The role of plant odors in the host finding behaviors of aphids was investigated by examining the olfactory responses of 3 Uroleucon species, U. gobonis (Matsumura), U. adenophorae (Matsumura) and U. kikioense (Shinji), against host and non-host plant odors. All the tested Uroleucon species were attracted to their host plant odors. U. gobonis was repelled by Aster scaber, a non-host plant. However, U. gobonis and U. adenophorae were attracted to the non-host plant, Platycodon grandiflorus, and U. kikioense was attracted to the non-host plant, Adenophora triphylla. Therefore, the aphids tested were allowed to choose between their host plants and non-host plants having attractancies by using an olfactometer. No differences in the preferences of aphids were observed in the olfactory responses between host and non-host attractive plants. However, all the tested aphid species settled and reproduced on these non-host plants only rarely. These results indicated that plant odors are not the only factors that these aphids use in finding host plants. The olfactory behaviors of oligophagous and polyphagous aphids belonging to genera other than Uroleucon to host plant odors were also examined in order to investigate the relationship between the olfactory behaviors of aphids against their host plant odors and the host range. Polyphagous aphids such as Myzuspersicae (Sulzer), Aphis gossypii Glover, A. craccivora craccivora Koch and Macrosiphum euphorbiae (Thomus) were not attracted to host plant odors, while Capitophorus formosartemisiae (Takahashi), an oligophagous aphid, was attracted to its host plant odor. The findings suggest that oligophagous and monophagous aphids tend to find their host plants by both visual and olfactory cues, while polyphagous aphids have a tendency to find their host plants mainly by visual cues.
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