Bottom-up effects of conventional and organic soil fertility management on Ostrinia nubilalis development and resistance of corn plants to herbivory

Background/Question/Methods

Under USDA National Organic Program certification standards, farmers rely partly on soil nutrient management practices to suppress insect pests.  Consequently, a growing body of literature has investigated relationships between crop nutrient profiles and insect herbivore response.  One practice, known as the basic cation saturation ratio, or soil balance (BAL) concept, maintains that altering soil calcium and magnesium ratios facilitates nutrient uptake by crops, optimizing yield and reducing herbivorous insect damage. However, we found in a greenhouse experiment that the European corn borer (Ostrinia nubilalis) preferentially oviposits on plants reared in organic BAL soil over plants reared in soil farmed using standard organic (STD) practices and conventionally-farmed (CONV) soil.  To test larval response of O. nubilalis to corn reared in different soils, and to test the resilience of these plants to increasing herbivory, we collected the three soil types from plots in Arlington, WI, which have employed these soil fertility management systems in a corn-soybean-alfalfa rotation for at least 4 years.  Corn plants were reared to maturity in the three soil types in a greenhouse, after which each plant was infested with 0-40 O. nubilalislarvae for 17 days.  Larval performance and proportion of kernels damaged were then assessed per plant.

Results/Conclusions

Ostrinia nubilalis exhibited signs of competitive stress, with proportion survival and mean development stage of the larvae significantly decreasing as a function of increased infestation.  Mean development stage was significantly higher in larvae consuming BAL-reared plants than in larvae on STD plants, with CONV showing intermediate larval development.  However, there was no significant interaction of survival or development stage with increasing larval density among soil types.  Proportion of kernels damaged per corn plant significantly increased as a function in increasing O. nubilalis density, but this relationship was not significantly different among soil types.  Total yield of the corn plants that had not been infested also did not significantly differ.  These results refute the basic cation saturation ratio hypothesis, as plants reared in the BAL soil type failed to produce higher yields in the absence of O. nubilalis, and actually hastened development rate of O. nubilalis larvae compared to the STD soil type without reducing their larval/pupal mass or survivorship.  The relationships between these results and both soil and plant tissue nutrients will also be discussed.