The control of reproductive diapause in Nebria salina (Coleoptera: Carabidae)
1. Adult male and female Nebria salina were collected in summer, when in reproductive diapause, and subjected to three different photoperiod regimes, LD 18:6 h, LD 12:12 h, and LD 6:18 h, to assess the role of light regimes on sexual development.
2. Two alternative hypotheses were tested: (i) development rates increase in response to time restriction and (ii) development rate is at a maximum on the day length approximating to that in the field during reproduction.
3. Gonad development was assessed after 1 and 2 months' exposure to the experimental day lengths. In female N. salina, photoperiod and length of time of exposure both influenced reproductive development. Females required at least 2 months' exposure to photoperiods of ≤ LD 12:12 h to mature. Development of the ovary did not take place on LD 18:6 h, irrespective of the length of exposure. Sexual development occurred in all the males after 2 months, irrespective of photoperiod.
4. At the end of the study the activity of the remaining beetles was measured and compared. Significantly higher activity levels were measured in periods of darkness, and consequently beetles exposed to the two shorter day lengths were found to be more active than those kept on the longest photoperiod. Comparing the 3-h period 06.00 – 09.00, the last 3 h of darkness for all three regimes, activity was significantly higher on LD 12:12 h and LD 6:18 h than on the LD 18:6 h regime.
5. In order to place the experimental studies in context with the life cycle under natural conditions, pitfall traps were used to compare the seasonal activity of N. salina in the field.