Pestipedia

Pecan Nut Casebearers

**Pecan Nut Casebearers** (*Acrobasis nuxvorella*) are the single most devastating insect pest of pecan production in North America. The damage is caused by the larval stage (caterpillar), which bores into and destroys the developing pecan nutlets. Early-season damage can cause the entire crop to drop, leading to $50$ to $100$\% loss if untreated. The pest’s habit of tunneling into the nut makes chemical control challenging, requiring precise timing for effectiveness.

Taxonomy and Classification

Pecan Nut Casebearers belong to the family Pyralidae (snout moths) in the order Lepidoptera. They undergo complete metamorphosis. This moth is a specialist feeder, targeting only pecan and hickory species. There are typically three or four overlapping generations per year, with the first generation being the most destructive to the developing crop.

Physical Description

The adult moth is tiny, dark, and nondescript, with a wingspan of about $2/3$ inch.

  • **Adult Moth:** Grayish-black with a prominent, transverse ridge of dark scales on the forewings. They are nocturnal and are rarely seen unless captured in a trap.
  • **Larvae (Caterpillars):** Olive-green to dark brown, about $1/2$ inch long when mature, and typically found inside the developing nut.
  • **Overwintering Site:** Larvae overwinter in small, tightly woven, protective silk shelters (**hibernacula**) at the base of terminal buds on pecan twigs.
  • **Damage Sign:**
    • **Webbing:** Frass and silk webbing near the base of the nut cluster.
    • **Nut Drop:** The most characteristic sign is the complete drop of entire nut clusters (nutlets), often with a tiny bore hole at the base of the remaining stem.

Distribution and Habitat

Pecan Nut Casebearers are found throughout the native and cultivated range of pecans in the U.S. Their habitat is the pecan tree canopy. The pest life cycle is intrinsically linked to the development stages of the pecan nut, with the most severe damage occurring shortly after pollination.

Behavior and Damage

The first generation larvae emerge in spring and first bore into the terminal shoots, but quickly migrate to the newly set nutlets.

  • **First Generation Destruction:** The larva bores into the base of a pecan nutlet, consuming the contents. A single larva can destroy an entire cluster of nutlets (often $2$ to $5$ nuts), causing immediate and massive crop failure. This is the only generation that justifies chemical treatment.
  • **Later Generations:** Subsequent generations attack larger, developing nuts later in the summer, where they cause less total loss but can still damage the shucks and kernels.
  • **Nesting:** The larva cements frass and silk around the entry point, creating a protected case, making it hard to treat once inside.

Management and Prevention

Control is highly focused on preventing the devastating first generation from entering the nutlets.

  • **Pheromone Trapping (Key):**
    • Pheromone traps are used to monitor the flight of the adult males of the first generation. This data is used in conjunction with **degree-day models** to predict the exact time of the first larval hatch.
  • **Timing of Treatment:** The insecticide application must be precisely timed to target the larvae **after** they hatch from the egg but **before** they bore into the nutlet clusters. This window is often only a few days wide and is the single most critical treatment decision in pecan production.
  • **Dormant Pruning:** Clipping and destroying terminal shoots containing the overwintering hibernacula during the dormant season provides minimal, but sometimes useful, control in small orchards.
  • **Chemical Control:** Control relies on highly effective, fast-acting insecticides delivered by air or ground sprayers to the nut clusters.

    • Conservation and Research

    Pecan Nut Casebearers are managed as the most important insect pest in the industry. Research focuses on refining the precision of pheromone and degree-day models, evaluating new biological control agents (like parasitic wasps), and developing nut varieties that are naturally resistant to the borer.