Twigs falling from pecan trees

    This afternoon I decided to trim up some seedling pecan trees growing along the south edge of my farm. These trees are growing in the worst soil on my farm, an eroded Dennis silt loam that is shallow and very prone to drought. However, I can't bear to cut down  these squirrel-planted pecan trees even if I'll never harvest a nut. I can still enjoy the shade.
    When I drove up to the trees I noticed the ground was littered with small branches that look like they had been cut from the tree with the leaves still attached. I picked up four of the fallen shoots and brought them back to the house to photograph (photo at right).

    If you look closely at the cut end of the branch you will note that the outer portion of the stem was cut smoothly but the center portion actually broke off in a gust of wind (photo at left). This type of limb pruning is caused by a long-horned beetle called a "twig girdler". You can see a photo of this beetle in a previous post.

     As I carefully inspected each of the four limbs I collected , I found one with a partially completed girdle (photo at right). I bet the insect got frustrated by constantly backing into that small branch just below and decided to try a different location. However, this partially completed girdle gives us a good idea how deep twig girdlers chew into pecan branches.

   Looking even closer at the twigs, I noticed a oviposition site at the base of old leaf scar (photo at left) . Twig girdlers carve a crescent shaped hole in the twig and deposit their eggs inside. Looking at the entire stem I found several oviposition sites, always placed just below a leaf scar.
    Twig girdlers adults are active in late summer (August-Sept.). They girdle pecan stems at this time of year so that all the carbohydrates created by the leaves can not be translocated down to the roots and end up accumulating in the stem above the girdle. Eggs laid in crescent-shaped scars hatch the following spring, with the resulting larvae feeding on the nutrient rich twigs.
    In commercial orchards, twig girdler is rarely a problem because sprays aimed at controlling pecan weevil during the month of August also control twig girdler. For non-sprayed pecan trees, twig girdler populations can reduced by picking up all fallen twigs during the winter months and burning them (thus killing the next generation of girdlers).  

Broken twigs caused by long-horned beetle

   This week, I've been back up in pecan tree canopies collecting nut samples from our breeding plot. When I'm up in the hydraulic lift I get to see things I might not notice from ground level. High-up in one tree, I found a branch that looked like it was cleanly broken in two with the leaves above the break dessicated to a crispy brown (photo at right).

     Moving in for a closer look, I found that the branch had a clean break--almost like it was partially cut by a pruning tool. This type of clean break is the tell-tale sign that the limb had been attacked by a long-horn beetle called the twig girdler. You can find a photo of an adult twig girdler and the details of this beetle's life history in a post I made previously.
     The female beetle lays eggs in the branch above the girdle. These eggs will hatch next spring and larvae will feed on the recently killed twig. If you find neatly broken off limbs on the ground under your pecan trees this harvest season, it is a good idea to pick up those twigs and burn them to destroy next year's crop of twig girdlers.

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.