Abstract: In order to verify whether the alarm signal was released when the insect attacked by its natural enemies， Erigonidium graminicolum， behavioral responses of the brown planthopper （BPH）， Nilaparvata lugens Stl， to the volatile substance released by the same kind of insects in danger were studied with Y-tube olfactometer. The results showed that BPH nymphs had no response to the volatile chemicals of E. graminicolum released， but escaped when received the chemicals released by the same kind of insects attacked by E. graminicolum. However, such responses varied in different instar nymphs. The 2nd to 3rd instar nymphs of BPH responded strongly to the volatile released by the same kind of insects， while no obvious response was made by the 1st， 4th to 5th instar nymphs of BPH to the volatile. The results above indicated that 2nd to 3rd nymphs of BPH in dangerous environment could release volatile substances and give a dangerous alarm signal to the same kind of insects， but 1st， 4th to 5th nymphs had not obvious alarm behavior. There might be reasons that mature nymphs of BPH could not be easily predated by E. graminicolum， or the youngest nymphs could not release a strong alarm signal while attacked by the natural enemy．

Key words: Nilaparvata lugens, Erigonidium graminicolum, Y\|tube olfactometer, volatile substance, behavioral response