The potato aphid, Macrosiphum euphorbiae (Thomas), (Homoptera: Aphididae), is a polyphagous aphid known to feed on over 200 plant species across 20 families. Although this aphid is known as one of the potato (Solanum tuberosum L.) colonizing aphids, field observations for populations of M. euphorbiae in southern Idaho indicated that they preferentially settled and colonized hairy nightshade, Solanum sarrachoides (Sendtner), plants rather than potato plants. Laboratory investigations revealed that M. euphorbiae survived only on S. sarrachoides and not on potato. A series of subsequent laboratory experiments conducted using laboratory-reared and field-collected aphids confirmed that M. euphorbiae survived only on S. sarrachoides. Experiments also showed that M. euphorbiae produced more winged morphs (alatae) than Myzus persicae on S. sarrachoides when reared alone under similar conditions. Furthermore, we documented intraguild competition between M. euphorbiae and M. persicae on S. sarrachoides. Results suggested that M. euphorbiae can potentially suppress M. persicae on S. sarrachoides. Competition studies reiterated that M. euphorbiae produced more alatae than M. persicae. Results also indicated that M. euphorbiae may have specialized on S. sarrachoides. The loss of polyphagous abilities of this M. euphorbiae “biotype” is intriguing and the factors responsible for specialized host utilization pattern remain to be elucidated.