Curculio caryae
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Description
The adult is a reddish-brown weevil that appears light brown or gray due to the pale-colored scales on the body. Adults are about 2 inch (12 mm) long and have an extremely long beak. The female’s beak is as long or longer than the body. The male’s beak is about 2/3 as long as the body. Immature larvae are white. Mature larvae are fat, yellowish-white grubs, 3/8 - 2 inch (9-12 mm) long, with small reddish-brown heads.
Damage
Damage may be of two types, depending on the stage of nut development. Prior to nut hardening (water stage), adults puncture nuts with their snout and feed through the shuck and unhardened shell. This injury causes the nuts to shrivel, turn black inside, and drop. Damaged nuts can sometimes be recognized by a tiny dark puncture that extends through the shuck and unhardened shell. However, similar malformations are caused by other insects and mechanical injury. Prior to shell hardening, the presence of a larva in the nut indicates damage by some other insect, usually nut curculio or hickory shuckworm. Pecan weevil grubs are not found in nuts with unhardened shells. Early feeding damage by adult weevils can be serious if the nut crop is light. After pecan shells harden, female weevils drill holes through the shucks and hardened shells and place an average of about 3 eggs each in separate pockets within the kernels. Grubs hatch and feed on the kernels, destroying the interior of the nut. Infested nuts may be recognized by emergence holes about 1/8 inch (3 mm) in diameter through which mature grubs escaped (if there are not grubs in the nuts), or by the adherence of shuck to the shell.
Early (water stage) damage often passes unnoticed but the second type of damage is generally noticeable at harvest time and may affect most of the crop in seasons where large numbers of weevils are present.
Late maturing varieties (Success, Tache, Mobile, Van Deman) sometimes are not attacked if early maturing varieties (Stuart, Schley, Mahan, Moneymaker) contain enough nuts to supply weevils with adequate food and egg laying sites. Trees growing in low areas and near hickory trees are more likely to be infested. Certain trees may be infested year after year, while nearby trees, even of the same variety, may be almost weevil free. Trees with history of infestation should be checked when sampling for weevils. Weevil emergence is dependent upon soil moisture and is usually higher following rains.
Seasonal History
Adult pecan weevils emerge from the soil from July to October with most emergence from August 1 through September 20. Egg-laying begins about the last week of August (or when kernels gel). Between late September and late December, or sometimes later, the grub cuts the 1/8 inch (3 mm) diameter hole through the shell, drops to the soil, and enters the soil to a depth of 4-12 inches, depending on soil type and type of orchard floor. They construct earthen cells where they remain for 1 - 2 years. They pupate between September and October and become adults after only about 3 weeks as pupae. These adults remain in the soil until the following summer. The complete life cycle requires 2 - 3 years.
When to Control
Unfortunately, we cannot yet define the precise number of pecan weevils that must be present (as detected by traps, etc.) in order to justify control. Applications must be started when weevil emergence is heavy or when unacceptable damage is evident. Sample weevil emergence in each orchard. Masonite board traps (placed under trees with whitewashed trunks), cone cage "Malaise"-type traps placed in the crotches of trees or cone cage traps placed on the ground under trees are best. Trunk traps (sticky band or burlap bag band) can also be used to tell when weevils are emerging. Knock-down sprays are also an effective method to determine when weevils are present in trees. A combination of sampling methods is advisable (see section on checking for pecan weevils).
Prior to shell hardening, treat if high numbers of weevils are seen or if nut drop caused by weevil feeding is considered excessive. After shell hardening, treat when weevils begin to emerge and continue sprays at 7 - 10 day intervals, especially following rains, until weevils are no longer found.
See also
- Archived article Curculio caryae.
