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Abstract As an abundant element in the Earth’s crust, sodium plays an unusual role in food webs. Its availability in terrestrial environments is highly variable, but it is nonessential for most plants, yet essential for animals and most decomposers. Accordingly, sodium requirements are important drivers of various animal behavioural patterns and performance levels. To specifically test whether sodium limitation increases cannibalism in a gregarious lepidopteran herbivore, we hydroponically manipulatedHelianthus annuushost plants' tissue-sodium concentrations. Gregarious larvae of the bordered patch butterfly,Chlosyne lacinia, cannibalized siblings when plant-tissue sodium concentrations were low in two separate experiments. Although cannibalism was almost non-existent when sodium concentrations were high, individual mortality rates were also high. Sodium concentration in host plants can have pronounced effects on herbivore behaviour, individual-level performance, and population demographics, all of which are important for understanding the ecology and evolution of plant-animal interactions across a heterogeneous phytochemical landscape.