Relative humidity as an environmental factor determining the microhabitat of the nymphs of the rice brown planthopper,Nilaparvata lugens (St?l) (Homoptera: Delphacidae)

Summary Microhabitat of the nymphs and the adults of the rice brown, planthopper,Nilaparvata lugens is known to be the lower parts of rice plants. In this study, possible environmental factors determining the range of the microhabitat were experimentally analyzed by using the nymphs under laboratory conditions. Thirty individuals of the 1st or the 3rd instar nymphs were released to a potted rice plant covered with a transparent acrylic cylinder. When the top of the cylinder was kept opening (open condition), the temperatures in the cylinder were almost constant, and relative humidities in the cylinder decreased with the increase of the height from the water surface of the pot. In the open condition, most nymphs and all exuviae were found on the basal parts of rice plants where the humidity was more than ca. 90% r.h. When the top of the cylinder was kept closing with parafilm (closed condition), the temperatures in the cylinder were almost constant, and relative humidities in the cylinder were more than 95% r.h. In the closed condition, the nymphs and the exuviae were distributed sparsely to the whole parts of rice plants. In both of the open and the closed conditions, patterns of nymphal distributions on rice plants during the dark regime were the same as those during the light regime under 25±2°C and 16L∶8D. Four different temperatures ranging from 20°C to 35°C did not influence on the microhabitat in the open condition. It was concluded from the results that relative humidity is the important environmental factor to determine the microhabitat of the nymphs ofN. lugens which showed to prefer very humid condition more than ca. 90% r.h.     

Humid microenvironment prerequisite for the survival and growth of nymphs of the rice brown planthopper,Nilaparvata lugens (St?l) (Homoptera: Delphacidae)

Nymphs ofNilaparvata lugens were experimentally reared from the 2nd instar in a cage covering part of the leaf sheath of an individual rice plant grown in a Wagner pot. Plants were covered with the cage from the water surface of the pot to 10 cm above the surface (lower cage-group) or from 10 cm to 20 cm above the surface (upper cage-group). Temperatures measured at three different parts of the cage remained fairly constant in both groups at around 25°C (23.7–25.2°C in mean value). In the lower cage-group, relative humidities measured at the three heights in the cage in (76.3–90.5% in mean value) markedly increased with the approach to the water surface. The nymphs of this group, particularly during the molting period, aggregated close to the surface. Eighty-two percent of the released nymphs emerged in this group. Relative humidities measured at three heights of the upper cage-group were 69.5–72.7% in mean value, and all the nymphs in this group died within 3 days after their release although half of them stayed on the rice plants within 6 h after their release. The role of relative humidity as a limiting factor on the range of the microhabitat and the population density ofN. lugens in rice fields was discussed on the basis of the results.