THE WITHIN-FIELD DYNAMICS OF RICE TUNGRO DISEASE IN RELATION TO THE ABUNDANCE OF ITS MAJOR LEAFHOPPER VECTORS

Seasonal patterns of immigration and population development for the th ree major leafhopper vectors of rice tungro viruses were examined in r elation to tungro disease incidence over four seasons in the Philippin es. Early immigration of leafhoppers into rice plots was greatest in w et seasons (WS) and was also high in a late-planted dry season (DS) cr op which was out of synchrony with surrounding rice fields. Peaks in p opulation density occurred by 50-65 days after transplanting in wet se asons, but varied more widely in dry seasons. Nephotettix virescens (D istant) was the most important vector species because of its earlier c olonization of rice plantings, more rapid population development and h igher transmission of tungro viruses than Nephotettix nigropictus (Sta l) or Recilia dorsalis (Motschulsky). Early infected rice hills were r andomly distributed and became increasingly aggregated as tungro disea se developed, indicating that much secondary spread occurred within pl antings. Tungro disease incidence was not directly related to the size of leafhopper vector populations. Rapid spread of tungro disease was recorded in the 1991 dry season, when leafhopper numbers were relative ly low but infection pressure was high. The results suggest that vecto r numbers are not an accurate indicator of tungro disease risk, unless account is also taken of inoculum pressure. It is proposed that a tun gro management strategy aimed at reducing the amount of inoculum is mo re likely to be effective in controlling the disease than a strategy b ased on reducing leafhopper numbers through insecticide applications. Copyright (C) 1996 Elsevier Science Ltd.