Leaf-feeding insects are among the most common, destructive, and often downright annoying pests encountered on shade, ornamental, and urban landscape trees. Many species are ravenous feeders capable of completely defoliating trees, thus destroying their ornamental and environmental values in the landscape. Loss of foliage also retards tree growth, and may cause dieback in crowns and, in some cases, tree mortality.
AAES research on tree insects has identified many leaf feeders associated with Alabama trees. Among the most destructive and annoying of these are caterpillars, the larvae of moths and butterflies. Typically, populations of these leaf feeders vary over time, both in species and in number of insects present. Commonly, a species will be abundant for one, two, or three seasons, then scarce for several. Several species, however, are more common than others, and infestations tend to occur more frequently. The following presents some of the more common species found on trees in the urban forest landscape.
The yellownecked caterpillar (Photo 1) occurs commonly on oaks but will feed on several trees and shrubs, including apple, basswood, birch, honeylocust, and maple. The larvae typically feed in colonies, defoliating one branch, then moving as a group to another. Young larvae (Photo 1, left) are basically red with yellow or white stripes; the full-grown larva (Photo 1, right) is about 2 inches long, black with similar longitudinal stripes, and has a yellow collar or "neck" behind the head. Caterpillars are normally present from late July or early August until mid-October. There is only one generation per year.
The walnut caterpillar (Photo 2) is closely related to the yellownecked caterpillar. Its favorite food trees are pecan, hickory, walnut, and butternut. Walnut caterpillars, like the yellownecked, feed in colonies, moving from branch to branch. Young larvae are red to purplish with white longitudinal stripes (Photo 2, left); fully grown larvae (Photo 2, right) are black, 2 to 2¼ inches long, and covered with long white hairs. Two caterpillar broods occur each year, one in late May and June, and one in late summer (mid-July through September).
The greenstriped mapleworm (Photo 3) feeds primarily on maple and boxelder. Early and mid-stage caterpillars feed gregariously; older larvae tend to separate. Young larvae (Photo 3, left) are yellowish cream with black heads and faint greenish stripes along the body. The full-grown caterpillar (Photo 3, right) is about 1¾ inches long. The head is cherry red, and the body is yellow-green with dark green longitudinal stripes. There are two distinctive long, black spines behind the head and rows of shorter black spines along the body. Two broods usually occur each year, one during May-June and another during August-September.
The spiny oakworm (Photo 4) is a close relative of the greenstriped mapleworm, and bears the pair of long black spines typical of this group of caterpillars. Oaks, as the name implies, are the primary food trees. Larvae feed in a group until about half grown, then tend to separate to feed. Like the preceding caterpillars, color changes as larvae develop. Young caterpillars (Photo 4, left) are creamy white to green with black heads; the fully grown larva (Photo 4, right) is brownish orange, sometimes tinged with pink, and about 2 inches long. There is one brood each year. Caterpillars usually begin to appear in the last half of July or early August, and may be present until early October.
The fall webworm is one of our most common leaf feeders. Larvae feed gregariously and are responsible for the conspicuous silken webs (Photo 5) common on host trees in summer and fall. Larvae and webs are commonly found on pecan, persimmon, sweetgum, willow, and mulberry. In most years, populations are relatively low, but periodically, the fall webworm occurs in "outbreak" numbers, as it did in some areas of Alabama in 1997. During outbreaks, webs become numerous, and are found on a wide variety of hosts, including the above trees and cherry (both native and introduced), sourwood, redbud, sycamore, blackgum, and cypress. Larval activity actually begins in late April or early May; however, webs are most common in late summer and fall, thus the common name.
There are two races, or forms, of fall webworm larvae, orange and black. Orange race larvae (Photo 6, left) have orange heads and orange tubercles on the body, black race larvae (Photo 6, right) have black heads and tubercles. Full-grown larvae of both races are about 1 inch long and thickly clothed with whitish hairs.
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Photos courtesy of Professor L.L. Hyche