Thursday, May 17, 2012

Pecan Nut Casebearer 2012

    Pecan nut casebearer is one of the primary nut feeding insects that pecan producers must monitor closely (photo at left. Note frass and webbing). During years of low nut production, casebearer larvae can completely wipe out what few nuts are set on a tree. In contrast, casebearer feeding can act as a beneficial nut thinning agent during years when trees set an excessive crop.
    With the this growing season nearly 3 weeks ahead of normal, we got way behind in our normal spring work and failed to get pheromone traps up early enough to capture the flight of first generation adults. However all is not lost. From our experience of monitoring casebearer and pecan nut development for more than 30 years we can predict that first nut entry will occur next week most likely, May 22. Since all northern pecan areas experienced the same early spring this year, The normal differences we see in the timing of budbreak, nut development, and insect appearance from south to north have been nearly erased. As a consequence, everyone should expect first nut entry sometime next week.   
     How can we tell about casebearer by just looking at nuts?   Pecan nut casebearer does not attack until the nuts have been fertilized and the nut begins to swell in size. Look at this pair of photos.
The nuts cluster on the left is pollinated but the pollen tube has yet to grow down to the ovary to fertilize the egg. Note the small size of the nutlet and the fact that the 4 leafy bracts just below the stigma stick outwards. The nuts pictured on the right are fertilized. The small nuts have enlarged and the 4 bracts are now sticking straight up. The nuts in our grove are nearly all fertilized and are now ready to for egg laying by female casebearer moths.

    To spray or not to spray, that is the question. We've been following nut set and casebearer damage levels for many years and have developed a decision tree to determine if spraying for casebearer is really necessary (pictured above).  We start by counting the number of nuts in a cluster on at least 300 nut bearing terminals. If the average cluster size is above 2.9 we have determined that spraying for pecan nut casebearer is not cost effective and in fact nut casebearer  may act as a beneficial thinning agent.  This year our native pecan trees have an average cluster size of 3.8. We may be looking at a record breaking crop in 2012.
    This decision tree was developed for native pecans. Improved cultivars most likely have a higher cluster size threshold for not treating than native trees. However, with the nut set I'm seeing on all our trees, I won't be spraying anything for casebearer this year.