PESTS ASSOCIATED WITH FOLIAGE
Black Pecan Aphid
The black pecan aphid is much more destructive than the two species of yellow aphid. Three black pecan aphids per compound leaf can cause severe leaf damage and defoliation. Like yellow aphids, the black pecan aphid feeds on the undersides of leaves and occurs throughout the pecan growing region of Texas.
While feeding, black pecan aphids inject a toxin that turns the leaf tissue between major veins bright yellow. These damaged areas, up to 1/4 inch across, turn brown and die. Infested leaves soon fall. Premature defoliation reduces nut fill and the next year’s production.
The black pecan aphid is pear-shaped. Nymphs are dark olive-green while adults, which may be winged, are black. Like yellow aphids, all summer forms are females that reproduce without mating. Male and female forms appear in fall and females lay eggs that overwinter on branches. Densities often are very low until August or September, when infestations often increase rapidly.
Monitor the orchard frequently for black pecan aphids and their characteristic leaf injury. Because these aphids feed singly and can be damaging in low numbers, examine leaves closely. Examine the interior of the canopy, where infestation often begins. In general, treat when black pecan aphids average two to three per compound leaf. In most cases, black pecan aphids are easier than yellow aphids to control with insecticides. Natural enemies are important in maintaining low numbers of black pecan aphids.
Feeding by the immature stages of the pecan phylloxera, Phylloxera devastatrix causes galls or knots to form on the woody portion of the new growth. This includes stems, leaf petioles and midribs, nutlets and catkins. Heavy infestations of this insect can cause nut loss and defoliation.
The immature stage hatches from eggs in the spring that have overwintered on the tree. The immatures, also known as “stem mothers” migrate to the new growth where they settle and begin to feed. As the phylloxera feed, gall formation is initiated with tissue forming around the insect. If an insecticide is needed, it should be applied before the immatures become embedded in the new tissue. Insecticides only need to be applied to a tree if a tree had galls the previous season. The time of treatments should be when the foliage is at the stage shown in the picture. Recommended insecticides can be found in the Texas Agricultural Extension Service publication “Managing Insect and Mite Pests of Commercial Pecans in Texas” B-1238. Since the last printing of the pecan insect control guide, the insecticide imidacloprid, (Provado 1.6F or Admire 2F) has received a label for pecans. Provado 1.6F, the formulation for foliar application has provided good control of phylloxera. In addition, this product does not have a grazing restriction.
Adult sawflies are small, bee-like in appearance and are about 1/4 to 1/3 inch long. Larvae feed on foliage and leave holes in the leaves or consume the whole leaf.
Sawfly larvae resemble caterpillars but are actually larvae of a wasp. Larvae of moths and butterflies have 1 to 4 sets of abdominal prolegs, whereas sawfly larvae have 6 sets. Once larvae finish feeding, they drop to the ground and burrow into the soil to overwinter. There is only one generation per year.
Fall webworm caterpillars build large silken webs in pecan trees. A hundred or more caterpillars may be found inside the web, where they feed on pecan leaves. Large infestations may cover the tree with webs, causing severe defoliation.
Mature larvae are about 1 inch long, pale yellow or green, and covered with tufts of long, white hairs. The adult is a white moth with dark spots on the wings. Female moths emerge in spring and deposit eggs in masses of several hundred on the undersides of pecan and other tree leaves. The greenish-white eggs are covered by gray hairs left by the female. There are two to four generations each year, depending on location in the state. The last or fall generation is usually the most damaging.
Many insect parasites and predators feed on and reduce the number of fall webworm larvae. Also, insecticides applied for other pecan pests help reduce webworm densities. If webs are common and the potential defoliation appears unacceptable, spot spraying of infested trees may be practical. The insecticide spray must penetrate the web to be effective.
Walnut caterpillars feed together in large numbers on pecan leaves, but do not build silken webs like fall webworms. Larvae eat leaves, leaving only the mid-ribs and leaf stems. Large infestations can defoliate entire trees. This insect is found throughout Texas east of the Pecos River. Although economic infestations are uncommon, severe and widespread outbreaks of walnut caterpillar have occasionally occurred in Texas.
Walnut caterpillar moths emerge in spring, de-positing eggs in masses of 500 or more on the undersides of leaves. The egg masses are round, about the size of a half-dollar and are not covered with hairs or scales. Eggs hatch in about 10 days; larvae feed for about 25 days. Young larvae are reddish-brown with yellow lines running the length of the body. Full-grown larvae are about 2 inches long, black with grayish lines and are covered with long, soft, gray hairs. Larvae congregate in large masses on the trunk and scaffold branches to shed their skins before crawling back to complete feeding on leaves. These final-stage larvae consume most of the foliage, and defoliation can occur very quickly. Mature larvae crawl to the soil to pupate. A generation is completed in about 6 to 8 weeks. There are two to three generations each year.
Because walnut caterpillars do not build tents or webs, infestations often go unnoticed until leaf damage becomes obvious. To detect infestations early, look for egg masses or leaf feeding. Egg masses can be detected at night by shining a flashlight on the undersides of leaves and looking for white spots about the size of a half dollar.
The pecan leaf scorch mite is the most important spider mite attacking pecans. Large numbers of these tiny mites feed on the undersides of pecan leaves. Mites suck plant sap, causing irregular brown spots on infested leaves. Infestations often develop first along the leaf midrib. Damaged leaves appear russeted or scorched. Large infestations can result in leaf loss, especially if trees are under moisture stress.
Scorch mites overwinter as adults in the rough bark of limbs. Adult females begin laying eggs in spring. Mites can complete a generation in 5 to 15 days and are more numerous during hot, dry weather. Natural enemies of scorch mites, including predatory mite species, are important in controlling these pests.
Because scorch mites prefer the shady, interior portion of the tree, significant damage can occur before infestations are detected. Check water sprouts and shady, lower branches to detect early mite infestations. Mites may increase after some insecticides (e.g., Sevin ® ) are applied for hickory shuckworm, aphids or other pests. Monitor the orchard for mites when the weather is hot and dry and after insecticides are used. Spray when mites are present and damaging leaves. Mark infested trees or areas to determine if spot treatment is practical.