1CIAT, A.A. 6713, Cali, Colombia, 2FEDEARROZ, A.A. 6713, Cali, Colombia

Background and objectives
Rice hoja blanca virus (RHBV) is member of the tenuivirus group. RHBV epidemics are cyclic, occurring every 8-15 years. During the peak of epidemics, yield losses at the country level have been estimated to be between 25% and 50% [1]. Peru, Costa Rica, Dominican Republic, Surinam and Colombia are currently experiencing outbreaks of RHBV. The virus replicates in the planthopper, Tagosodes orizicolus (Muir), and is transmitted in a propagative manner. When RHBV is present, the farmers use up to ten insecticide applications to control the vector. Multiple insecticide applications reduce the number of predators and parasites of T. orizicolus, and this can lead to higher than normal vector populations. If the planthopper becomes resistant to insecticides, the populations of the planthoppers can reach very high levels. The object of this research is to develop integrated pest management strategies to mitigate the losses due to RHBV.

Results and conclusions
A RHBV survey of the major rice growing regions in Colombia was started in 1995. The incidence of the disease and the percentage of viruliferous vectors were determined. During the survey three zones were identified as the areas at greatest risk of RHBV outbreaks. The northern region of the Tolima valley was one of the zones having high levels of RHBV infected plants and viruliferous vectors. During last year, the incidence of RHBV and viruliferous vectors has increased in the central region of Tolima.

Insecticides and entomopathogens were evaluated for their effect on the populations of T. orizicolus, its predators and parasites. The pesticides tested included highly toxic general pesticides, moderately toxic selective pesticides and biological control agents. One of the moderately toxic selective pesticides does not kill spiders and has been adopted by many rice growers. Farmer surveys indicate that some planthopper populations may be developing insecticide resistance. Efforts are being made to encourage the judicious use of insecticides in a manner that prevents the development of pesticide resistant planthopper populations.

Several commercial varieties of rice were tested to determine their level of field resistance to RHBV. Varieties with intermediate or high levels of resistance were recommended for areas with a 5% or greater incidence of RHBV. In addition, new lines and varieties of RHBV rice are being tested and promoted. Since all the conventionally bred resistant varieties are susceptible to RHBV during the first 20 days after planting, reducing vector populations during this time is a required IPM strategy. Transgenic rice with resistance to RHBV has been produced [2]. The type of resistance in the transgenic plants complements the conventionally bred resistance. The first crosses to pyramid these resistance sources have been made.

The survey to monitor levels of RHBV incidence is continuing. In those regions identified at risk, farmers are encouraged to plant RHBV resistant varieties and to apply, as needed, a selective pesticide only during the early part of the growing cycle. Better varieties and biological control agents continue to be evaluated as part of the comprehensive strategy to end the cycle of RHBV epidemics.

1. Morales F, Niessen A, 1985. ABB Descriptions of Plant Viruses. No. 299.
2. Lentini Z, Calvert L, Taberes E., Lozano I, Cuervo, M., Roca, W. Ramirez, BC, 1997. General Meeting of the International Program of Rice Biotechnology. Abstracts p. 61.