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United States Department of Agriculture

Agricultural Research Service

Title: Head Capsule Widths of Larval Instars of the Boll Weevil

Authors
item Reardon, Brendon
item Suh, Charles
item Spurgeon, Dale

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 26, 2002
Publication Date: N/A

Interpretive Summary: The boll weevil is a very devastating insect pest of cotton. Boll weevil larvae are legless grubs that pass through three distinct phases (instars) during development. Studies of this pest often require the ability to accurately distinguish between these three stages, and techniques based on head width have been used. However, our observations of field-collected boll weevil larvae indicated a greater variation in the widths of the head than was previously reported. Therefore, we examined the head widths of field-collected larvae to develop more easily applied rules for distinguishing the different stages. Average head widths of first, second, and third stage larvae were 0.4, 0.6, and 0.9 mm respectively. Ranges in head width were from 0.36 to 0.44 mm for first stage larvae, 0.56 to 0.72 mm for second stage larvae, and 0.76 to 1.08 mm for third stage larvae. Because these ranges more accurately reflect the variation in head widths within a given stage, but do not overlap among the three stages, they represent an improvement in accuracy and ease of application over previously published rules. These findings will aid boll weevil researchers in their efforts to better understand how boll weevils develop, and will ultimately facilitate work to more effectively manage this destructive pest.

Technical Abstract: Studies of the ecology or population dynamics of the boll weevil, Anthonomus grandis grandis Boheman, often require the ability to accurately identify the larval instars. During ecological studies of boll weevil larvae, we observed that variation in head capsule width within instars was too great for confident application of previously published rules for distinguishing instars. Therefore, we examined head capsules for field-collected larvae to establish more robust guidelines for identification of larval instars. The mean (+/- SD) head capsule width for first, second, and third instars was 0.4 +/- 0.019, 0.6 +/- 0.049, and 0.9 +/- 0.077 mm respectively. Ranges in head capsule width were from 0.36 to 0.44 mm for the first instars, 0.56 to 0.72 mm for second instars, and 0.76 to 1.08 mm for third instars. Overlap of the ranges of head capsule widths among the instars was not observed. These observations indicate greater within-instar variation in boll weevil head capsule widths than was previously documented, and provide a more accurate and robust guide for distinguishing among the instars of field-collected larvae.

Last Modified: 9/22/2014
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