The status of weed-associated populations of the brown planthopper, Nilaparvata lugens (Stål) - host race or biological species?

Populations of the brown planthopper, Nilaparvata lugens (Stål), from the Philippines were collected from and reared on the grass weed, Leersia hexandra Swartz. Six different populations from five localities were studied and compared with a sympatric rice-feeding population. Experiments on survival, virulence and oviposition preferences demonstrated that populations from one host plant survived poorly on the other and showed a strong preference for the species from which they derived. Crosses between the two populations showed some barriers to hybridization, but viable and fertile hybrids were obtained. Mate choice experiments demonstrated that, when a choice was available, no heterogametic matings occurred. Pulse repetition frequencies of female and male calls differed consistently and significantly between rice- and Leersia-associated populations. Hybrid calls were intermediate between the parental calls. Playback experiments showed that the call differences were primarily responsible for maintaining the identities of the host-associated populations. It was concluded that the populations from rice and from L. hexandra represent two distinct, but very closely allied, sympatric species in the Philippines.