Aphids and other phytophagous insects often show intra-specific variations in relation to host plant utilization. In several instances, intra-species variations lead to host-plant specialization. These are considered to be important source of speciation. In a recent study (Foottit RG et al. 2010. Zootaxa 2358: 25–38) two forms of the banana aphid, Pentalonia nigronervosa f typica Coquerel (Hemiptera: Aphididae) from banana hosts and P. nigronervosa f. caladii van der Goot collected from Zingiberaceae and Araceae respectively were described as separate species, P. nigronervosa Coquerel and P. caladii van der Goot, based on morphological and molecular differences. A study was undertaken to examine the ecological and biological characters in asexual wingless morphs of the two forms of P. nigronervosa sensu lat. using taro (Araceae) and banana (Musaceae) as host plants. The results showed consistent differences between the two forms. In biological characters, the apterous morphs off. caladii from taro host plants were found to be significantly more fecund, showed a higher net reproductive rate, longer reproductive duration, and their adults lived longer than the f. typica aphids from banana host plants. In ecological characters, f. caladii aphids formed bigger colonies and in significantly less time on taro plants in comparison to f. typica aphids which formed smaller colonies in significantly more time on banana plants. Reciprocal transfer of the two forms of P. nigronervosa aphids between their host plant species lowered performance on the transferred host plants. These results confirmed that P. nigronervosa f. typica from banana hosts and P. nigronervosa f. caladii from taro hosts are indeed two different species in relation to host plant utilization and suggested that the observed differences in their fitness characters represented distinct genotypes.

Banana aphid Pentalonia has already been described in its two taxonomic fo rms fro m hosts of Zingiberaceae and Araceae were regarded as separate taxa, P. nigronervosa Coquerel and P. caladii van der Goot, respectively, based on morphological and molecu lar differences. Between the two species of Pentalonia tested in our earlier study, the nigronervosa species expressed fitness for banana host plants and the caladii species for taro host plant suggesting strong genotype(aphids)-environment(host plants) interactions and increased genetic variation. This study shows the morphological variations of the two species and the isozyme variations of the two taxa fro m Araceae and Musaceae plants, respectively, an indicative of this separation as the laboratory-reared clones of banana aphid from the host plants also occur as two variants of the aphid species. In the existing conditions of information, the two species can be certainly considered as separate species.

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