The critical photoperiod for diapause induction in the predator Geocoris uliginosus (Say) and the stage/instar(s) sensitive to diapause-inducing photoperiods were determined in the laboratory. Six photoperiods were used for the critical photoperiod experiment (all at 24 ± 1°C): 10:14, 11:13, 12:12, 13:11, 14:10, and 15:9 (L:D) h. The critical photoperiod was ≈13.2 h of light d−1. At 12:12 and 11:13 h, 86–88% of the population entered diapause, whereas 20% or less entered diapause at longer daylength. To determine the stage/instar(s) sensitive to diapause-inducing photoperiods, cohorts of bugs were reciprocally switched between two photoperiods, 15:9 (not diapause-inducing) and 11:13 h (diapause-inducing), at the beginning of the second, third, fourth, and fifth instar, and at the beginning of the adult stage. A given cohort was switched only once during the experiment. Two additional cohorts were reared as controls under the respective photoperiods with no switching throughout the experiment. Exposure to diapause-inducing photoperiods from the beginning of the fifth instar through the adult stage induced reproductive diapause in 100% of the females in this experiment, whereas exposure from the beginning of the adult stage induced only 42% diapause. Thus, when daylength shortens to the critical photoperiod in late summer, most individuals that are in the nymphal stage will subsequently enter reproductive diapause, whereas a majority of those that have reached the adult stage by that date will reproduce.
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Vol. 99 • No. 2