The biology and ecology of rice-stem borers in Sarawak (Malaysian Borneo).
The following is based largely on the author's summary. Observations were made in 1964-68 on the biology and ecology of rice stem borers in Sarawak (East Malaysia) [cf. RAE A 56 305], where the principal species in the wet rice area were Tryporyza incertulas[Scirpophaga incertulas] (Wlk.), Chilo suppressalis (Wlk.) and Sesamia inferens (Wlk.), while T. innotata (Wlk.) and C. auricilius Dudgn. were commoner in dry plantings. Data on the head widths of the larvae in different instars and the duration of the immature stages are given. The mean duration of development was 59, 61, 49 and 61 days for T. incertulas, T. innotata, C. suppressalis and S. inferens, respectively, at 24-29°C and 72-98% R.H. Observations were made on certain aspects of their reproductive biology. The mean fecundity of T. incertulas, T. innotata, C. suppressalis, C. auricilius and 5. inferens was estimated at 143, 142, 212, 285 and 253 eggs per female, respectively, and these figures represented 60-90% of the total number of eggs in the ovaries. The figures for fecundity and generation length (the sum of the development and preoviposition periods and half the oviposition period) were used to calculate the potential rates of increase, and populations of C. suppressalis were shown to have the greatest growth potential. In all species, fecundity was positively correlated with pupal or adult size, and these characters were used to predict fecundity in the field. The size of the borer populations was estimated over five seasons in one area. Egg-masses were extremely scarce, and this was attributed to predation by two Tettigpniids of the genus Conocephalus, particularly C. longipennis (Haan), and three Gryllids of the genus Anaxipha, which was confirmed by precipitin tests. Larval numbers varied from one season to another, but T. incertulas was generally the dominant species in the main crop of wet rice, while Chilo suppressalis predominated in late plantings. The mean maximum numbers of T. incertulas, C. suppressalis and S. inferens for five seasons were 134, 171 and 45 larvae/200 plants, respectively. Mortality during the development period exceeded 98% in most seasons. This was attributed to the action of predators and egg parasites, including Telenomus rowani (Gah.), T. dignus (Gah.), Tetrastichus schoenobii Ferrière and Trichogramma sp. [cf. 59 3346]. Larval parasitism rarely exceeded 2%; the parasites included Bracan chinensis Szépl., Apanteles flavipes[Cotesia flavipes] (Cam.) and Rhaconotus schoenobivorus Rohw. Pupal parasitism was somewhat higher, reaching 37% in C. suppressalis; the parasites recorded were Xanthopimpla stemmator (Thnb.) and Tetrastichus israeli (Mani & Kurian). Information was obtained on the spatial distribution of the larvae in the field and on seasonal infestation patterns. The distribution of the larvae of all species was non-random and approximated to the negative binomial series. Clustering was greatest in C. suppressalis and was attributed to lack of dispersal from the hatching sites. Infestation of the rice crop was usually light prior to the flowering phase. Light-traps were used to determine moth abundance, but the data obtained bore little relation to population trends in the crop, as both the species and the sexes were unequally attracted. During the off-season, there was evidence of diapause and quiescence in mature larvae of Tryporyza innotata[Scirpophaga innotata] and T. incertulas, respectively, but small breeding populations of these species, as well as of C. suppressalis and S. inferens, were present on self-sown and ratoon rice plants.