Host plant nutrition drives fitness outcomes in the cactus specialist Drosophila mettleri

Organisms must navigate complex interactions with host plants, microbial communities, and environmental cues to ensure their survival and reproductive success when adapting to novel environments. Host plant specialists can be used to study how these interactions affect fitness due to their ecological constraints. In specialist species, such as cactophilic Drosophila , it remains unclear how feeding preferences, nutritional value of the substrate, and microbial interactions collectively shape fitness outcomes. We used diverse laboratory media formulations to examine the fitness implications in Drosophila mettleri , a species that exclusively uses Saguaro cactus ( Carnegiea gigantea ) as its natural host plant. We found that D. mettleri did not preferentially feed on one medium compared to others, nor on C. gigantea supplemented media compared to controls. Microbial communities, especially on banana media with C. gigantea additives, appear to improve developmental success and can influence D. mettleri post-pupation success. Taken together, our results highlight the behavioral and environmental interplay between host plant substrate and microbial communities.