The combined effects of temperature and insecticide on the fecundity of adult males and adult females of the brown planthopper Nilaparvata lugens Stål (Hemiptera: Delphacidae)

Abstract

Extreme temperatures are thought to be one of the main factors suppressing natural population growth of the brown planthopper Nilaparvata lugens (Stål) (Hemiptera: Delphacidae). In contrast, sublethal doses of some pesticides (such as triazophos) stimulate reproduction of adult males and adult females of N. lugens. However, the combined effects of high or low temperature and sublethal doses of pesticides on the reproduction of adult males or adult females have not been investigated. This study examined the effects of high (34 °C), typical (26 °C) and low (20 °C) temperature and the insecticide triazophos on the protein content of male accessory glands (MAGs) and adult female ovaries and on the fecundity of adult females. The study demonstrated that temperature and triazophos concentration significantly influence the protein content of MAGs and of adult female ovaries before and after mating. The protein content in MAGs before mating attained a maximum value at 26 °C (45.1 μg per adult male, grand mean of all concentrations), followed by an intermediate value of 38.0 μg at 20 °C and a minimum value of 10.4 μg at 34 °C. These results indicate that the reproduction of males is more sensitive to high temperature compared to adult females. Protein content in MAGs after mating at 26 °C remained at a maximum value (25.6 μg per adult male, grand mean of all concentrations), followed by 21.9 μg at 20 °C and a minimum value of 8.3 μg at 34 °C. The protein content of MAGs after mating decreased with increasing triazophos concentration at the three temperatures. This result indicates that treated males transferred more MAG protein to adult females through mating. For example, MAG protein after mating at 26 °C for 0, 10, 20, 40 and 80 ppm triazophos treatments decreased by 16.5, 39.2, 42.8, 53.5 and 58.2%, respectively, compared with the corresponding values before mating. The protein content of adult female ovaries both before and after mating at three temperatures for all concentrations was significantly higher than in the control. However, the increase of ovary protein (percent) both before and after mating for all concentrations attained its maximum value at 34 °C, an intermediate value at 26 °C, and its minimum value at 20 °C, compared with the control. Similarly, percent increases in the number of eggs laid for 10, 20, 40 and 80 ppm triazophos treatments at 34 °C and 20 °C (except for 80 ppm) were greater than those at 26 °C.