No abstract

Resurgence of the brown planthopper, Nilaparvata lugens (Stål) on rice was induced by applications of decamethrin, methyl parathion, and diazinon. Differential mortality of predators and hoppers did not appear to be the primary factor for resurgence. Hoppers appeared to be attracted to methyl parathion and decamethrin treated plants because of plant growth. Improved plant growth, however, did not compensate for the increased feeding of the insecticide-treated hopper population, and plants treated with resurgence-causing insecticides succumbed to feeding injury earlier than untreated plants. Population increases were due in part to stimulation of reproduction of the hopper, either by contact action of the insecticides or through increased plant growth. Reduction in the length of the nymphal stage and increased adult longevity resulting in a shortened life cycle and longer oviposition period, respectively, were additional factors contributing to resurgence.

Rice varieties having a moderate level of resistance to the brown planthopper, Nitaparvata lugens, were evaluated to determine the mechanisms of resistance. The host plant-insect interactions were studied in the greenhouse and insectary to determine the levels of antibiosis and tolerance in the moderately resistant varieties. Parametersmeasured as the tolerance indicator were plant damage, plant weight loss, and yield reduction due to insect feeding. The insect biomass production, growth index, and population growth rate on the test varieties were used as antibiosis indicators. A regression of plant weight loss, due to N. lugens feeding, on N. tugens weight for five varieties indicated that the variety 'Utri Rajapan' has a high level of tolerance and no antibiosis. The resistance in 'IR46,' 'Kencana,' and 'Triveni' is due to a combination of antibiosis and tolerance. The relative amount of each of the two resistance components was measured by using the tolerance-antibiosis index. Field studies confirmed results obtained in the greenhouse and insectary studies. Although 'Utri Rajapan' had field populationsof N. tugens equal to that of the susceptiblecheck, yield reduction was only 25'7c, compared with 100'70 in the susceptible check.

No abstract

The release of an insect pest-resistant variety and subsequent adaptation of the insect species to that variety through the process of biotype selection has occurred on many cultivated crops. The present study was designed to obtain detailed information on some of the processes involved in the selection of biotypes of Nilaparvata lugens (Stål) on two rice varieties with different major genes for resistance. The N. lugens population selected for the study, which had originally been collected in the field and reared on a susceptible variety for 12 years (ca. 140 generations), initially had a low survival on the resistant rice varieties Mudgo (Bph 1 gene) and ASD7 (bph 2 gene) and high survival on the susceptible variety, Taichung Native 1 (TN1). Monitoring the survival and length of the developmental period for seven generations indicated a progressive increase in the survival and shortening of the nymphal period in each generation on the heretofore resistant varieties until they were similar to those on TN1. The shift in the population to a more virulent biotype was also confirmed by the seedling bulk test. The selected populations reared on Mudgo or ASD7 were not able to kill Mudgo or ASD7, respectively, in the fifth generation but by the eighth generation were able to do so. In addition, adult longevity and fecundity on the resistant varieties increased during the selection process. Nilaparvata lugens (Stål) is one of the major insect pests of rice and has had a distinct economic effect on rice production in many Asian countries (Dyck and Thomas 1979). It is a monophagous pest, limited to feeding on Oryza sativa and wild rice species. The epidemic of outbreaks of N. lugens in 1972, 1973, and 1974 in several Asian countries including the Philippines stimulated rice scientists to accelerate the development of rice varieties resistant to this pest. Genetic analysis of several rice varieties resulted in identification of two