The influence of physiological age of Argas reflexus larvae (Acari: Argasidae) and of temperature and photoperiod on induction and duration of diapause.

The occurrence of diapause and quiescence was investigated in Argas reflexus engorged larvae, nymphs I and nymphs II. For diapause experiments, larvae were maintained at five different locations: at constant 20°C long day (LD; 17 h light:7 h dark) or short day (SD; 10 h light:14 h dark), at two locations with natural photoperiod and temperature and at one location with natural photoperiod but constant 15°C. At 20°C, diapause incidence was low in physiologically young larvae, increased with larval age, and then decreased to zero in specimens of increased physiological age. This pattern, observed both at constant LD and SD, suggests that the propensity to diapause changes with the physiological age of the unfed larva. The duration of diapause decreased with increasing larval physiological age at all locations, resulting in a seasonally synchronized moulting pattern. The results suggest that A. reflexus larvae are photoperiodically sensitive both before and after feeding and that decreasing daylengths may be particularly strong inductive stimuli. The developmental zero and thermal constant of the larvae were determined as 13.24°C and 220 degree-days, respectively. Degree-day measurements revealed that larval A. reflexus may enter a diapause of different length when fed between August and December and kept at natural daylength. Development of engorged nymphs I and nymphs II, but not of larvae, was ultimatively restricted at a temperature of 37.5°C, but immediately resumed at 25°C, demonstrating the occurrence of quiescence at high temperatures. Similarly, at a low temperature of 15°C, many nymphs I and II did not develop within 58 months, but did so successfully after transfer to 25°C, without additional food intake.