The nutritional needs of animals largely depend on their ecology and habitat. Phenotypes and general performance often depend on the synergistic influence of multiple nutrients. These effects are currently studied within the geometric framework of nutrition. Contrary to its close relative Drosophila melanogaster, the invasive Spotted-Wing Drosophila, Drosophila suzukii, attacks fresh, undamaged fruit devoid of microbial growth. Different oviposition habits suggest different nutritional needs by the two species. We investigated the combined influence of carbohydrate and protein concentrations on the larval performance of a D. suzukii population. Proportions of individuals that survived until the adult stage were maximal at intermediate protein and low sugar concentrations. Larval development was shortest under high protein diets. Observations on this population are congruent with what is known of D. suzukii larval ecology, as ripening, undamaged fruit is generally poor in sugars and proteins in comparison to ripe, yeast-colonized fruit. We discuss the limitations of fly nutrition experiments based on laboratory food, such as ours, where the natural dynamics of microbial growth is neglected.