Drought also causes failure to fill, if it occurs late in the growing season. The shell is formed earlier and filling occurs later in the season. In many years, lack of pollination causes the greatest loss of nuts. Since pecans are wind-pollinated, excessive rain during bloom prevents pollination and the unpollinated nuts fall. Weather conditions in some seasons cause the male and female flowers to mature at different periods, and pollination fails to occur.
Some cultivars shed their pollen before the female flowers are receptive.
Insect damage also can cause premature loss of nuts. Besides the nut casebearer, black aphids, pecan weevils, stink bugs and shuckworm also cause losses. Pecan weevil feeding causes nut drop. Scab, powdery mildew, blotch and other diseases cause nut loss. Scab is usually bad if rainy, humid conditions occur during the early part of the season.