Effects of photoperiod and temperature on diapause induction of Lobesia botrana

【Objective】Many insects utilize diapause to cope with various environmental stresses, and photoperiod and temperature play significant roles in diapause induction for numerous insect species. Lobesia botrana (Denis & Schiffermüller) has a serious potential threat to the grape industry in Turpan region. The study aimed to refine the understanding of the effects of different photoperiod conditions on the growth and development of L. botrana, and to further investigate the significant differences in its photoperiodic responsiveness, as well as the critical photoperiod and sensitive life stages for its diapause pupae so as to identify the patterns linking outbreaks and disasters of L. botrana in its third and fourth generations to the photoperiodic and temperature variations in the Turpan region.【Methods】 During the larval outbreak period of L. botrana, grapes infested with larvae were collected from multiple vineyard areas affected by the pest in Turpan City. The grapes were brought back to the laboratory, and the larvae were separated from the fruit. The larvae were reared until they pupated and subsequently eclosed (emerged as adults). These adults were then mated and allowed to lay eggs, with the resulting offspring being used as the test insect source for subsequent generations. The larvae were placed in incu-bators to observe and record their developmental duration and survival rate. The eggs, larvae, pupae, and adults of L. botrana were reared under five different photoperiod gradients (16L∶8D, 14L∶10D, 12L∶12D, 10L∶14D, and 8L∶16D), maintained at a temperature (Tm) of 28±0.5 ℃, relative humidity (RH) of (45±5)%, and light intensity of 800-1200 lx, to observe their growth, development, and diapause under these varying photoperiod conditions. The diapause individuals were identified based on external morphological characteristics, specifically the color, position, and size of the compound eye, after pupation. The individuals that had not eclosed after 15 days were classified as diapause pupae. The weight, length, and width of the pupae, as well as their diapause status, were observed and recorded under different photoperiod conditions. The diapause rate was then statistically calculated.【Results】Under the conditions of Tm (28±0.5) ℃ and RH (45±5)%, different photoperiod treatments had a significant impact on the developmental duration and survival rate of L. botrana eggs, pupae, and adults (P＜ 0.05). Specifically, the 8L∶16D and 16L∶8D photoperiods resulted in significant differences (P＜0.05) in egg duration, with short-day conditions (16L∶8D) leading to a shorter egg duration. However, these photoperiods had no significant effect on larval duration (P＞0.05). The 8L∶16D and 12L∶12D photoperiods caused a significant difference (P＜0.05) in adult lifespan, with short-day conditions (8L∶16D and 12L∶12D) leading to a longer adult lifespan. The lowest larval survival rate was observed under the 10L∶ 14D photoperiod, while the lowest pupation rate and adult emergence rate occurred under the 12L∶12D photoperiod. The highest average egg production per female was recorded under the 8L∶16D photoperiod; however, no significant difference (P＞0.05) was found in egg diameter or egg hatching rate across different photoperiod treatments. The pupa was the diapause stage of L. botrana. Diapause pupae were significantly larger in size compared with non-diapause pupae. The female diapause pupae induced under the 14L∶10D photoperiod were the largest in size, and under the same photoperiod, the female diapause pupae were larger than the male diapause pupae. Different photoperiods also influenced the morphology of the non-diapause pupae: the male pupae reared under the 8L∶16D photoperiod had the longest average pupal length; the pupae reared under the 16L∶8D photoperiod were the widest; and the male pupae induced under the 8L∶16D photoperiod were the heaviest, with significant differences in pupal weight observed between treatments (P＜0.05).Under conditions of 25 ℃ or 28 ℃ and RH (45± 5)% , the 12L∶12D photoperiod resulted in the highest pupal diapause rate, reaching over 80% . At 28 ℃, a photoperiod of 12-14 hours induced diapause in over 50% of pupae, while at 25 ℃, diapause was induced in over 50% of pupae within a range of 8-14 h. These results indicated that L. botrana exhibited facultative diapause. The critical day lengths for pupal diapause were 14.17 hours at 25 ℃ and 14.03 hours at 28 ℃. The L. botrana's response to photoperiod showed that increasing photoperiod duration decreased diapause incidence. The short-day conditions had a clear cumulative effect on inducing diapause in L. botrana. Two most sensitive periods were identified: the initial instar larval stage and the late stage of the mature larva.【Conclusion】The developmental duration and survival rate of the L. botrana eggs, pupae, and adults were all influenced by photoperiod. Under short- day conditions, the egg duration was the shortest, the adult lifespan was the longest, and the egg production was the highest. The short-day conditions also significantly increased the egg production rate per female. However, different photoperiods did not have a significant effect on the egg diameter or egg hatching rate. The pupation diapause could be induced under conditions of 25 ℃ temperature and photoperiods of 8-14 hours, indicating that L. botrana was a facultative diapause insect. Both photoperiod and temperature were important factors in diapause induction; the long-day conditions significantly would reduce the diapause rate. Among these, photoperiod might play the dominant role, while temperature might act in conjunc-tion with photoperiod. These results also could explain why the first and second generations of L. botrana typically had lower population numbers. The reason would be that some pupae would enter diapause under short- day conditions. As day length increased and temperatures rised, diapause would break around the third and fourth generations, leading to an outbreak and disaster of adults.