Treat Now to Avoid Holes in Pecans
Adult pecan weevils are beginning to emerge. These small beetles with long slender snouts and thin legs are gray and only 0.3 to 0.5 inch long. The snout is longer than the body on the females and slightly shorter on the males.
The pecan weevil is the most serious late-season pest because it attacks the nut. Pecan weevils cause two types of damage. The first type occurs when adult weevils puncture the nuts while feeding before the shell hardens. This can result in the nuts dropping from the tree. The larva feeding within the nut causes the second type of damage. The larva feeds for about a month before maturing and chewing a circular hole through the shell. This circular exit hole allows the grub to fall to the ground and burrow into the soil to a depth of four to twelve inches. This stage of the pecan weevil remains below ground for one to two years. The larva pupates into the adult stage and can remain in the soil for an additional year.
Adult weevils emerge from the soil from late July to October depending on many factors including the soil type and moisture. A heavy rain often triggers their emergence. The heaviest emergence usually extends from the middle of August through the middle part of September. Adults crawl to the trunk, or fly to the tree trunk or fly directly into the tree canopy in search of food and a mate. Feeding begins by both male and females pecan weevils as they reach the tree canopy.
A mated female feeds by drilling holes through the shuck and shell to the kernel where she excavates a small cavity in the developing kernel. She places two to four white eggs in each nut with her ovipositor on the developing kernel. The life cycle continues as the larvae pass through four stages feeding inside the nut.
The key to managing the pecan weevil is to prevent the laying of eggs and the timing of control measures is very important. Growers benefit from learning to recognize the vulnerable stages of this insect. Female weevils begin laying eggs about 5 days after they emerge from the soil. Adult emergence should be monitored starting the first of August.
Insect traps can be used to catch adult weevils and monitor their emergence. There are two ways to monitor for pecan weevils using traps. First is by placing cone cage traps under tree drip line and recording the number of adult weevils collected. A second way is to use a band around the trunk of the tree as a trap. Wrap burlap around a pecan tree 3 to 4 feet above the soil and secure at the bottom. Lap the remaining burlap over and secure it at the top. The weevils will be slowed down as they must walk over each flap to reach the canopy. This extra time will help a grower observe weevil emergence.
Traps should be checked daily noting the numbers of weevils indicating when to time a control spray. One study has found that pecan weevils move from the orchard floor to the pecan tree near dusk.
Ground sprays of carbaryl (Sevin) during emergence are recommended for pecan weevil control. Weevil emergence is observed in traps to help identify the critical periods for well-timed sprays. Multiple applications are recommended every week until emergence stops.