Wednesday, June 25, 2014

Following the evidence: Pecan nut casebearer damage development

    This spring I've put together a separate page on this website to keep you posted on pecan nut casebearer activity (see tab above). But today, I wanted to show how the activity of casebearer larvae can be traced by careful observation of frass piles (frass=insect manure).
    As casebearer larvae feed on the inside of pecans, they push all their frass outside into a nice neat pile. In the photo above, the red arrow marked "A" points to a pile of dark, granular frass created by a casebearer feeding inside that nut.
   But that's not the entire story. If you look carefully, the red arrow marked "B" points to a small frass pile at the base of a bud. When casebearer larvae first hatch, they often feed on buds before moving onto the nut cluster. Note that the frass pile at the base of the bud is much smaller than the pile formed at the base of the nut. The smaller pile was created by a smaller, first-instar larvae. By the time the insect moved into the nut, the larvae had grown larger and was eating far more plant material.

    The photo at left is a close up of the frass pile found on the nut seen in the photo above. There are two things to notice. First, fine strands of white webbing are strung between the nut and the pedicel. This webbing serves to prevent the damaged nut from dropping off the tree,  keeping the insect within the tree's canopy.
   Next, notice that there are two different colors of insect frass in the pile. The lighter colored frass was just expelled from the nut betraying the fact that there is still an actively feeding larvae inside the nut.
    The photo at right shows a clear evidence trail of how a single casebearer larvae destroyed three nuts in this cluster. The larvae started at the base of the cluster. At the time of first nut entry, the basal nut was still small and could not provide enough nourishment for the growing larvae.  Once the caterpillar ran out of food in the first nut its moved onto the next nut up the stem. This second nut had grown slightly larger but still didn't meet all the nutritional needs of casebearer. So the larvae moved onto a third nut.
   Looking at the piles of frass at the base of each nut, notice that the size of the granules changes from nut to nut. The first nut attacked has the smallest frass granules indicating a small larvae had fed inside the nut. In sharp contrast, the frass pile at the base of the third nut has large granules that are light in color. Just by looking at the frass, I can tell a full sized larvae is actively feeding on this nut and will probably pupate inside.
    The photo above also demonstrates how webbing, spun by the larvae, holds the nut onto the tree even after a damaged nut aborts. The lowest nut in the cluster is literally holding on by a thread. At a later date, the last nut damaged by casebearer will also abort from the tree. Again, insect webbing will hold this nut in the tree but this time the nut will contain a pupal case. By not dropping to the ground, the survival of both pupae and the emerging adult moths is increased.