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Priestley, Amanda L.

Plant and Soil Science

2006

MS

European corn borer (ECB) (Ostrinia nubilalis) Hübner is the most destructive corn pest in North America. Injury caused by larval feeding results in physiological damage, consequently rendering contaminated corn unusable. Control has largely been achieved through the use of chemical insecticides; however, repeat applications are necessary over the growing season and from year to year, with associated risks to the environment and pesticide applicators. Alternative management technologies include transgenic crops that express insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) Berliner. In many Bt-crops, including silage corn, the insecticidal toxin is expressed throughout the entire plant, eliminating the need for total spray coverage at various times in the pest's life cycle. The persistent expression of the toxin provides season-long protection but raises concerns over possible side-effects on nontarget beneficial organisms. Bt-toxins are exuded from corn plant roots into the rhizosphere and may accumulate to levels that are detrimental to the success of these organisms. The current 2-year study (2003, 2004) evaluated effects of two-silage corn varieties: Pioneer var. 38A25 (Bt-corn expressing the Cry1Ab toxin) and Pioneer var. 38A24 (parent isoline) on species diversity and abundance of Collembola and carabid beetles. The experimental site was located at the University of Vermont farm in Burlington, VT. Treatments were arranged in a complete randomized design, and replicated five times in eight-row transects aligned N-S. Pitfall traps used to collect surface-dwelling species were set on a bimonthly schedule within each transect for 48h at a time. Soil core samples used to collect subterranean species were taken once a month, and placed in Berlese funnels for extraction. All individuals were recorded and identified for analysis in the Simpson's D and Shannon-Wiener H' diversity indices. Evenness was measured using Simpson's E', after which dominant species were analyzed in a multivariate ordination analysis. Results showed Bt had no negative treatment effect on any of the organisms analyzed; however, a significant treatment by year interaction, and replication within treatment effect was found within collembolan populations that were analyzed.
To determine whether Bt-toxins have the potential to directly impact Collembola, two bioassay experiments were carried out on the collembolan Folsomia candida Willem, to assess effects of different toxins derived from Bt on various life parameters, i.e., survival, fecundity, and population growth. Experiments were conducted with spores and toxins-produced by whole bacteria (Bt var. kurstaki) and truncated Btk toxin expressed in the roots of a transgenic corn hybrid (Pioneer var. 38A25). Results from the whole protein experiment showed no direct differences among treatments on egg production of F. candida; differences appeared to be associated with food quality. Similarly, transgenic corn expressing the Cry1Ab toxin had no significant effect on population growth of F. candida. The physiological growth stage of the maize plants at the time of sampling appeared to have the greatest influence on population development in the assay cells. In both experiments, nutritional quality of the food appeared to have the most influence on the life-parameters of F. candida.