Detailed knowledge of boll weevil, Anthonomus grandis Boheman, chemical ecology on cotton, Gossypium hirsutum L., is needed to improve pheromone trapping methods. Recent studies using headspace collections have indicated that most pheromone is not in the feces as previously assumed. We used headspace collections to examine the diel patterns of boll weevil pheromone production and to determine whether the timing of food replacement influenced those patterns. Pheromone production of individual 9- and 11-d-old weevils was estimated during 4 daily periods (0730–1130, 1130–1530, 1530–1930, and 1930–0730 h) under temperatures of 27 ± 2°C and with a 13:11 (L:D) h photoperiod that began at 0700 h. Weevils fed daily at 0730 h produced more pheromone (2.83 ± 0.25 μg h−1) than weevils fed at 1530 h (1.95 ± 0.25 μg h−1), and 11-d-old weevils produced more pheromone (2.62 ± 0.22 μg h−1) than 9-d-old weevils (2.16 ± 0.23 μg h−1). Furthermore, weevils fed at 0730 h exhibited a clear diel pattern of pheromone production with maximal and minimal production occurring between 1130–1530 h and 1930–0730 h, respectively. Weevils fed at 1530 h exhibited a similar periodicity, but the pattern was less distinct. Pheromone composition of weevils fed at 0730 h was relatively consistent among daily periods and the ratio of the 4 components was approximately 45:42:3:10 (I:II:III:IV). However, pheromone composition varied significantly among daily periods for weevils fed at 1530 h. Our results demonstrate a diel periodicity in pheromone production by the boll weevil, and illustrate the need for careful control of weevil diets in such studies.

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