch long and dark brown. Adults emerge the following spring.
Check branches for leaf feeding by groups of caterpillars in July and August. Inspect undersides of branches and trunks for silky masses of cast skins as evidence of group molting.
For small trees, strip groups of feeding caterpillars from branches by hand or use a strong stream of water to knock them down. Few will make it back to the branches. Strip individual leaves of egg masses when found. Tree Tanglefoot or other sticky materials can be applied to the bases of larger tree limbs to ensnare larvae as they move to the larger lower limbs and trunks to molt. Groups of caterpillars found on tree trunks or branches during molting also can be removed and destroyed by hand. Another option, Bacillus thuringiensis (Bt), is effective against young larvae.
Chemical control is seldom necessary, especially on large, well-established trees. Tree mortality due to this pest is rare, but small, newly planted trees that are infested and under drought stress could be damaged.
The fall webworm, Hyphantria cunea, is a white to pale yellow, fuzzy, black-spotted caterpillar that strips green tissue from leaves. It infests over a hundred different species of trees and shrubs, including fruit trees and ornamentals.
The worms are usually present in large numbers and living in a protective silken web, or "tent." The protective tent may hold as many as 400 feeding caterpillars. As the worms grow in size, the web becomes larger and may cover two or more individual branches. The web is very messy and contains old caterpillar skins and large amounts of dark-colored excrement pellets, or "frass."
Larvae feed for 4-6 weeks before dropping to the soil surface to pupate. Larvae have the unusual habit of twitching in unison when they are threatened, a habit that supposedly frightens away predators and parasites.
If easily accessible, simply strip webbing and worms from infested branches and knock them to the ground. Few will find their way back to feed on the tree. Remember that the loss of the leaves of a few branches is not likely to seriously harm the tree. Webs are a nuisance and are not attractive to look at, and these may be pruned from the tree as well.
Nearly any insecticide that is approved for use on trees and shrubs should easily control the larvae by simply wetting the webbing with a spray mixture. For the latest information on available nonchemical and chemical controls, consult the woody ornamentals page at the UNL Entomology site. When using a pesticide, always follow label directions.