The pecan weevil, Curculio caryae, is the most significant insect pest for pecan production across North America. Its life cycle is adapted to the pecan harvest, requiring precise timing and a multi-faceted approach for control. Protecting the yield depends on understanding this pest’s behavior and applying targeted interventions. The most effective management strategies combine diligent non-chemical practices with well-timed applications of insecticides.
Identifying Pecan Weevils and Their Life Cycle
The adult pecan weevil is a reddish-brown to gray beetle, approximately one-half inch in length, distinguished by its long, slender snout. Female weevils use their snout, which is slightly longer than their body, to bore into the developing nut to lay eggs. Early-season feeding punctures by adults cause immature nuts to drop prematurely from the tree, often around early August.
The most common sign of infestation is the presence of creamy-white, legless grubs, or larvae, found inside harvested nuts. The larvae chew a circular exit hole, about one-eighth of an inch in diameter, through the shell after feeding on the kernel.
Adult weevils emerge from the soil over a prolonged period, typically from early August through September, with emergence often triggered by significant rainfall. After emerging, females climb or fly into the tree canopy, waiting for the pecan kernel to reach the soft dough or gel stage before laying eggs.
Once the larvae fully develop within the nut, they chew their way out and drop to the ground, usually during October and November. They burrow into the soil, constructing earthen cells where they remain dormant. The majority of the population spends one to two years underground, often at depths of four to twelve inches, before pupating and emerging as adults.
Non-Chemical Strategies for Control
Promptly collecting and destroying pecans that drop prematurely in the late summer is a key sanitation practice. This intercepts developing larvae before they can exit the nut and burrow into the soil.
Physical barriers placed around the tree trunk can effectively trap adults that crawl up from the soil after emerging. Applying a sticky material to a band wrapped around the trunk prevents climbing weevils from reaching the canopy. While this method does not stop flying weevils, it significantly reduces the population moving from the soil to the nuts.
Specialized devices, such as pyramid or circle traps, can be used to monitor and capture emerging adults. The traps are placed on the ground under the canopy of infested trees, collecting weevils as they emerge. Data from these traps helps determine the exact timing of adult emergence for scheduling insecticide applications.
For small-scale removal, physically shaking or jarring the tree trunk causes adult weevils to drop from the canopy. These dislodged adults must be collected immediately from a ground cloth or tarp laid beneath the tree and destroyed. This manual method is most practical for a few small trees and must be repeated regularly during the emergence period.
Targeted Insecticide Application
For established infestations, chemical control is necessary and must be precisely timed to target the adult weevils before they lay eggs. The application window begins when the earliest maturing nuts enter the susceptible gel or dough stage, typically around mid-August. Applying treatment before this stage is ineffective, as the nuts are not yet suitable for egg-laying.
The primary objective of the insecticide application is to coat the nuts and foliage to eliminate the adult females before they bore into the nut to deposit their eggs. Since an adult female can survive for several weeks and does not begin laying eggs until approximately five days after emerging, a well-timed treatment can prevent an entire generation of larvae from developing.
Because adult weevils emerge from the soil over a period that can span two months or more, a single treatment is rarely sufficient for control. Repeat applications are usually required at intervals of seven to ten days, depending on the product label and continued weevil emergence activity. Monitoring with traps or observing new signs of emergence is important to determine if additional treatments are needed through mid-September.
Environmental factors, particularly soil moisture, greatly affect the emergence schedule, with drought conditions delaying adult movement until the soil is softened by rain or irrigation. This variable emergence pattern necessitates careful monitoring and flexible treatment schedules, sometimes requiring three or four applications in a season. Consult a local agricultural extension office for guidance on approved insecticide formulations and proper application rates.