Dietary Mixing in a Generalist Herbivore: Tests of Two Hypotheses

It has been demonstrated in several taxa that generalists grow better when they ingest a mixed diet than when they are restricted to just one or two items, but there are few cases that provide definitive evidence for how this benefit is achieved. Two hypotheses are addressed concerning the possible benefits of feeding on a variety of foods: (1) mixing foods increases the quality of the overall diet by improving the nutrient balance and (2) mixing improves the diet due to dilution of any one plant secondary compound. The generalist grasshopper Schistocerca americana was used in a series of experiments distinguish between the two hypotheses. Experiments using artificial foods with complementary nutrients demonstrated that dietary mixing improves growth while the amount ingested is decreased. Nutritionally identical foods with differing plant secondary compounds led to different growth rates, but insects able to mix generally grew at intermediate rates. Experiments with domestic or wild plant mixtures gave results generally closer to the pattern found with complementary nutrients than the pattern found with differing poisons, but both hypotheses are supported to some extent.