Bionomics of overwintering generation of Chilo suppressalis (Walker) in Hunan Province

Chilo suppressalis (Walker) is a serious and destructive rice pest in Hunan Province, China. The diapause larvae of C. suppressalis can successfully overwinter in Hunan Province and the number of overwintering larvae corresponds to the amount of destruction caused in the following year. To properly manage C. suppressalis, it is crucial to survey the number of overwintering larvae and overwintering host plants. Populations of overwintering larvae and overwintering host plants were observed in field surveys from December 2012 to March 2013. Surveys were conducted on grasses, such as Oryza L. (rice) and Zizania latifolia (Griseb.) Turcz. ex Stapf, as well as on weeds and shrubs growing in a rice cultivation area in 14 prefecture-level city in Hunan Province. C. suppressalis larvae were collected from different host plants, including Oryza, Z. latifolia, Zea mays L., Xanthium sibiricum Patr, Miscanthus floridulus (Labnll.) Warb and Coix lacryma-jobi L. These collections were made at different latitudes in Yueyang, Xiangtan, Hengyang and Chenzhou prefecture-level cities in Hunan Province. The pupae were weighed, assessed for differences in overwintering adult fecundity and second generation hatching rate. We found that during this year there were more overwintering larvae in Loudi, Xiangtan and Changsha than other sites in Hunan Province. The largest populations were 15,366 and 11,230 (per 667 m2) overwintering larvae, recorded in Xiangtan and Yueyang, respectively. The larvae's December survival rate ranged between 75.38% and 93.87%, and decreased slightly over the next few months, which fluctuated between 70.26 and 89.27. Eleven different plant species in Hunan Province, such as rice, Z. latifolia, and field ridges were chosen by larvae as hibernaculum. This study is the first record of X. sibiricum, M. floridulus, Nelumbo nucifera Gaertn., C. lacryma-jobi, Ambrosia artemisiifolia Linn. and Bidens pilosa Linn. as hibernaculum for C. suppressalis in Hunan Province. The developmental progression of larvae was highly synchronous in that most larvae were at the 4th instar stage in December and January, while half of the larvae were in the 5th stage in February and 60% of the larvae reached the 6th stage in March. There were no significant differences in pupal weight among the 14 prefecture-level cities surveyed in Hunan Province, but there were temporal differences in larval development. In southern Hunan Province (Hengyang, Youzhou and Chenzhou) the 50% pupal (April 6th–12th), adult emergence (April 17th–May 24th) and peak hatching (May 1st–5th) stages occurred 10 days earlier than they did in populations in the northwestward sites (Jishou and Zhangjiajie). However, significant differences in C. suppressalis pupal weight were observed when they were contrasted according to their hibernacula. Average pupal weights were highest on Zizania caduciflora (64.21 mg) and Oryza (51.52 mg). However, neither the hibernacula nor the latitude corresponded to significant differences in overwintering adult fecundity and second generation hatching rates. Our results indicate that many C. suppressalis choose rice stubble as a hibernaculum and treatment of rice stubble on a large scale would be a useful component to the integrated control of this pest. Besides rice stubble, the diversity of overwintering host plants may serve as trap crops to control populations of C. suppressalis in rice fields and there should be further research in this area. In the meantime, our results suggest that prefecture-level city plant protection station staff should invest in surveys of overwintering C. suppressalis larvae population sizes and overwintering host plants as reasonably accurate forecasts of C. suppressalis populations for the coming year. These surveys can act as a basis for planning local cropping systems and selecting the proper varieties of rice to plant.