The potato aphid, Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae), is a polyphagous aphid known to feed on over two hundred different host plant species across twenty different plant families. Although this aphid is known as one of the potato colonizing aphids in Southern Idaho, field observations indicated that M. euphorbiae preferentially settled and colonized hairy nightshade, Solanum sarrachoides (Sendtner) plants rather than potato, Solanum tuberosum (Linnaeus) plants. Laboratory investigations revealed that M. euphorbiae survived only on hairy nightshade plants and not on potato plants. A series of subsequent laboratory experiments confirmed that M. euphorbiae survived and reproduced only on hairy nightshade when compared to four other previously recorded host plant species besides potato. Currently our M. euphorbiae “strain” seems to exhibit monophagy and the exact reasons for such a change in host utilization and feeding status remains unknown. Such a phenomenon is intriguing and remains uninvestigated. Both potato and hairy nightshade plants are also hosts of several economically important potato viruses, and these viruses are also vectored by M. euphorbiae. Life history studies indicated that viral infection significantly altered M. euphorbiae’s performance on different host plants, thus increasing the complexity of the whole pathosystem. The results and implications of various biological, behavioral, and ecological experiments conducted to understand the underlying causes of M. euphorbiae’s host utilization pattern will be presented.