Phylloxerans are tiny, aphid-like insects that cause galls on plants as a result of their feeding. Once the feeding damages the leaf tissue, the plant tries to compartmentalize the wound. Then a gnarly unattractive gall develops like a blister to limit the spread of the wound. The growth typically encapsulates this insect within the gall, providing a safe environment for young insects to feed and develop.
There are three species of phylloxera commonly encountered on pecan trees in the Southeast. They can be distinguished by the features of the galls they form, as the insects themselves are too tiny to inspect without a microscope.
Pecan leaf phylloxera (Phylloxera notabilis Pergande)
The galls of pecan leaf phylloxera are usually 2 to 20 mm across, forming circular blisters that span both the upper and lower surfaces of a leaf. In mid-May, the galls on the underside of the leaf split open into 4 to 8 bracts and release adults (sexuals) that lay overwintering eggs and nymphs that spread to other leaves to form new galls. There are at least three generations per year. Pecan leaf phylloxera is most prominent in nursery stock and young trees.
Southern pecan leaf phylloxera (Phylloxera russelae Stoetzel)
The galls of Southern pecan leaf phylloxera split open in late May to mid-June. They also split from the underside of the leaf, and usually, the fissure forms three bracts. This species is primarily a pest of mature trees, and light populations do not cause significant damage.
Pecan phylloxera (Phylloxera devastratrix Pergande)
The galls of pecan phylloxera are hard and tumor-like, reaching up to an inch across. They form on leaves, stems, leaf petioles, catkins, and nuts. The galls split open in late May or early June and release winged females, which lay overwintering eggs in protected places on the trees. There is only one generation per year. Heavy infestations can cause defoliation, crop loss, and overall reduced tree health. Among the three phylloxera species, pecan phylloxera is the most damaging.
Even though the damage created by these insects is unattractive and alarming at first glance, it may not hurt the tree or impact nut production. As with all pest and disease issues, there needs to be an acceptable level of damage to warrant action. Therefore, if there is only minor damage, treatment may be unnecessary. However, if the damage is extensive and begins to impact nut yield, it may be possible to take action to get the population to a manageable level.
Cultural Control: Keep the trees as healthy as possible. Reduce other stresses to the pecan tree by fertilizing the tree in late February and liming in the fall, according to a soil test report. Additionally, pecan trees should be watered weekly during periods of drought for the best development of the pecan nut crop. Since some eggs overwinter in the galls, fallen leaves must be correctly disposed of (such as through raking and burning) to prevent additional infestation by this insect.
Chemical Control: Chemical control of phylloxera species is usually not practical for homeowners. Effectively treating a tree once it has become large is generally unsafe and impossible without professional equipment and training. The cost of hiring a reputable tree company to treat this condition can be substantial.
Homeowners should only attempt pesticide application if the pecan trees are small and they can safely follow all label directions for use. The most appropriate timing is a narrow window of opportunity early in the growing season, from bud break to when the leaves are just starting to emerge. This timing coincides with when newly-hatched insects crawl from the cracks and crevices of the bark up to the developing leaf buds. Proper treatment consists of spraying the entire trunk and large branches with an insecticide labeled for controlling phylloxera, such as products containing zeta-cypermethrin or imidacloprid. As outlined on the product’s label, two to three additional applications may be necessary. Follow label directions for use.
Currently, no treatment is available for managing these insects once the galls appear and the leaves are damaged. Spraying insecticide once galls are present will have no benefit and is a waste of time and money as the sprays will not reach the insects inside the galls. Additionally, spraying of broad-spectrum insecticides could reduce the abundance of natural enemies (such as lady beetles), which will lead to outbreaks of other insect pests of pecan, such as aphids, mites, weevils, and caterpillars.
Southeastern Pecan Growers’ Handbook. Available for purchase: https://extension.uga.edu/publications/detail.html?number=B1327
Originally published 07/16