Predicting the outcome of competitive interactions is a fundamental goal in ecology. Ecological stoichiometry, which relates nutrient balance to ecological processes, provides a framework for identifying mechanistic links among macronutrient availability, nutritional physiology and competitive performance. Because carbohydrates serve as a principal metabolic fuel, carbohydrate scarcity may impinge upon behaviours affecting competitive dominance (e.g. aggression, activity) to a greater extent than deficiencies of protein or other nutrients used preferentially for growth. Here, we tested this prediction with a diet manipulation study involving laboratory colonies of Argentine ants (Linepithema humile), a widespread and aggressive invasive species. The availability of both sucrose and insect prey influenced brood production and worker survival after three months. However, colonies became less aggressive and less active only when deprived of sucrose (but not prey). Scarcity of sucrose (but not prey) was also associated with reduced fat mass in individual workers. These data provide the first experimental support that carbohydrate scarcity compromises aggression and activity in ants, and illustrate, in principle, how access to carbohydrate-rich resources (e.g. plant exudates, hemipteran honeydew) might influence behavioural investments that contribute to competitive performance. Such investments might be especially important for invasive ants, given their aggressiveness and tendency to interact with honeydew-producing Hemiptera.