Response of Green Rice Leafhoppers to Rice-Planting Practices in Northern Thailand

Abstract

Recent changes in rice-seeding practices in Thailand have altered the traditional density and spatial dispersion of rice plants in a field. Typical transplanted fields have a low density of uniformly spaced plants, whereas broadcast or direct-seeded fields have a high density of irregularly dispersed plants. Effects of these variations in plant texture on the abundance and movement of important rice pests, the green rice leafhoppers, Nephotettix virescens and N. nigropictus, were examined in controlled field experiments in Chiang Mai, Thailand, in 1989 and 1991. Nephotettix colonized broadcast fields more quickly than transplanted fields and peaked in both types of fields when rice plants reached panicle initiation, suggesting phenological synchrony with crop development. Although leafhoppers did not respond consistently to treatment changes in plant or tiller density, leafhopper load per plant was highest in transplanted rice fields of low plant density and uniform dispersion. Visual stimuli during colonization and subsequent phenological changes in plant quality appeared to influence seasonal patterns of leafhopper abundance. Nephotettix movement was greater in uniform transplanted fields than in irregular broadcast fields throughout most of the crop season, regardless of plant density. Eggs were more evenly distributed in transplanted fields than in broadcast fields, where they were more frequently found in border plants. Results suggest that, despite the delayed establishment of Nephotettix populations in transplanted fields, the combination of higher leafhopper load per plant and more frequent within-field movement could lead to greater incidence of leafhopper-transmitted pathogens in transplanted than in broadcast rice. The greater risk of crop loss to leafhopper-transmitted viruses would be influenced by the amount of viral inoculum in surrounding areas and possibly partially offset by higher yields in transplanted rice. Factors associated with seeding practice, rather than plant or tiller density per se, most influenced the leafhoppers’ abundance and movement patterns.