Sunday, April 15, 2012

Twig girdler damage on pecan

    Between rain showers, I spent most of this weekend grafting pecan trees on my farm. While wading through the water and wet grass, I came across a tree that I grafted last year. At first, the tree looked like it had suffered terminal dieback possibly from last year's drought. But when I took a closer look, I noticed a profusion of new shoots all growing out below a nicely cut grove in the stem (photo at right).  This groove was cut late last summer by an insect called a twig girdler.
   The twig girdler is a long horn beetle that is 5/8 to 3/4 inch long (photo at left). The most distinguishing feature of this insect are the long antennae that are as long or longer than the entire insect. Female twig girdlers lay eggs in August through September in small pecan branches usually just below the point where a leaf is attached to the stem.
    After laying eggs, the female cuts a grove in the stem. The girdling of the stem causes all the carbohydrates manufactured by the leaves above the girdle to become concentrated in the stem rather that being set down to the roots. The eggs she lays in late summer don't hatch until the following spring. But when they do hatch, the larvae feast on the carbohydrate enriched twig.
     Twig girdlers are most often a problem in non-bearing pecan groves. Once pecan trees start to bear, insecticides applied in August and September to control pecan weevil also control twig girdler. In young orchards, picking up girdled branches every spring and burning them will help reduce the population of twig girdlers in the future.