Survival rate and stylet penetration behavior of current Korean populations of the brown planthopper,Nilaparvata lugens,on resistant rice varieties

To understand the development of host plant resistance-breaking ability of the current BPH populations in Korea, we conducted nymphal survivorship tests and electrical penetration graph (EPG) studies on susceptible and resistant rice varieties with four different BPH populations, which were collected in the early 1980s (S-BPH) and in 2005, 2006, and 2007. The S-BPH had low survival rates on resistant rice varieties carrying either Bph1 or bph2. However, the current BPH populations have high resistance-breaking ability on the varieties with their elevated survival rates, whereas their survival rates were still low on the other resistant varieties, Gayabyeo (Bph1  the other unknown gene) and Rathu Heenati (Bph3). The EPG analysis also revealed that the ratio of BPH that could reach the phloem sap ingestion waveform (N4-b) within 15 h on the resistant rice varieties containing Bph1 or bph2 was higher in the current BPH populations (16.7–50%) than in the S-BPH population (0-4.2%). However, the pre-reaching time from the penetration start to the first N4-b waveform in the current BPH populations was significantly longer on resistant varieties (Bph1 or bph2) than on susceptible varieties. Furthermore, the total duration of N4-b waveform was significantly shorter on the resistant varieties.From these results, we suggest that, although the current BPHs collected in Korea have a high resistance-breaking ability through the increase of survival rate on resistant rice varieties carrying either Bph1 or bph2, they still have some difficulties feeding on the phloem sap of the resistant rice varieties.