Grain hardness and protein content as key determinants of barley resistance to Sitophilus granarius Linnaeus, 1758 (Coleoptera: Curculionidae)

Postharvest infestations by the granary weevil, Sitophilus granarius Linnaeus, 1758 (Coleoptera: Curculionidae), pose persistent challenges to barley storage, Hordeum vulgare L. (Poaceae), worldwide. While host plant resistance offers a sustainable control strategy, the mechanisms by which grain traits suppress pest development are still unclear. This study aimed to identify which grain characteristics impair S. granarius performance and to describe the biological cascade by which host traits affect the nutritional physiology and population growth of the pest. Ten barley cultivars differing in grain hardness, protein, starch, and moisture content were evaluated. Pest responses were assessed using life history traits, nutritional indices, and digestive enzyme activity. Marked differences in pest fitness were observed among cultivars. A trait–physiology–performance cascade was used to interpret resistance patterns. Strong negative correlations were found between grain hardness and protein content with fecundity (r = −0.821), intrinsic rate of increase (r = −0.789), and ECI (r = −0.738). Digestive enzyme activity did not correlate with pest success, indicating limited physiological plasticity. These results support a mechanistic model in which grain traits trigger a cascade of physiological impairments, ultimately limiting the fitness of the pest. Barley resistance to S. granarius emerges from sequential biological effects, beginning with grain traits and culminating in reduced population growth. Resistance in specific barley cultivars arises primarily from physical and nutritional characteristics, particularly hardness and protein content, rather than enzymatic interference. This cascade provides a practical and cost-effective approach for resistance screening and cultivar deployment in postharvest IPM systems.