Cottonwood Leaf Curl Mite
Aculus lobulifera Keifer

Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. 1975. Insects and Diseases of Cottonwood. Gen. Tech. Rep. SO-8. New Orleans, LA: U.S. Dept of Agriculture, Forest Service, Southern Forest Experiment Station. 41 p.

Attacks by leaf curl mites seriously reduce growth and vigor of young cottonwoods in nurseries and plantations. Mites feed on terminal foliage and stems, causing stunting and malformation of leaves, terminal tips and buds as well as the loss of immature foliage. The pest has been observed in cottonwood throughout most of its commercial range.

Leaf damage symptoms appear in early summer and intensify as the hot, dry weather continues. Heavily attacked leaves become stunted with red veins and crinkled, purplish-green blades, which are brittle and curled. The petioles become scaly and brown. Terminal shoots are also stunted, scaly, and brown. Small, developing leaves commonly break off, leaving several inches of the terminal shoot leafless.

Typical foliage and stem damage by leaf curl mited. Photo by Theodor D. Leininger, USDA Forest Service

Leaf curl mites are minute, four-legged, and straw-colored; they look like dust flecks on the leaves. They develop two alternating forms: hibernating mites and foliage-feeding mites, the primary form. The majority of the mites perish with leaf fall; but a few find hibernation shelters in bark crevices, branch scars, and at the base of the trunk. Early the next spring, the hibernating mites leave their shelters, feed on green tissues, and lay eggs, which produce primary forms on the new foliage. The primary mites multiply rapidly; and all stages, eggs to adults, are found together on the foliage and tender terminals. Heavy populations develop during dry periods, especially from June through August and in October.

Heavy rains disseminate mite populations, and new growth with normal foliage may follow; however, this pest may reappear with dry weather in the fall. No other natural controls are known. Some improved cottonwood clones show evidence of possible resistance, and these are being investigated further. Artificial controls are available and may reduce populations significantly.