Pecan Insect Causes Galls On Trees

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Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

No other tree holds such promise and so frequently underperforms for residents in Angelina County as the pecan. Truthfully, we live in a harsh area for the growth of pecans.  Much of our soils drain too poorly.  Our moisture and humidity increase the likelihood of fungal disease.  And our naturally high insect levels can also make it difficult.

This year, the insect call the pecan phylloxera is making its presence known in the form of several knots. Beginning in mid-April, galls or knots appear on the leaf veins, leaf rachises, catkins; current seasons shoot growth and nuts of affected pecan trees.  These galls are caused by the feeding of small aphid-like insects known as the pecan phylloxera, Phylloxera devastatrix.

Already this year, some galls may be extremely numerous, covering the entire tree and giving the limbs a knotty appearance.  This unsightly mess is truly more of an eyesore for homeowners than it is a health concern for the tree.  That said, below is the life cycle of the this insect.

Galls can remain on the twigs for several years. High infestation levels of this insect cause the current season’s shoots or twigs to become deformed, reducing their rate of growth. In some cases, severe infestations can lead to dieback of the current season’s shoots. Galls also can form on the nuts, causing nut deformity and premature nut loss.

Pecan phylloxera overwinters as a single egg within the body of a dead female insect. Prior to dying, the female seeks shelter on the tree under dead bark, within old galls or even under the carapaces (shells) of dead scale insects.

The overwintering eggs begin hatching in early spring at about the time the pecan buds are beginning to open. In eastern Texas, this can begin as early as mid-March. Upon hatching, the nymphs move from the overwintering sites to the opening buds.

Once on the buds, the insects begin feeding.  As they feed, a gall begins to form around the insect, eventually enclosing it within the gall (Figure 4). It is only this generation that forms a gall. Once the stem mother reaches maturity, she begins to lay eggs within the gall.  The number of eggs laid ranges from 300 to 1,300 per gall.

The young that hatch from these eggs feed and develop within the gall.  As they mature, they develop into wingless and winged females.  The winged variants often are referred to as winged migrants.

The winged migrants emerge as the galls begin to split open in May.  They disperse within the tree or, with the aid of the wind, are carried to other trees within the orchard. Soon after emergence, egg laying takes place.

The small, light-yellow eggs are deposited on the upper and lower leaf surfaces.  When infestation levels are high, the leaves often take on a yellowish tint because of the high numbers of eggs deposited on the leaves.  The eggs deposited by the winged migrants hatch into sexual males and females.

Almost immediately after hatching, the male and female phylloxera mate, and a single egg forms within the body of the female. Prior to dying, the female seeks shelter in a protected area on the tree, usually under the bark, in old galls or under dead scale insects.  The egg will remain dormant within the body of the dead female until the egg hatches the following spring and begins another cycle.

Infestations of pecan phylloxera do not occur on a regular basis, nor are all pecan cultivars susceptible to attack. Before an insecticide application is made, it is important to determine if phylloxera are present and on what cultivars.

If phylloxera are present, insecticide applications should be made after the tree has budded out, where the leaves have begun to unfurl, and early leaf expansion has started. It is suggested that one insecticide application be made per year. Insecticide applications need to be made prior to gall formation, because once the insects are enclosed in the galls, control is no longer possible.

Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu

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