Chemical ecology of a plant-herbivore-entomopathogen interaction

摘要

The effects of variation in host plant suitability to two polyphagous insect herbivores was studied using two Apium graveolens var. rapaceum (L.) cultivars that differed in their suitability for the growth and development of Spodoptera exigua (Hubner) and Trichoplusia ni (Hubner). The role of host plant variation in mediating an herbivore-entomopathogen interaction was also studies using the toxins produced by the bacterium, Bacillus thuringiensis subsp. kurstaki (Berliner) (Bt). The genotypic differences in host plant suitability observed previously under laboratory conditions were also observed under field conditions with the combined effect of host plant genotype and Bt being additive. In laboratory studies, host plant suitability varied between host plant genotypes and over time for each genotype. This variation affected the plant-herbivore and herbivore-pathogen interactions although the interaction between host plant genotype and Bt was generally additive. S. exigua larvae reared on the more suitable host plant genotype, survived longer and grew faster than larvae reared on the less suitable genotype when fed Bt treated foliage in spite of the fact that they received higher Bt doses because they consumed more treated foliage. The chemical basis for the differences in suitability between the two host plant genotypes was isolated using a bioassay-driven extraction procedure and identified as sedanenolide. Foliar concentrations of sedanenolide varied seasonally over a broad range but the concentrations found in the less suitable genotype were significantly greater than the concentrations found in the more suitable genotype. The role of sedanenolide in mediating host plant suitability was confirmed in both direct and correlational studies. Like the interaction between host plant genotype and Bt, the interaction between sedanenolide and Bt is additive.