Test Your Knowledge - February

Pecan weevil larva & larval exit holes
Pecan weevil larva & exit holes
Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Yes, this is a pecan weevil larva inside a pecan nut, and the holes are the larval exit holes.

The pecan weevil (Curculio caryae) is a reddish-brown weevil about 3/8-inch long and is one of the most destructive pests of pecan nuts. The adult has a long snout, which is used to feed on the developing pecan nut during the water stage prior to shell hardening. This feeding causes many damaged nuts to drop to the ground within two to three days. The damaged kernel inside will become black and shriveled.

Adult pecan weevil
Adult pecan weevil
Clemson University/USDA slide series, Bugwood.org

In addition, many more pecan nuts are damaged by the feeding of the pecan weevil larvae (immature stage) within the developing nuts. The adult female inserts two to four eggs into the developing nut, the eggs hatch and the larvae devour the kernel. After several weeks of feeding, many of the infested nuts fall to the ground, and within one to two weeks the larvae bore circular, 1/8-inch exit holes in the shells and enter the soil. The larvae will then burrow four to twelve inches deep into the ground to overwinter.

Pecan weevil larvae in nut
Pecan weevil larvae in nut
HC Ellis, University of Georgia, Bugwood.org

Each larva will remain in the soil for one to two years before developing into a pupa inside an earthen cell. The pupal stage only lasts about three weeks, during which the pupa develops into an adult. Each adult will remain in the soil until the following summer and emerge between mid-August and late-September. Drought conditions can delay emergence, as hard, dry soil prevents the adults from moving to the soil surface.

The adult pecan weevils fly or walk up the pecan tree trunk to feed, mate and lay eggs on the developing nuts. Egg laying begins as early as five days after adults leave the soil.

Female pecan weevil laying eggs
Pecan weevil female ovipositing (laying) eggs into pecan nut
Jerry A. Payne, USDA ARS, Bugwood.org

It is not practical for a homeowner to spray the canopy (branches with leaves) of large pecan trees in order to stop weevil feeding and egg laying. However, cultural controls significantly help control this late season pest. Prematurely fallen pecan nuts should be picked up weekly and discarded. This practice helps break the cycle by preventing larvae from entering the soil and completing their life cycle.

Research has revealed that 5% of adult weevils walk up the trunk, 77% fly onto the trunk to a height of six to eight feet, and 15% fly directly into the lower tree canopy. Once they reach the canopy, they begin feeding, find a mate, and lay eggs. Removal of low-hanging limbs reduces the ability of the adult weevils to reach the canopy. Tree limbs should be trimmed to six feet above the ground.

By encircling the trunks with a sticky coating at a height of eight feet, the majority of the adult weevils will be trapped before reaching the canopy. Tanglefoot is an extremely sticky paste made from natural gum resins, castor oil and wax, and is labeled for use on pecan trees for pecan weevil trapping. It is normally applied in a three inch wide band around the tree. For easy removal at the end of the season, apply Tanglefoot onto Tangle Guard or another banding material, such as duct tape. Alternatively, Tanglefoot is available on Pre-Coated Sticky Tree Bands. These bands have a peel-off glue strip which allows the band to stick to the pecan tree bark. Tanglefoot bands should be removed at the end of the season or when full of insects. Tanglefoot products can be purchased through most farm supply stores, or ordered from mail-order companies, such as Planet Natural (800-289-6656) or Peaceful Valley Farm & Garden Supply (888-784-1722).

Again, spraying the canopy of large pecan trees by homeowners is impractical. However, an alternative to the use of Tanglefoot is to securely wrap a band of folded burlap around the trunk at a height of eight feet, which is then sprayed to saturation with a carbaryl (Sevin) spray should help control any pecan weevils advancing up the trunk. In addition, spray the trunk below the burlap band to the point of run-off. The burlap band and tree trunk should be rewet with the carbaryl spray every 10 to 14 days corresponding to adult weevil emergence during August and September. If the soil has been hard and dry during the late summer, repeat the spray application on the day after any rainfall of ½ inch or more.

For more information on pecan problems and pecan tree culture, please see HGIC 2211, Pecan Diseases; HGIC 2213, Reasons for Poor Quality Pecans; EIIS/FV-2, Pecan Weevil (http://entweb.clemson.edu/eiis/pdfs/fv2.pdf) and HGIC 1356, Pecan Planting & Fertilization.

Joey Williamson, Ph.D.
Home & Garden Information Center

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.