Thursday, August 10, 2017

Pecan leafminers

   Have you every noticed a brown spot on the upper side of a pecan leaflet (photo at right)?  What looks like a foliar disease is actually the home of a small caterpillar. The upper-surface blotch leafminer (Cameraria caryaefoliella) feeds on leaf cells just under the epidermis of the leaf and creates an irregular shaped  brown blotch. Look carefully at the blotch and you'll see that under the papery thin epidermis you'll find a pile of black frass left by the caterpillar.

     I carefully pealed back the upper layer of the blotch to see if I could locate the caterpillar underneath (photo at left). I found the worm and moved him out to the leaf surface. The larva was black in color and very sluggish. The color and behavior of this caterpillar indicated that this poor fellow has been parasitized by a miniature wasp. That's one leafminer that won't be metamorphosing into a adult moth.
   Less common in my orchard is the pecan serpentine leafminer, Stigmella junglandofoliella (photo at right). This leafminer also feeds on leaf tissue just under the epidermis of pecan leaves but creates a meandering tunnel. As the larvae grows in size, the tunnel becomes wider making it look like a small snake is sitting in the surface of the leaf. Eventually the larvae in the tunnel will pupate and a small moth will emerge for the widest portion of the tunnel.

    Leaf miners are typically a non-economic pest. I commonly find them on young trees that receive little or no insecticide treatments. Once trees start bearing nuts and are regularly treated for major nut feeding pests, leaf miners largely disappear from the orchard.