Abstract: The rice stem borer, Chilo suppressalis (walker) is an important pest to rice that threatens the rice growth by boring into stems. Steinernema carpocapsae N-Yz1, the natural enemies isolated from dead larvae of rice stem borer, are considered a potential biological control agent for C. suppressalis. Based on the invasion process, the infection to different instar larvae, and field effect, the control effect and using method of N-Yz1 to C. suppressalis was explored. The analysis of invasion process revealed that N-Yz1 possessed great infective action and could enter the body of larva by penetrating from the skin. Free water was essential for the movement of N-Yz1 on the surface it contacted. Infection ability was significantly different among different individuals of N-Yz1, which caused a different rate and ratio of invasion to the larvae of pest. The nematode N-Yz1 had a highly lethal effect on all the instar larvae, while juvenile larvae (1— 2 instar) were sensitive to elder larvae (3—5 instar). Lab tests indicated that the control effect of N-Yz1 on field larvae of C. suppressalis was not high at the stage of rice elongation, the mortality of the larvae was 37.3% at a N-Yz1 dose of 2.4× 10⁵ IJs/m². In contrast, simulation of control wintering larvae by N-Yz1 was much higher, with larval mortality of 85.2% at the same dosage. Field tests further demonstrated that the effect of N-Yz1 on C. suppressalis was just 21.23% at the stage of second generation of pest in summer, although higher mortality might be caused by an increasing dose of N-Yz1. It was also confirmed that, using N-Yz1 against wintering larvae in rice stems at spring, the mortality was 53.7% at 15 d at a dose of 1.2× 10⁵ IJs/m², and increased to 79.1% at 45 d. It was indicated that N-Yz1 can be used to control the wintering larvae in rice stems which is main multiply source of the pest, reduce the base quality and effectively control the damage caused by the borer pest.

Key words: Chilo suppressalis, entomopathogenic nematodes, Steinernema carpocapsae, control effect