So, your pecan trees are coming along nicely, but now you notice these little blister-like bumps on many of the leaves. What’s up with that?
The bumps on pecan tree leaves are actually galls caused by the leaf’s response to small insects called pecan phylloxeras (Fill-OX-er-ahs). These are true bugs (order Hemiptera), an aphid like insect. There are three different species that feed almost exclusively on pecan trees – carya illinoensis. All are about 1/8 inch long, soft-bodied and pale yellowish in color. The actual insects are rarely seen, but the galls can be conspicuous.
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Types of Phylloxera
The pecan leaf phylloxera (Phylloxera notabilis) produces small greenish galls along the midribs and secondary veins of the leaf tissue. The galls are often reddish on their undersides. This species attacks only the pecan leaves on the tree growing. It rarely causes much damage other than minor defoliation and leaves turning brown.
The southern pecan leaf phylloxera (P. russellae) produces small flattened galls between the secondary veins on the terminal leaflets. The galls may show a reticulated pattern. This species also rarely causes much damage other than minor defoliation.
The pecan stem phylloxera (Phylloxera devastatrix) is the worst. It makes large green galls on leaves, stems, twigs, and even on the nuts. The pecan stem phylloxera is mainly a pest on mature pecan trees, but it sometimes infests young trees too.
Light infestations are usually not worrisome. Severe infestations can destroy an entire nut crop and weaken trees so that the following year’s production is reduced as well. The pecan varieties ‘Elliot’, ‘Candy’, and ‘Jenkins’ show some resistance phylloxeras on pecan trees.
Phylloxera Life Cycle
The phylloxeras have very interesting life cycle histories involving both asexual and sexual stages. Pecan stem phylloxeras die in the winter after leaf drop. But beforehand the females secrete themselves into crevices, under bark, or within cracked twigs.
There each stem mother matures producing a single egg that remains in her dead body until it hatches the following spring. Just as the leaf buds are beginning to open, the eggs hatch and the emerging nymphs (called stem mothers) crawl off to bore into nearby developing leaf buds. As the stem mothers feed on the young leaves, a leaf gall forms around them, protecting them from predators (and insecticides!).
After a few weeks of sucking nutrients from the leaf, each stem mother lays inside her protective gall 300-1000 eggs and then dies. The eggs hatch into female insects which feed and develop within the gall. Later in the season the galls split open and the virgin females disperse to other leaves on the same or nearby trees. They then lay hundreds of eggs that will hatch into males and females. The males and females then mate sexually and each female produces a single egg that remains in her dead body until early spring when the cycle begins again.
Pecan Phylloxera Control
If you see pecan phylloxera galls on your tree, it is already too late to do anything about it this year. Instead, mark the trees that have the worst infestations, and plan to treat them and adjacent trees early next growing season. It’s mainly the trees with an infestation this year that will have it bad again next year.
Use a hose-end sprayer made for trees. Spray a dormant oil in late winter before bud break to smother many of the overwintering eggs. Spray insecticide just as the leaves are beginning to unfurl to kill many of the stem mothers as they crawl to fresh leaves.
Appropriate insecticides include Imidacloprid (Bayer Advanced® is one product), Carbaryl (such as Sevin ®), or a malathion product labeled for pecan trees. Be careful with the spray drift, as these insecticides are very toxic to fish and beneficial insects in the grove.
Pecan phylloxera are weak fliers and don’t spread very far, so if you can get them under control one year, you should be good for a few more years. They have been nicknamed “the devastator” because of the amount of destruction they can cause so they must be managed! The bumps on pecan tree leaves can become a huge issue if left untreated.
Contact us with any further questions you may have about pecan phylloxera or other pecan tree diseases.