BPH clearly responds differently to different rice varieties spending more than 80% of its time exhibiting the non-ingestion waveform types such as non-penetration or pathway in the varieties previously identified as resistant by Brar et al . A similar result of resistance characterization based on EPG was also found by previous researchers using other varieties such as IR56 , ASD7 and IR 62 . However, in susceptible varieties such as TN1 , BPH OTX015 ingested phloem sap for a long period without interruption. Therefore a longer duration for N4-b waveform could easily be found. Interestingly, N4-a salivation activity for last 5 h period was not significantly different between resistant and susceptible rice varieties, indicating that BPH could reach the sieve element region in both resistant and susceptible, but could only ingest the phloem sap in susceptible genotypes. These results support the suggestion of Hattori that resistance to BPH is determined by phloem related characters. Phloem based resistance may have its basis in phloem chemistry where silicic, oxalic and phenolic acids , sterols and apigenin-C-glycosides have been implicated in resistance to BPH. The low level of essential amino acids in the phloem could influence BPH feeding perhaps representing phago-stimulatory cues. The interaction of plants and herbivorous insects is complex and still not well understood and further advances may require molecular approaches . A clear picture of resistance based on EPG waveform and honeydew drop data has been presented by cluster analysis. The twelve rice varieties could be PCI-32765 clinical trial classified into three groups. Group 1 was classified as the susceptible group because the average percentage duration of N4-b EPG waveform was found to be the highest. In contrast, EPG waveform NP, pathway, N5 and N6 of group 1 showed the lowest values. These results clearly indicate that BPH could easily feed on the phloem sap in this group. As we expected, the common control rice variety, TN1 was classified in this group 1. The other three varieties in the susceptible group are Azucaena, Nipponbare and IR694. Groups 2 and 3 have a much closer relationship, but with group 3 being more resistant than group 2. Consistent with this, the varieties in this group have previously been found to contain the resistance genes bph1 in IR64 , bph4 in Babawee and bph3 in Rathu Heenathi and the F1 .