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Verifying Pecan Weevil Damaged Pecans Through Microscopic Examination


Pecan weevil larval exit holes. (Photo by Pat Porter, Texas A&M AgriLife Extension)

Pecan weevil is one of the most important nut-infesting insects of pecan and as such this insect is not only a producer issue but also a state and industry problem. Making positive identifications of an infestation can be important for making management decisions, purchasing property for future orchards, determining new infestations/new county records, and following quarantine regulations. The most obvious sign of a pecan weevil infestation will be the exit holes created by emerging pecan weevil grubs in the pecan shell. However, holes in a pecan shell can also be created by birds, rodents, other insects and by mechanical means. With the aid of a compound dissecting scope or hand lens you can determine if a pecan was damaged by pecan weevil.

Pecan weevil grubs and a damaged pecan.

Pecan weevil grubs and a damaged pecan. (Photo by Bill Ree)

1. Pecan weevil infested nuts typically contain three or four larvae with all larvae becoming ready to exit the nut at the same time. It is thought that each larva begins to cut an exit hole in the shell. Typically, the one that breaks through first completes it and other larvae exit through it too; occasionally a nut will contain two pecan weevil exit holes.

Bird peck damage to pecan shells

Bird peck damage to pecan shells. (Photo courtesy of Bill Ree)

2. Bird pecks and rodent gnawing can scar or damage the exterior nut surface but seldom leave a distinct circular hole the size of the PW head capsule. With this type of damage, there will be kernel (or nut meat) still present in the pecan.

Pecan weevil larval exit hole

Pecan weevil larval exit hole. (Photo by Bill Ree)

3. This is an exterior view of a pecan weevil larval exit hole. The circular hole size corresponds to the round, hard head capsule of the larva which presses through the hole and is followed by the larger fleshy body.

The circular, head capsule size hole with mandibular scars and beveled edge around the exterior edge of the shell are considered definitive evidence of pecan weevil nut infestation.

hole in pecan shell created by unknown source

This hole in a pecan shell was created by an unknown source. (Photo by Bill Ree)

4. This exterior view of a hole in a pecan shell, although almost circular it is irregular in shape with sharp, defined edges, not characteristic of pecan weevil larval emergence.

A view from inside a pecan looking at a pecan weevil exit hole.

A view from inside a pecan looking at a pecan weevil exit hole. (Photo by Bill Ree)

5. This is an interior view of the pecan weevil larval exit hole. Note the round beveled appearance. The small mandibles of the larvae cut the exit hole through the shell which leaves scars on the inside surface around the exit hole.

The view from inside a pecan looking at a hole that was not created by pecan weevil.

The view from inside a pecan looking at a hole that was not created by pecan weevil. (Photo by Bill Ree)

6. This is an interior view of a hole created in a pecan shell by an unknown source. Note the irregular shape and the sharp edges of the hole.

For additional information on pecan weevil, visit the Pecan ipmPIPE website.

Author Photo

Bill Ree

Bill Ree is an Extension Program Specialist – IPM (Pecan) at Texas A&M AgriLife Extension. bill.reejr@ag.tamu.edu