Mixture models of soybean growth and herbivore performance in response to nitrogen–sulphur–phosphorous nutrient interactions

Summary 1. It is widely established that plant‐mineral nutrition is an important determinant of herbivore developmental performance and behavioural preference. Unfortunately, the specific effects of minerals on herbivory have been variable and few unifying principles have emerged. Advances in this area may be impeded in part by an experimental approach that emphasises single nutrients without regard to nutrient ratios. 2. In this study, mixture‐design experiments were adapted to the study of mineral nutrition and herbivore performance. The interactive effects of nitrogen, sulphur, and phosphorous on the development of soybean looper (Pseudoplusia includens) and two‐spotted spider mite (Tetranychus urticae) feeding on soybeans ( Glycine max ) were quantified by polynomial regression. 3. Although significant effects of individual minerals were measured, the actual responses to these nutrients depended on the proportions of the other components in the nutrient solution. For example, over a range of decreasing nitrogen concentration, resulting soybean looper pupal mass first declined then increased when replaced by a high sulphur : phosphorous blend, but just the opposite response was measured when replaced with high phosphorous : sulphur ratio. 4. Moreover, responses to mineral proportions were generally nonlinear and the effects of mineral proportion were not only species‐specific, but varied for different responses within a species. 5. These studies demonstrate that understanding the role of mineral nutrients in host‐plant quality requires that mineral proportions be considered in addition to concentration. Mixture modelling, which is largely unknown to ecologists, is a powerful new tool that could significantly advance the study of the interactive effects of mixture components, such as in plant‐nutrient blends.