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|Adult pecan weevil on a mature nut
University of Missouri
The adult pecan weevil typically emerges from the soil as early as July 25, frequently two to three days after a heavy rain. Adults cause two types of nut damage, depending on the stage of nut development during attack. Adults feeding on nuts before the gel stage (i.e., in the water stage, usually before shell hardening) induce kernel shriveling and blackening and premature nut drop. You sometimes can recognize nuts damaged in this way by a tiny, dark puncture that extends through the shuck and unhardened shell and a tobacco-like stain around the feeding wound. The presence of a larva in the nut, prior to shell hardening, indicates damage by another insect, usually nut casebearer or hickory shuckworm. Pecan weevil grubs are not found in nuts with unhardened shells.
The second type of nut damage is caused by weevil grubs feeding in partially matured nuts. Females oviposit two to four eggs in separate pockets within each kernel, after the nuts have entered the gel stage (about mid-August) until shuck split. Damaged mature nuts neither bleed nor drop. Pecan weevil grubs feed on the kernels for approximately 30 days and then exit through a one-eighth of an inch emergence hole beginning in late September.
The pecan weevil remains in the larval stage for one to two years in earthen cells 4 to 12 inches underground. They pupate in early autumn and metamorphose into adults in about three weeks. These adults remain in the soil until the following August. The complete life cycle requires two to three years.
Adults are light-brown to gray and about one-half inch long (see Figure 3). The beak of the male is half the length of the body, and the beak of the female is slightly longer than the body. Larvae have no legs or prolegs and are creamy-white, C-shaped grubs with reddish-brown heads measuring up to one-half inch long.
The pecan weevil is considered to be the most serious late-season pecan pest. Pecan varieties differ widely in their susceptibility to attack. Early ripening varieties that enter the gel stage in early August are most commonly infested. Ordinarily, weevils do not move far from the tree under which they emerge from the soil (provided there is a crop of nuts on that tree). Consequently, certain trees may be infested year after year while other adjacent trees of the same variety may not be attacked. You can spray pecan trees that have a history of pecan weevil damage with an insecticide at gel stage and then spray again 10 to 14 days later.
Cone-shaped emergence traps are the best way to detect first emergence of pecan weevil adults. Place the pecan weevil traps (four per tree, near the drip line) under suspected "weevil trees" by July 25. The economic threshold is five pecan weevil per trap when the nuts have reached the gel stage.
George S. Smith and Maureen H. O'Day
Department of Entomology, University of Missouri-Columbia
Kansas State University