Jw. Busch et Pl. Phelan, Mixture models of soybean growth and herbivore performance in response to nitrogen-sulphur-phosphorous nutrient interactions, ECOL ENT, 24(2), 1999, pp. 132-145
1. It is widely established that plant-mineral nutrition is an important de
terminant of herbivore developmental performance and behavioural preference
. Unfortunately, the specific effects of minerals on herbivory have been va
riable and few unifying principles have emerged. Advances in this area may
be impeded in part by an experimental approach that emphasises single nutri
ents without regard to nutrient ratios.
2. In this study, mixture-design experiments were adapted to the study of m
ineral nutrition and herbivore performance. The interactive effects of nitr
ogen, sulphur, and phosphorous on the development of soybean looper (Pseudo
plusia 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 a
ctual responses to these nutrients depended on the proportions of the other
components in the nutrient solution. For example, over a range of decreasi
ng nitrogen concentration, resulting soybean looper pupal mass first declin
ed then increased when replaced by a high sulphur:phosphorous blend, but ju
st the opposite response was measured when replaced with high phosphorous:s
ulphur ratio.
4. Moreover, responses to mineral proportions were generally nonlinear and
the effects of mineral proportion were not only species-specific, but varie
d for different responses within a species.
5. These studies demonstrate that understanding the role of mineral nutrien
ts 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 stu
dy of the interactive effects of mixture components, such as in plant-nutri
ent blends.