Utah State University Extension
Walnut husk fly (Order Diptera, Family Tephritidae; Fig. 1) is the most common insect pest of walnuts in Utah. Husk fly larvae (maggots) tunnel in walnut husks, causing them to soften and decay, and stain the shell (Fig. 2). Damaged husks are difficult to remove. If husk fly infestation occurs early in kernel development, nuts may shrivel, darken, become moldy, and drop from the tree (Fig. 3).
Alston, Diane; Murray, Marion; and Barnhill, James, "Walnut Husk Fly" (2015). All Current Publications. Paper 980.