Arthropod populations and rice yields in direct-seeded and transplanted lowland rice in West Africa

Abstract

Two methods for planting rice in irrigated lowland were evaluated during the wet seasons of 1994 and 1995 to determine their effect on rice arthropod numbers, insect-caused rice plant damage and rice grain yield. The six treatments tested were: hand transplanting of seedlings at spacings of 14 cm × 14 cm, 20 cm x 20 cm, and 30 cm x 30 cm; and direct-seeding of rice at 60 kg seeds ha -1 , 90 kg seeds ha -1 and 120 kg seeds ha -1 . The most abundant arthropods in the study were the diopsid flies, Diopsis longicornis Maquart and D. apicalis Dalman; the green leafhoppers Nephotettix spp.; the white leafhoppers Cofana unimaculata (Signoret) and C. spectra (Distant); spiders; dragonflies and damselflies; and stem borers. There was no overall difference between transplanting and direct-seeding, or between plant densities, in regard to sweep net counts of Cofana spp. and spiders. Diopsis longicornis and D. apicalis adult numbers were highest in the 20 cm × 20 cm transplanted plots in 1994, but no significant differences occurred in 1995. Nephotettix spp. adult numbers were highest in the 120 kg seeds ha -1 direct-seeded plots in 1995 but no significant differences occurred in 1994. The percentage of tillers infested with stem borers was highest in the three transplanted and the 120 kg seed ha -1 direct-seeded treatments in 1994 and the three transplanted treatments in 1995. In 1995, the percentage of whiteheads (empty panicles) caused by stem borer feeding was highest in the direct-seeded treatments, increasing from the low rate of 60 kg seeds ha -1 to the highest rate of 120 kg seeds ha -1 . Grain yields were generally similar in the transplanted and direct-seeded plots. Implications of planting methods and plant density as management practices in rice IPM and labour requirements for rice production are discussed.