Interactions Between Rice Water Weevil (Lissorhoptrus Oryzophilus Kuschel) And Rice Stem Borers, And Reproductive Properties Of Summer Weevils

The rice water weevil (RWW), Lissorhoptrus oryzophilus Kuschel (Col., Curculionidae) is an alien insect species in China. It has spread to over ten provinces along seashore and inland in this country. The basic population biology of this weevil has been well known to date, but little is understood about their interspecific relations with indigenous insect species, reproductive behaviors, and many other aspects contributing to their successful invasion to Asia. The general objective of this study was to enrich our understanding on the invasive biology of this insect.1. Interactions between RWW and rice stem borersInteractions between RWW and rice stem borers (RSB), which are serious indigenous pests of rice in China, were studied in a rice field in Yueqing, southern Zhejiang in 2004, aiming at determining any plant-mediated interspecific interactions. In this area, the major RSB species include striped stem borer (SSB), Chilo suppressalis (Walker) (Lep., Pyralidae), and pink stem borer (PSB) Sesamia inferens (Walker) (Lep., Noctuidae). Adult feeding and oviposition or underground density of RWW, combined injury levels of SSB and PSB, and growth status of rice plants were surveyed separately at tillering and booting stage, and correlations of these indices were analyzed. At tillering stage, no evident competition was observed between RWW feeding or oviposition and RSB injury. At booting stage, both RSB injury and weevils tended to occur on plants that had more productive tillers and larger root system, suggesting that insect competition would take place at this stage that might be mediated by plants.2. Reproduction of summer RWW females in ChinaOogenesis in summer females (first-generation females) and its relations to temperature were studied. Females were collected from a rice field in southern Zhejiang in early and mid-July 2005, and were reared on young rice plants under 28, 31 and 34±1℃ in the laboratory. Percentage of females having oocytes (oogenesis percentage), number of oocytes of different developmental stages (stage-I, from early previtellogenesis to middle vitellogenesis;stage II, late vitellogenesis;and mature-oocyte stage), and ovariole length were determined every 10 d of feeding. Under each temperature, oogenesis took place in over 40% of females in both groups after feeding for 20 d, but only 0.0-3.3 stage-I, 0.0-0.8 stage-II and 0.0-1.1 mature oocytes on average were observed in the females feeding for 20-40 d. High temperatures (31-34℃) could induce the formation of stage-I oocytes, but had no significant effect on stage-II and mature oocytes. Conclusively, in southern Zhejiang, summer RWW females may become reproductive individuals, but their oogenesis is very low with being little affected by temperature.Reproductive responses of summer females to starvation threat were studied. Three groups of females were collected rice fields in Tanghai, Hebei in mid July, late July and mid-August, and their oogenesis and ovarian length were compared on the collection date, and after 10 and 15 d of starvation. Starvation led to certain changes in oogenesis percentage, number of grade-I and mature oocytes, and ovariole length, indicating that summer females had certain sensitivity to starvation threat.Reproductive responses to starvation were compared between summer RWW weevils collected from rice fields and those collected from aestivating sites in mid-August. Females were starved for 10-15 d or reared with rice for 20-30 d. For females collected in rice fields, starvation/rearing dadsignificant effects on their oogenesis percentage, ovariole length, and number of stage-I oocytes, with ovariole being lengthened and oogenesis promoted. But neither of these indices in female collected from aestivating sites was significantly by starvation. This suggests that summer RWW females in the two habitats might have different response to starvation with those in rice fields being more sensitive.Responses to starvation were also observed in females that had fed on young rice for a period of time and thus supposed to have enough nutrient accumulation. Females being previously fed on rice for 40 d had an oogenesis of nearly 50%, but after three days of starvation oogenesis was observed in all of them, indicating that starvation had induced the production of oocytes in another 50% of females. Moreover, it was also found that 3~5 d of starvation could induce the formation of mature oocytes.SDS-PAGE was used to identify proteins in haemolymph of summer RWW females. Three were three proteins that had comparatively higher concentrations, with their molecular weight estimated as 84, 74 and 22 kD respectively. Each of these three proteins increased as females fed on rice for 1 wk at 31 or 34°C. No protein could be identified that was induced by high temperatures (31-34 °C).Conclusively, the oogenesis in summer RWW females is very low with being little affected by temperature. They have certain reproductive responses to starvation threat, and these responses may different among females in different habitats. For females with good nutrient conditions, reproduction appears to be stimulated by starvation with oogenesis occurring in more females.