A comparison of the pathological effects on the midgut of the European corn borer treated with Bacillus thuringiensis and Bacillus subtilis

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Eichman, Phillip L.
Zimmack, Harold L.
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Thesis (D. Ed.)
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The purpose of this study was to compare the pathology caused by two bacterial species, Bacillus subtilis and Bacillus thuringiensis, in European corn borer larvae. The study consisted of: (1) a determination of the mortality of larvae that had ingested various concentrations of bacterial spores; (2) light microscopic examination of midgut tissues of larvae treated with various concentrations of bacterial spores, and (3) examination of midgut tissues of treated larvae with transmission electron microscopy.The results of the mortality study indicated that B. subtilis did not cause mortality in corn borer larvae. Light and electron microscopy did not reveal any pathological damage to midgut tissues of larvae treated with B. subtilis. It was observed that there was an increase in the amount of peritrophic membrane produced in the larvae treated with B. subtilis. These larvae were also observed to be sluggish and have diahrrea, suggesting a mild disease state.Mortality in larvae treated with B. thuringiensis varied with the concentration of spores ingested, and ranged as high as 100 percent. The mortality data were used to select three spore concentrations, corresponding to 25 percent, 50 percent, and 90 percent mortality levels. Larvae treated with B. thuringiensis spores in these concentrations were then examined by light microscopy. The pathology observed included slight swelling and vacuolation of the epithelial cells, cellular disruption, and complete breakdown of the epithelial tissue and subsequent bacteremia. It was observed that the degree of pathology increased with an increase in the bacterial spore concentration.Electron microscopic examination of midgut of B. thuringiensis treated larvae revealed the following pathological symptoms: decrease in apical microvilli, decrease in microvilli of goblet cells, and decrease in basal infoldings of plasma membrane.