Soil fertility practices, plant nutrient profiles, and mycorrhizal colonization affect oviposition response of Ostrinia nubilalis to corn plants

Background/Question/Methods

As USDA National Organic Program certification standards require farmers to utilize natural pest suppression in crops, much research has been conducted on the use of soil nutrient management practices to suppress insect pests.  An approach used in both conventional and organic farming, known as the basic cation saturation ratio hypothesis or soil balance (BAL) approach, claims that modifying soil Ca/Mg  ratios aids plants with calcium and other nutrient uptake, maximizing yield and reducing insect damage. To test if O. nubilalis vary their oviposition in response to corn grown in organic BAL, standard organic-farmed (STD), or conventionally farmed (CONV) soils, we collected soil from plots in Arlington, WI, which have implemented these three practices for at least 4 years. Corn plants were reared in these soils to at least the V5 stage in the greenhouse. We then positioned one plant of each soil type in large cages and introduced 5 female adult O. nubilalis to each cage.  After 2 days, total number of eggs and number of egg masses were recorded per plant. Soil and plant tissues samples from each plant were collected to assess nutrient content and root samples from each plant were stained and assessed for arbuscular mycorrhizal colonization.      

Results/Conclusions

Ostrinia nubilalis showed significantly different oviposition preference among all 3 soil types, laying the greatest number of eggs on plants grown in the BAL, fewer on STD plants, and the fewest on CONV plants.   Female moths also showed a positive oviposition response to increased amounts of Ca, Mg, S, and Fe in plant tissues, although only S and Fe significantly differed among the soil types.   There was an additional response of oviposition preference to AM colonization by soil type, in that the number of eggs and the numbers of arbuscules or vesicles were positively correlated with AM colonization in the BAL and CONV soil types, but negatively correlated in the STD soil.  However, mean mycorrhizal colonization itself did not significantly differ among soil types.  Our results seem to run in opposition to what the basic cation saturation ratio proposes in that O. nubilalis preferentially oviposited on BAL corn; however, oviposition response of O. nubilalis appears to be correlated with a number of factors, only a few of which appear to be associated with soil type.