The within-field dynamics of rice tungro disease in relation to the abundance of its major leafhopper vectors

Abstract

Seasonal patterns of immigration and population development for the three major leafhopper vectors of rice tungro viruses were examined in relation to tungro disease incidence over four seasons in the Philippines. Early immigration of leafhoppers into rice plots was greatest in wet seasons (WS) and was also high in a late-planted dry season (DS) crop which was out of synchrony with surrounding rice fields. Peaks in population density occurred by 50–65 days after transplanting in wet seasons, but varied more widely in dry seasons. Nephotettix virescens (Distant) was the most important vector species because of its earlier colonization of rice plantings, more rapid population development and higher transmission of tungro viruses than Nephotettix nigropictus (Stål) or Recilia dorsalis (Motschulsky). Early infected rice hills were randomly distributed and became increasingly aggregated as tungro disease developed, indicating that much secondary spread occurred within plantings. 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 relatively low but infection pressure was high. The results suggest that vector numbers are not an accurate indicator of tungro disease risk, unless account is also taken of inoculum pressure. It is proposed that a tungro management strategy aimed at reducing the amount of inoculum is more likely to be effective in controlling the disease than a strategy based on reducing leafhopper numbers through insecticide applications.