Southern Pecan Leaf Phylloxera

Prepared by Mark Arena, Specialty Crop Agent, Clemson Extension, Clemson University Cooperative Extension Service. 07/16.

HGIC 2212

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Warty-like bumps on pecan leaves can start popping up in May. Resembling chickenpox, these bumps are a result of a tiny aphid-like insect feeding on the foliage of the pecan leaves. Once the feeding damages the leaf tissue, the plant tries to compartmentalize the wound. Then a gnarly unattractive gall develops like a blister, in an effort to limit the spread of the wound.

Close up of leaves infected by Pecan phylloxera (Phylloxera devastatrix).
Close up of leaves infected by Pecan phylloxera (Phylloxera devastatrix).
Mark Arena ©2016 Specialty Crop Agent, Clemson Extension

The growth typically encapsulates this insect within the gall. This response occurs for two reasons. First to protect the leaf tissue and damage that has occurred. Secondly, it provides a safe environment for young insects to feed and develop. This is known as a co-evolved response.

Even though the damage created by these insects is unattractive and concerning at first glance, it generally does 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 it may be best to treat for this issue to get the populations to a manageable level if possible.

Pecan phylloxera (Phylloxera devastatrix) insects feeding within a gall (magnification = 30X).
Pecan phylloxera (Phylloxera devastatrix) insects feeding within a gall (magnification = 30X).
Mark Arena ©2016 Specialty Crop Agent, Clemson Extension

Treatment must be timed appropriately in order to be effective. The proper time to either treat or prevent this issue from occurring is early in the growing season when the leaves are just starting to emerge. Proper treatment consists of spraying the entire trunk and large branches with an insecticide labeled for controlling aphids. The trunk is sprayed because a majority of the eggs are laid in the cracks and cervices of the bark. The eggs hatch in the spring, when the leaves start to develop on the tree. Therefore, by applying an insecticide at this time, a majority of the young crawlers are terminated. It is also recommended to make two to three additional sprays as outlined on the label of the product used. It is also important to clean up all the leaves on the ground at the end of the year, since some eggs do overwinter in the galls. These leaves need to be properly disposed of to prevent additional infestation by this insect. Currently, there is no treatment available for this condition once it is noticed and the leaves are damaged. Please understand that treating this condition once it is notice will have no benefit! You will only be wasting your time and money. This condition needs to be treated as noted under the Cultural and Chemical Control sections of this publication!

The biggest challenge for homeowners is safely and effectively treating a tree once it becomes large in size. Currently, and for commercial production, the primary method for treating these insects is by using an insecticide that is sprayed onto the tree’s trunk and large branches. For most homeowners this is impractical and the cost associated with hiring a reputable tree company to treat for this condition would not be financially affordable. Also, most homeowners are not willing to invest in the equipment needed to properly spray the tree on their own. Below are the cultural and chemical controls that homeowner might consider.

Cultural Control: Keep the trees as healthy as possible. Other stresses to the pecan tree should be reduced, 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. Rake and burn or dispose of fallen leaves in autumn.

Chemical Control: 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 leaf galls to kill the insects. Commercially, pecans are sprayed one or two times in the spring shortly after bud break when the eggs hatch and the small insects are crawling to the developing leaf buds.  Homeowners could attempt this if the pecan trees are small, using carbaryl with two applications at 7 to 10 days apart. Follow label directions for use. However, it is impossible for a homeowner to spray a large tree and get good coverage in a safe manner.

If Southern pecan leaf phylloxera severely damaged the pecan the previous year, a homeowner can spray a 4% horticultural oil solution (10 tablespoons oil per gallon water) to thoroughly wet the trunk, limbs and smaller branches before bud break (before new spring growth) during the winter or early spring. The winter is spent in the egg stage on the bark in cracks and crevices, and in the crevices of old galls on fallen leaves. This is a much safer approach to control the pest, and the oil spray will smother the eggs and any nymphs that have hatched.

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