Abstract: To provide the theoretic basis for the scientific application of insecticides controlling the small brown planthopper, Laodelphax striatellus (Fallén), the bioactivity of chlorpyrifos to L. striatellus was determined by stem immersion method in the laboratory. Parafilm sachet and filter paper funnel were used to measure the effects of sublethal dose of chlorpyrios on such indices as honeydrew excretion, weight gain and egg production. The results obtained through parafilm sachet showed that honeydrew excretion and weight gain of single female L. striatellus treated with sublethal dose of chlorpyrios (0.21 mg/L a.i.) increased by 10.99% and 22.22% compared with the control, respectively, but with no significant difference (P>0.05）. The number of eggs laid per female L. striatellus was 79.6±26.4, with a 12.27% increase compared with the control and an extremely significant difference (P<0.01). The result of filter paper funnel method indicated that the honeydrew spot area of female L. striatellus treated with sublethal dose of chlorpyrios was 119.74±5.90 mm², with a 13.06% increase compared with the control and a significant difference (P<0.05). According to the test results, sublethal dose of chlorpyrios may cause the aggravation of the virulence of L. striatellus since its honeydew excretion, and weight gain and egg production could be promoted. The findings have scientific value for ascertaining sublethal effects of insecticides on the population of L. striatellus.

Key words: Laodelphax striatellus, chlorpyrifos, sublethal dose, sublethal effects, virulence, productivity