Round galls 1/4 inch to 1 inch in diameter can sometimes be found on twigs and leaves of pecan trees in late April, May and early June. These galls are formed by a dot-size insect called phylloxera, according to LSU AgCenter horticulturist Dr. John Pyzner.
The galls generally split open in late May and early June and release small, greenish, winged aphid-like insects. The galls then dry, which causes twig dieback and leaf drop. Severe infestations of this insect can cause loss of the pecan crop for the current year and also for the following year.
Pyzner says no effective control of phylloxera is available once the galls are present. Sprayed insecticides will not reach the insects inside the galls, and systemic insecticides will usually not reach high enough concentrations in the galls to kill the insects.
Spraying for phylloxera after the galls split open will kill some of the insects, but control at gall split is not very effective because of the extended period over which the galls release insects. Spraying at gall split will not reduce any current injury to the tree, since phylloxera insects cause damage only during April when they start feeding on the new shoot growth. Phylloxera can damage tree appearance and reduce tree vigor; however, the insects rarely kill a tree unless the tree has other major problems.
Pecan phylloxera insects overwinter as eggs in protected places on branches. The young phylloxera resembles a tiny orange aphid and appears in the spring about the time the buds unfold. The young insect inserts its beak into the new growth and injects a toxin. This causes abnormal tissue growth, which forms a gall around the insect.
Phylloxera matures within the gall and deposits numerous eggs in the gall that hatch and develop into greenish winged phylloxera insects. In late May or in early June, the galls split open to release the insects.
The winged phylloxera deposits eggs on bark and leaves. The eggs shortly hatch into wingless male and female insects that do not eat. These insects mate, and the females produce one egg that is hidden in bark crevices.
The most effective control of pecan stem phylloxera occurs in the spring when the eggs hatch and the small insects crawl to the buds. One or two sprays in early April are usually effective in controlling this insect.
Pyzner recommends insecticides Provado 1.6F, Warrior 2.56 and Lorsban 4E for good phyloxera control. Phylloxera are delicate insects and usually do not spread very far; therefore, good control one year will often keep phylloxera damage low for several years unless infested trees are near.
For related horticulture topics, look for Gardening and Get It Growing links in the Feature section of the LSU AgCenter Web site: www.lsuagcenter.com.