The Walnut Caterpillar (aka Walnut Datana) (Datana integerrima) is a common pest of pecan trees. The adults are moths with light-brown wings marked with dark-brown, wavy lines. The hind wings are lighter brown and without lines. The moths are about 1.5 to 2 inches long, but this is not usually the life stage that is noticed by growers. It is the larvae that cause significant defoliation to plants. Immature larvae are reddish-brown with narrow, cream colored lines that extend the length of the body. Mature larvae are black, about 2 inches long and are covered with long, white or grayish hairs.
The Walnut Caterpillar overwinters as a pupa in the soil. The moths emerge in the spring and deposit white eggs in masses on the underside of leaves. These eggs result in the larvae that form a compact mass. These caterpillars molt several times during development. The larvae feed in groups, but unlike fall webworms (also common on pecans) they do not form webs. They are capable of eating all the leaves on small trees or entire limbs of larger trees.
Although common on walnut and pecan trees, the realsurprise this year is that I have reports of them feeding on blueberries (see photos below).
Above are three photos of the Walnut Caterpillar larvae. Below is the one photo of the resulting damage they can do. Significant defoliation can lead to poor winter hardiness and possible reduced fruitfulness next year.
So, what to do about them? There are a few options: cut out the branches with worms; use products that contain Spinosad in them (like common Leafminer & Tent Caterpillar Spray); or spray with Sevin (this also kills beneficial insects though). Most of the time the best option is the one to just remove the section of the plant with the larvae and dispose of it. Below are more links that cover this insect.
Kansas State: http://www.hfrr.ksu.edu/doc1741.ashx