SOIL-FERTILITY MANAGEMENT AND HOST PREFERENCE BY EUROPEAN CORN-BORER,OSTRINIA-NUBILALIS (HUBNER), ON ZEA-MAYS L - A COMPARISON OF ORGANIC AND CONVENTIONAL CHEMICAL FARMING

It has long been argued by proponents of organic agriculture that crop losses to insects and diseases are reduced by this farming method, an d that reduced susceptibility to pests is a reflection of differences in plant health, as mediated by soil-fertility management. These repor ts although widespread are mostly anecdotal and largely without experi mental foundation. In this study, the effects of two parameters of soi l fertility on the host-preference behavior of an insect pest were mea sured: (1) the immediate effect of organic vs. inorganic fertilizers a nd (2) the long-term effect of soil-management history. Soils were col lected from three pairs of neighbouring farms, each pair matched for s oil type and comprising organic and conventional chemical production s ystems. Each soil was potted and amended with mineral fertilizers, ani mal manures, or left amended. After planting the amended soils with ma ize (Zea mays L.) in a greenhouse, European corn borer females (ECB), Ostrinia nubilalis (Hubner), Were released to determine egg-laying pre ferences. For each of the three farm comparisons, there was a signific antly higher level of ECB oviposition on plants in conventional soil. In two comparisons, there was also a significant amendment effect; how ever, the specific fertilizers leading to greater egg laying were not consistent among farm comparisons. Thus, the form of the fertilizer di d not have consistent effects on maize susceptibility to ECB, but soil -management history did. Moreover, there was significant variation in egg laying among fertilizer treatments within the conventionally manag ed soil, but for plants in the organic soils, egg laying was uniformly low. Pooling results across all three comparisons, variance in egg la ying was about 18 times higher among plants in conventional soil than among plants in organic soil. It is suggested that this difference is evidence for a form of biological buffering characteristic of organica lly managed soils. Also significant, ECB ovipositional preference did not correlate with plant biomass. Thus, these results suggest that soi l-management practices can significantly affect the susceptibility of crops to pests, and do so without adversely affecting plant productivi ty.