What’s causing these knotty growths on the twigs of my pecan tree and what can I do about it? The short answer is that the growths are galls caused by a small aphid-like insect called the pecan phylloxera and, if the tree has already leafed out, there is nothing you can do about it for the rest of this year. By the time these galls appear in April, the damage is done for the year and it is too late to treat. It is possible to successfully treat for phylloxera, but the time window for treatment is very narrow, lasting only a couple of weeks or less.
Phylloxera overwinter as eggs on the bark of the tree and egg hatch coincides closely with bud break. As the nymphs, which are all females, feed on the developing stem and leaf tissue, they stimulate the formation of gall tissue, which quickly encloses them to form the galls. These first generation females mature inside the galls and then lay many eggs that hatch into a second generation. These are also all females and continue developing inside the hollow galls. There may be many dozens of these second generation phylloxera within a single gall. When the galls split open in late May the mature second generation phylloxera exit the gall and lay eggs on the leaves. The resulting third generation, which consists of both males and females, does not cause galls. Mated females of this generation seek a protected site in a bark crevice and die with a single fertilized egg still inside their bodies. These eggs overwinter and hatch the following spring to begin the cycle anew.
Timing is critical for phylloxera control! Successful control depends on applying insecticides before galls have formed around the phylloxera - while she is still exposed. This means spraying as soon as you see the first new leaf growth emerging from the buds (inner scale split stage) and before new growth is ½ to ¾ inch long.
Of course, the real challenge in treating pecan trees is that they are tall and the new growth that needs to be treated is high above the ground. Urban and rural homeowners do not have the equipment to reach tops of mature pecans. I do not know anyone who does this kind of spraying either.
Severely damaged trees will usually recover in subsequent years. Sometimes a homeowner’s initial reaction to a serious phylloxera infestation is to think, “If this is going to happen every year, then I may as well cut the tree down.” Fortunately, phylloxera populations are cyclic and trees that are severely damaged one year will usually recover in subsequent years and then go many years without suffering a serious infestation. But this does not always happen the very next year, and trees that had phylloxera infestations one year may experience heavy infestations for another year or two before the infestation breaks. When possible, it is a good idea to treat trees that suffered heavy phylloxera infestations last year - but only if the treatments can be applied safely and at the proper time.
If you can find someone who can treat large trees, then apply an effective insecticide between the time the leaf buds break and before there is about ¾ inch of new growth, and don’t be late with this treatment. Only trees that were infested the previous year, or trees located next to previously infested trees, need be treated. If the tree was heavily infested the previous year, apply a second treatment approximately 10 days later.
Carbaryl (Sevin XLR Plus or other formulations of carbaryl), is a standard treatment for phylloxera in homeowner type situations. Carbaryl is still labeled, and still not restricted use, but it is difficult to find in small volumes that are practical for homeowners. (2.5 gallon jugs are readily available, enough to treat 2 or 3 acres). Products containing imidacloprid are somewhat better, but homeowners may have to consider generic formulations to be able to purchase imidacloprid in smaller quantities. There are many generic formulations of imidacloprid on the market and some of these are sold in 1 pint or 1quart bottles and are labeled for use on pecans (Macho 2.0 F is one example). The key is to be sure the product label gives specific instructions for use on pecans. {Use rate for Sevin XLR Plus (44.1% carbaryl) = 2 to 5 quarts per acre; Admire Pro (42.8% imidacloprid) = 1.2 to 2.4 fl oz per acre; Macho 2.0 FL (21.4% imidacloprid) = 2.8 to 5.6 fl oz per acre.
If you are planning to plant pecan trees in or around your lawn, you can choose varieties that are less prone to insect and disease problems. Varieties better suited for use in low maintenance situations include Amling, Candy, Elliot, Excel, Curtis, Forkert, Gafford, Jackson, Kanza, Lakota, McMillan, Melrose, Syrup Mill and several others. Generally, these varieties have smaller nuts than the commercial varieties, but they usually perform much better in unmanaged situations. Resistance to pecan scab is an especially important trait to consider when choosing varieties for low maintenance situations.
