Alfalfa (Medicago sativa L.) improvement under arid conditions increasingly requires integrating biotic stress tolerance with forage quality. This study evaluated six alfalfa genotypes-comprising native, exotic, and synthetic origins-for their response to infestation by the alfalfa weevil (Hypera postica), the primary insect pest affecting alfalfa productivity worldwide, under open-field conditions. Five genotypes consisted of half-sib progenies derived from either native (GAB) or exotic parental sources (AME, ABT, SAR, ETS), whereas the synthetic genotype (SYN) corresponded to a Syn-1 population obtained by mixing seeds of superior S-1 progenies selected from a polycross, with 60% originating from the native GAB background and the remaining 40% equally contributed by the four exotic families (10% each). Genotypic differences in infestation severity were substantial, ranging from 3.03% in the synthetic line SYN to 9.00% in AME and SAR, both half-sib progenies of exotic parents. The native-derived GAB exhibited intermediate infestation (6.58%) but maintained high crude protein - CP (23.65%) and moderate resilience in plant structure, marking it as a valuable contributor from local germplasm. The synthetic line SYN displayed the best integrated performance, combining the lowest infestation with sustained growth (internode length - LEN = 2.62 cm), pigment retention (SPAD = 44.1), and thermal stability (leaf temperature = 15.9 degrees C). In contrast, exotic-derived ABT, ETS, AME, and SAR showed sharper declines in structural and physiological traits under pest pressure. Across genotypes, infestation significantly increased fiber fractions (Neutral Detergent Fiber - NDF: + 7.2%; Acid Detergent Lignin - ADL: + 0.85%) while reducing digestibility (Dry Matter Digestibility - DMD declined to 62.3% in ETS) and SPAD index. Multivariate analysis (81.8% variance explained) revealed strong integration of morpho-physiological and nutritional traits. These findings underscore the breeding potential of native-derived progeny (GAB) and the synthetic line (SYN) for generating cultivars that jointly express pest resilience and forage value, offering strategic assets for sustainable alfalfa production in arid, pest-prone systems.