Ecology and control of rice planthoppers in Fiji

Abstract

Observations were made in Fiji on the ecology and control of two Delphacid planthoppers on rice, Sogatella furcifera (Horv.) and Nilaparvata lugens (Stål), the latter species having apparently been responsible for a devastating outbreak in 1959. The studies covered three rice seasons, 1961, 1962, and 1963, most of the research being done at Koronivia in the south-eastern part of Viti Levu. Samples were taken of planthoppers and their natural enemies in dryland (drilled) and wetland (transplanted) rice, as well as in rice seed-beds and areas with grass or mixed rice and grass. Insecticides applied at standard dosages were tested against the planthoppers.It was shown that S. furcifera was most common on young rice and that N. lugens predominated on older rice. Sogatella kolophon (Kirk.), long placed in the genus Sogata and confused with Sogatella furcifera, was abundant only in grassy areas. Populations of N. lugens and, to a lesser extent, of S. furcifera, were apt to reach damaging levels on transplanted rice growing in pools of standing water after a dry period. In drilled fields, the predatory Mirid Cyrtorhinus lividipennis vitiensis Usinger provided effective control by destroying eggs of N. lugens, and, by the time the rice was six months old, it usually outnumbered the females of N. lugens. Parasitism of planthopper eggs by Mymarids, Trichogrammatids, and Eulophids was very low. Parasitism of adults of S. furcifera and S. kolophon by Elenchid Strepsipterons averaged about ten per cent, and that by Dryinid wasps less than one per cent. Adults of N. lugens were virtually free from parasitism but were sometimes infected by fungi. Factors influencing outbreaks are discussed. These include the amount of rainfall, the type of cultivation, the age of the rice crop, the species composition of the planthopper populations, and the relative abundance of Cyrtorhinus.Insecticidal concentrates tested against both S. furcifera and N. lugens included malathion 50 per cent, emulsifiable concentrate, diazinon 20 per cent, e.c., Bogor 30 per cent, e.c., DDT 20 per cent, e.c., dieldrin 15 per cent, e.c., and Sevin 80 per cent, wettable powder. Dieldrin-resistant strains of N. lugens appeared during 1961, the third season in which dieldrin and BHC had been widely used. The other five materials gave good results, DDT being the least expensive and the most persistent, although it took more than 24 hours to reduce the plant-hopper populations. It was concluded that rice should be closely watched from its third to the end of its sixth month and that 50 oz. of DDT 20 per cent. e.c. in 50 gallons of water per acre, or an equivalent formulation of some other insecticide, should be sprayed on the rice stems if the planthopper density should exceed 10 per stem. Other possible measures for controlling rice planthoppers would include the introduction of Dryinid wasps parasitising N. lugens, the development of rice varieties resistant to planthopper attacks, and the use of controlled flooding in high-bunded fields, but on present evidence the first two appear to hold little promise of success.