Seasonal Fluctuations in Abundance of Various Rice Stem Borers on Luzon Island, Philippines12

Abstract

Monthly samples of rice stem borers were taken from the 1965-66 wet- and dry-season rice crops from 5 major rice-growing regions on Luzon Island, Philippines. The samples showed that, of the 5 species of lepidopterous rice stem borers previously recorded from the island, Tryporyza incertulas (Walker) and the Asiatic rice borer, Chilo Suppressalis (Walker), were the most abundant. S’samia inferens (Walker) and Chilotraea polychrysa (Meyrick) were relatively rare, while T. innotata (Walker) was not encountered at all. The borers attacked the crop when it was very young and reached their peak just before or immediately after harvest. Their overall abundance seemed to depend on the age and quality of the rice crop rather than on the small variations in climate found among the rice-growing regions on the island. From transplanting to harvest, T. incertulas was the dominant species in the rice fields in all 5 regions, comprising about 70% of the borer population. In the stubble after harvest the other 3 borers, especially Chilo suppressalis , increased considerably in numbers while T. incertulas declined. However, there are areas like the farms of The International Rice Research Institute and the College of Agriculture, University of the Philippines, at Los Banes, Laguna, where the former is more abundant than the latter.