Diapause induction and termination in the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper, Laodelphax striatellus, enters photoperiodic induction of diapause as 3rd or 4th instar nymphs. Photoperiodic response curves in this planthopper showed a typical long-day response type with a critical daylength of approximately 11 h at 25 °C, 12 h at 22 and 20 °C and 12.5 h at 18 °C. However, the high temperature of 28 °C nearly nullified the diapause-inducing effects of short daylengths; more than 90% individuals developed without diapause under short daylengths. The third instar was the stage most sensitive to the photoperiod. The photoperiodic response curve at 20 °C showed gradual decline in diapause incidence in ultra-long nights, and DD resulted in 100% development. The required day number for a 50% response was distinctly different between short- and long-night cycles, showing that effect of one short night was equivalent to the effect of three long nights at 18°C. By transferring diapausing nymphs induced under LD 11:13, LD 12:12 and LD 13:11 at 18, 20 and 22°C to 25°C combined with LD 15:9 to terminate diapause the rearing day length of 12 h evoked weaker intensity of diapause than did 10 and 11h. By transferring diapausing nymphs to a long photoperiod of LD 15:9 and a short photoperiod of LD 11:13 at 18 °C, the duration of diapause was significantly longer under the short daylength of 11 h than under the long daylenth of 15 h. By transferring diapausing nymphs to eight different temperatures of 18, 20, 22, 24, 26, 28, 30 and 32°C, it showed that the optimal temperature for diapause termination was 26 and 28 °C. Chilling at 5°C for different times did not shorten the duration of diapause but significantly lengthened it when chilling period was included. In autumn, 50% nymphs that hatched during late September-mid-October entered diapause in response to low temperatures below 20 °C. The critical daylength in the field was between 12 h 10 min and 12 h 32 min (including twilight), being nearly identical to the critical daylength of 12.5 h at 18°C. In spring, overwintering nymphs began to emerge during early March-late March when the mean daily temperature rose to 10°C or higher, suggesting that early or late termination of winter diapause mainly depends on the temperatures in March.