Effects of light intensity on pac choi (Brassica rapa) plant chemistry and growth responses of diamondback moth (Plutella xylostella)

The effects of light intensity on pac choi (Brassica rapa) chemistry and diamondback moth (Plutella xylostella) (DBM) body weights were assessed in four replicated greenhouse experiments (July 2009 to March 2010). Based on the results, male and female body weights, percent herbivory, foliar C:N ratio, protein, total phenolic content, caffeic and ferulic acids, light intensity, and temperature differed significantly (P<0.05) among the four experiments. Furthermore, light intensity, plant biomass, C:N ratio, total phenolic content, and sinapic and ferulic acids were significantly higher under ambient conditions (P<0.05), whereas protein content was significantly higher under shade conditions (P<0.05). Across light treatments in all experiments, ferulic acid was positively correlated with light intensity (P=0.008, r=0.46), and male pupal and adult weights were negatively correlated with ferulic acid (P=0.02, r=-0.46 and P=0.05, r=-0.39) respectively. Under ambient conditions, male pupal and adult weights were best-predicted by total phenolic content, and ferulic and chlorogenic acids. Under shade, total phenolics, chlorogenic acid, and protein were best predictors of adult weights, whereas total phenolics and temperature best-predicted pupal weights. Under ambient conditions at higher light intensities, ferulic acid was higher, and male DBM weights were correspondingly lower, compared to shaded conditions. Ferulic acid is known to lower protein digestibility; this could explain the lower body weights of males under higher levels of ferulic acid. Female weights were unaffected, possibly because of compensatory feeding behavior. This study demonstrates that light intensity may mediate plant-insect interactions by altering the composition of secondary metabolites in pac choi thus impacting DBM weights.