Micronutrient interactions on soybean growth and the developmental performance of three insect herbivores

Although the importance of plant mineral nutrition on insect herbivory has long been recognized, studies have focused almost solely on the effects of varying levels of a single nutrient. One aspect of the relationship between plant nutrition and herbivory largely ignored is the proportions among minerals. The mineral balance hypothesis Postulates a plant nutritional state with optimal levels and proportions among minerals that enhances plant growth and suppresses herbivore performance. As part of a systematic series of studies to determine the optimal proportions of minerals for soybeans (Glycine max), plants were grown in hydroponic solution with different proportions of boron (B), zinc (Zn), and iron (Fe), using a D-optimal experimental design for constrained mixtures. In this design, B and Fe varied from 0 to 0.05 mM in the nutrient solutions, whereas Zn ranged from 0 to 0.01 mM. Three soybean feeders, the polyphagous soybean looper (Pseudoplusia includens [Walker]), the oligophagous Mexican bean beetle (Epilachna varivestis Mulsant), and the velvetbean caterpillar (Ancarsia gemmatalis Hubner), were fed leaves front the plants and their developmental performance was evaluated using polynomial models. For all three species, developmental performance was highest on plants grown in solutions without B. Weight gain by velvetbean caterpillar larvae, whose feeding was terminated at 6 d, showed a linear response to mineral proportion (adjusted r(2) = 0.49), peaking at B:Zn:Fe = 0:20:80. Soybean looper pupal weight and larval developmental time showed predominantly quadratic responses determined primarily by B:Fe ratio (adjusted r(2) = 0.33 and 0.67, respectively). Mexican bean beetle showed the most complex response to mineral proportion, with cubic models of adult weight (adjusted r(2) = 0.66) and developmental time (adjusted r(2) = 0.45) that indicated significant interactions among all three minerals. Soybean shoot growth was described by a reduced cubic model (adjusted r(2) = 0.83), but unlike the insects, plants grew poorly in nutrient solutions lacking B. Rather, soybeans grew best at intermediate mineral proportions, and consistent with the mineral balance hypothesis, herbivores did relatively poorly on these plants. Also consistent with the hypothesis, the effect of Zn on plant growth and herbivore development was dependent on the ratio of the other two nutrients.