We have both black walnut trees as well as several varieties of English walnut trees in Delta County and the surrounding area. Have you ever noticed that in some years the husk that surrounds the walnut has black blotches (see picture)? It can be very difficult to separate the nuts from these husks when this kind of damage occurs. Sometimes the damage is so extensive the nut meat is made unfit to eat.
The cause of this damage is a little fly maggot produced from eggs laid by a banded-wing adult walnut husk fly (see picture). The fly has one generation a year with adults beginning to emerge from the ground beneath previously infested walnut trees in late June. Peak adult emergence occurs in our area between July 15 and 25.
The female lays eggs just below the husk surface. The maggots from early emerging flies can cause the most damage by penetrating through the soft husk to the nut meat itself. Later emerging flies tend to feed primarily in the husk. Maggots feed for three to five weeks before dropping to the ground to pupate.
If your infestation levels were low the previous year, you may not need or want to do anything to control this pest. But if many of your walnuts are difficult to remove from their black, blotchy husks or the nut kernels have been damaged and made unfit for consumption, you may want to do some preventative management.
I began trapping this fly last year in the Cedaredge area using a yellow sticky trap to determine when would be a good time to spray and if insecticide sprays could significantly reduce damage. My findings determined that a single application of a synthetic pyrethroid around mid-July could effectively minimize the damage from this pest. The use of a commercial applicator is recommended because of the size of bearing walnut trees.
I will be trapping for this fly again this year and hopefully will be able to give a heads up on the Cedaredge town website (http://www.cedaredge
colorado.com/) as to when an insecticide application would be most effective. One spray is generally all that is needed for most home orchardists. Since this fly overwinters beneath the tree it infests, control may not be needed every year if the insecticide spray was timed correctly.
A non-insecticidal approach to dealing with damaged husks that refuse to open to free up the walnut kernels is to place the damaged husk-enclosed nuts in a damp burlap bag for a few days. This can sometimes make husk removal easier. Also, by removing damaged nuts and husks beneath the tree as soon as possible, next year's fly numbers can be reduced.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.