Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. Insects and Diseases of Cottonwood. New Orleans, LA. U.S. Department of Agriculture, Forest Service; Southern Forest Experiment Station; State and Private Forestry Southeastern Area. Gen. Tech. Rep. SO-8. 1975. 41 p.
The pests described below are usually not serious threats to cottonwoods. However, large populations occasionally build up in limited areas or on individual trees and cause considerable damage. Normally natural controls are sufficient, but to suppress periodic buildups in localized areas, artificial controls may be required.
The first four insects are moths. The adults are inconspicuous and do no damage, but the larvae feed on cottonwood.
Blotch leafminers, Paraleucoptera albella. - This insect probably occurs throughout the commercial range of cottonwood. Eggs are laid in clusters of 3 to 12 on the upper leaf surface. Groups of small, white larvae tunneling between the upper and lower leaf surfaces consume the green tissues and cause conspicuous brown blotches. Periodic heavy infestations may destroy half the total leaf surface and reduce growth of young cottonwoods. Full grown larvae leave the mine and spin small white tent cocoons at leaf margins.
Serpentine leafminers, Gracillariidae. - The tiny, flat larvae construct winding mines in leaf blades and consume the green tissues. Their attacks late in the season are often noted, but their damage is usually negligible., Mature larvae spin small, flattopped, white cocoons in curved leaf margins.
Leaf rollers, Tortricidae. - Pale green larvae about 1/4 inch (6 mm) long fold over leaf margins and tips, fastening the edges with silk to form shelters in which they feed. Late in the season, new leaves may be conspicuously attacked, but damage to the tree is minor.
Epidermal miner , Murmura sp. - Tiny larvae mine immediately beneath the epidermis in phloem tissues and do not damage cambium or xylem. Larval mines appear as meandering brown lines on the smooth bark of l- to 2-year-old trunks and branches. The injury is probably insignificant, but its presence can cause rejection of cuttings subject to quarantine examination for shipment.
Aphids or plant lice (Aphididae). - These small sucking insects are widely distributed and often cause local damage. Most forms suck the sap from leaves and tender terminal tissues. While feeding, they exude droplets of honeydew, which attract ants. Sooty mildew develops on leaves and tips covered with honeydew, and serious growth retardation may result. Other aphids feed on bark in the spring, injuring the bark and cambium. Others cause the formation of galls on leaf petioles, where they are feeding, and may cause premature leaf fall.
Night-feeding leaf beetles, Metachroma sp. - These beetles appear sporadically and can damage new plantings by killing tender new leaves, terminal tips, and buds. Adults are brown and smaller than cottonwood leaf beetles. In Mississippi they appear after mid-May and disappear after mid- June. They hide during the day and feed at night, cutting many small holes in the leaves. Older leaves remain, but new leaves turn black and drop off; terminal tips also turn black and die. The larvae are root feeders, and little is known of their biology.
Leaf curl midge, Prodiplosis morrisi Gagne. - Maggots of a fly midge feed in the tightly rolled margins of developing cottonwood leaves and damage the tender tissues, causing them to turn black and die. The leaves cannot develop and usually drop off. Some may appear stunted and crinkled, with only their tips developed normally. Attacks typically occur in June and can slow the growth of first-year plantings. Heavy rains reduce outbreaks, and normal foliage development follows.
A host of other insects also attacks cottonwoods. Most are inoccuous and appear infrequently. Some are capable of building to damaging levels in large plantings. Such outbreaks can occur without warning and should be reported promptly so that potential damage can be anticipated and minimized.
