**Pecan Weevils** (*Curculio caryae*) are a serious, primary pest of pecan trees (*Carya illinoinensis*) and other true hickories across North America. The adult is a distinctive snout beetle, but the main damage is caused by the larval stage (grub), which bores into and consumes the developing nut kernels. Infested nuts drop prematurely, resulting in substantial or total crop loss in untreated orchards. The weevil’s long life cycle and larval activity inside the nut make control highly challenging.
Taxonomy and Classification
Pecan Weevils belong to the family Curculionidae (snout beetles or weevils) in the order Coleoptera. They undergo complete metamorphosis. Like all weevils, they are characterized by the elongation of the head into a prominent, tube-like snout (rostrum). Pecan Weevils typically take two to three years to complete their life cycle, spending most of that time underground in the soil.
Physical Description
The adult Pecan Weevil is $1/3$ to $1/2$ inch long, not including its extremely long snout.
- **Adult Appearance:** Mottled gray or brownish-gray, with hard wing covers (elytra). The female’s snout is about as long as her body and is used for boring holes into the pecan shell. The male’s snout is shorter.
- **Larvae (Grubs):** Legless, creamy white, C-shaped grubs with a reddish-brown head. They are found entirely inside the pecan kernel.
- **Damage Sign:** Pecans infested early (in the water stage) drop quickly. Later-infested nuts (in the dough stage) are characterized by the **perfectly round exit hole** chewed by the mature grub as it leaves the nut to burrow into the soil.
Distribution and Habitat
Pecan Weevils are found throughout the native and cultivated range of pecans in the U.S. and Mexico. Their habitat is the pecan orchard canopy during their brief adult stage, and the soil beneath the trees for their prolonged larval/pupal stage. They emerge from the soil primarily in late summer, timed with the hardening of the pecan shells.
Behavior and Damage
Adults typically emerge from the soil following summer rains. The female drills a hole through the pecan shell using her snout and deposits several eggs inside the kernel.
- **Kernel Destruction:** The larvae hatch and consume the developing nut kernel, leaving behind only frass and a worthless shell. A single female can destroy dozens of nuts.
- **Nut Drop:** The most visible sign of infestation is the premature drop of infested nuts, which often contain the feeding larvae.
- **Overwintering:** Larvae drop to the ground, bore into the soil, and spend 1–2 years in a pupal cell before emerging as adults, making them a recurring problem for years.
Management and Prevention
Control is critical and must be tightly timed to the brief adult emergence period before egg laying begins.
- **Monitoring (Key):** The most effective control relies on monitoring the **adult emergence** from the soil, often using screen cone traps placed beneath the trees. Treatment must occur immediately after the first adults emerge.
- **Chemical Control:** Control relies on multiple, repeated applications of a residual insecticide (often $3$ to $5$ sprays) to kill the adults as they emerge and before the females lay their eggs.
- **Sanitation:** Collecting and destroying all prematurely dropped nuts, especially during late summer, can reduce the number of larvae entering the soil to pupate.
- **Ground Barriers:** Placing non-porous ground covers (e.g., plastic sheeting) beneath the trees can prevent adults from emerging, though this is difficult for large operations.
Conservation and Research
Pecan Weevils are managed as a top economic threat to the pecan industry. Research focuses on developing effective pheromone traps to predict emergence, improving methods for soil sterilization, and utilizing specialized parasitic nematodes to kill the overwintering grubs in the soil.