Insect Attacks on Southern Pines

Insects harvest more southern pine timber than man and fire together. A group of bark beetles takes the blame for most of the damage. Some are primary sources of injury, destroying stems that otherwise appear healthy. Secondary insect attacks occur when fire, fungi, or vegetative competition for soil moisture, nutrients, and light weaken trees. The more common include the southern pine beetle, the Ips bark beetle, and the turpentine beetle. The initial attraction of insects to certain trees is thought to be due to the escape of volatile chemical aldehydes or esters, by-products of respiratory fermentation resulting from abnormal enzyme activity that occurs when trees suffer physiological stress. According to studies reported as early as 1931, the first few beetles that attack a tree introduce a yeast. This serves as a strong attractant, with beetles now moving to the area from far-distant locales. The inner bark of the pines is a favorable medium for yeast growth. More recently, pheromones (odor hormones), serving as sexual attractants for insects, have been isolated.

A question often asked is why the virgin forest did not suffer devastation by the chewing jaws of bark beetle larvae. Those stands were, of course, not subjected to management. Serious infestations, of which we have no accounts, probably did occur, but, for the most part, the low density

Figure 2.7 Damage to a loblolly pine stand by the southern pine beetle, Dendroctonous frontalis does much more damage to southern forests than does fire. (Texas Forest Service photo by Ron Billings)

of the pine component in the old-growth southern forest enabled more than an adequate amount of moisture, nutrients, and sunlight to sustain tree vigor. The trees were overmature and growth was slow, a typical stand probably numbering fewer than 10 or 20 trees per acre. The classical stands seen in historical photo collections were exceptions; it is the fact that they were exceptions when the pictures were taken that made the photographs worth saving.

Also worthy of note is the relation of southern pine beetle attacks and the occurrence of colonies of the endangered red-cockaded woodpecker. Management that maintains pines at wide spacing and that also reduces understory hardwoods produces colony stands in good health and, at the same time, trees highly resistant to beetle attack.6

Other insects of importance ecologically include the Nantucket pine tip moth. This moth leaves a sort of witches'-broom on most southern pine species (longleaf pine is the exception) as it bores into the terminal buds of twigs. Lateral buds then burst to extend many branches to compete for a leadership role in replacing the tree's terminal leader. (Many knots show in boards cut from such trees.) This, in turn, results in more compression wood being formed in the merchantable part of the bole. (Compression wood, exemplified by the fiber developed on the underside of a lateral branch, is inferior for all uses. Abnormal shrinkage and swelling of this denser wood causes warping and twisting of lumber cut from such trees.) Tree growth is stunted, and some stems are killed by repeated attacks.

The moth adults exhibit 1/4-inch wings showing brick-red and copper-colored patches separated by bands of gray scales. The eggs change from white to orange to gray color during incubation. They hatch to produce minute cream-colored caterpillars with black heads and hairy bodies (see Chapter 6). Some strains of southern pine species may be immune to tip moth attack. In that event, future southern forests that are artificially regenerated likely will be from genetically selected seeds carrying the immunity gene.

Notable among the insect pests of the pine forest, especially in the West Gulf South, is the Texas leaf-cutting ant, Atta texana. Ants in a colony may "work" 100 feet from their nests to cut needles of seedlings. Most of their effort is on sandier soils of well-drained sites with warmer western or southern exposures.

These ants march single-file along cleared trails that resemble miniature highways, carrying needle fragments over their heads to underground galleries. The intricate systems of chambers may extend more than 20 feet below the surface. The ants then carry small pieces of foliage into smaller cavities. There the fragments serve as a culture medium in "gardens" where a fungus is produced. The fungus, not the pine needles, provides food for the colony. The ants parade above ground only when the temperature is within a narrow range. Work ceases on hot, cold, cloudy, or rainy days.

Pine web worms in the larvae stage often defoliate and kill young pine plantations. The striped caterpillar lives in silken webs enclosing the main stems and tips of leaders and small branches. The web entraps the insects' excrement pellets with a frass-like material. After feeding on the foliage, the 1-inch-long larvae drop to the ground to pupate just under the surface of the soil.

Many other generally less serious insects also aid in keeping the southern forest a dynamic ecological enterprise. Some plague the woodsman and his woods for brief periods: pitch moths kill shoots, spider mites cause foliage to turn brown, and colaspis beetles make needles appear as though scorched by fire. Needle miners, pitch midges, scale insects, and aphids are other lesser pests that become serious menaces to stands of trees planted beyond their natural ranges or on sites for which they are ill-adapted.

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