The effect of photoperiod and temperature on Piezodorus guildinii (Westwood) adult body morphometry, color, lipid content, development of reproductive organs, and feeding activity at different ages was studied in the laboratory. Three different conditions were tested, each at 80 ± 10% RH: 14:10 h L:D at 25 °C, 10:14 h L:D at 25 °C, and 10:14 h L:D at 20 °C. Adults at 25 °C under 14 h photophase were larger and exhibited lower lipid content than those held at 20 °C under 10 h photophase. Highest percentages of females with clear pronotum band and connexivum were recorded at 10 h photophase regardless of the temperature. Development of the ovary was affected by both photoperiod and temperature. At 15, 30 and 45 days of age under 14 h photophase at 25 °C, the percentage of females with immature ovaries remained constant at 0%; in contrast, at 10 h (20 °C) this percentage declined from 60 to 33%. Testes size was affected by photoperiod and temperature; males at 14 h (25 °C) and at 10 h (20 °C) showed the largest and the smallest testes, respectively. The ectodermal sac condition was affected starting with males 15 days of age; the percentage of males with an expanded ectodermal sac was greater under the 14 h (25 °C) treatment than under the 10 h (20 °C) treatment. Feeding activity of adults under short photophase conditions (10 h) decreased. These laboratory results indicate that adults of P. guildinii maintained under a short (10 h) photophase at 20 °C enter into reproductive diapause, accumulate energetic reserves (lipids), show undeveloped reproductive structures and smaller size, clear coloration of the pronotum band and connexivum, and less feeding activity.