Critical role of the food K:Na ratio for ecology and evolution of herbivorous and detritivorous insects

The functioning of herbivorous and detritivorous insects is affected by nutritional limitation originating from general trait of plant tissues and products, i.e. low concentration of sodium (Na), required by consumers in high proportions. Therefore, Na availability influences survival, fitness, and abundance of consumers feeding on plant matter as well as shapes ecological interactions, and food webs. However, considering only one nutrient at a time is neither physiologically nor ecologically relevant, and the effect of Na on consumers is regulated by the potassium (K) to sodium ratio. Excessively high K:Na ratio in food have detrimental effect for consumers even if Na concentration is relatively high. Our preliminary studies on Apis mellifera have shown that, contrary to common knowledge, bees prefer specific K:Na ratio, not solely Na concentration in food and water. However, the concentrations of the two elements in pollen are closely linked and the K:Na ratio in pollen is stable and high. Moreover, the K:Na ratio in pollen is too high regarding the bee needs and Na must be supplemented from other sources to allow for adequate overall food K:Na balance. We propose use of the K:Na ratio in studies considering ecology and evolution of bees and other herbivorous and detritivorous insects to broaden our understanding of plant-insect interactions and lay the grounds for better understanding of: (1) general ecology and evolution of insects, (2) evolution of bees, especially regarding trade-offs experienced when evolving from carnivory to herbivory, and (3) the role of soil-plant-consumer pathway in nutrient cycling in ecosystems.