Print PageGypsy moth (Lymantria dispar) larvae are major defoliators of eastern North American forests, and they undergo cyclical outbreaks that may occur due to interactions of pathogens, predators, and host plant quality. As part of an ongoing project to understand these interactions, we continue to re-examine the effects of red oak foliar quality on gypsy moth larval performance and behavior.
Results/Conclusions
Red oak foliage was extracted with 70% acetone, the acetone removed from the extract, and the aqueous extract fractionated on a lipophilic Sephadex LH-20 column with a series of increasingly non-polar solutions of water, methanol, and acetone. The eight chromatographic fractions had the solvent removed by rotary evaporation, the aqueous extracts were freeze dried, and the resulting powders used in multiple feeding studies with gypsy moth larvae. In no-choice preference tests, third and fourth instar larvae showed variable response to the chromatographic fractions, depending on their diet prior to the test. In a 12 day feeding study, aqueous solutions of 4 mg/mL of the chromatographic fractions were added separately to gypsy moth artificial diet and fed to neonate gypsy moth larvae. The fresh mass of larvae feeding on the fraction-infused diet differed significantly and varied over twofold (ANOVA, P <0.001; 97.4 ± 11.6 mg vs. 44.6 ± 7.8 mg, mean ± SEM, fraction 1 vs. fraction 8). However, survivorship did not vary significantly (p = 0.157) and consumption of the different fractions did not seem to retard development, as most larvae had molted to the third instar before the experiment ended. Although none of the fractions contained any detectable condensed tannins, hydrolyzable tannin concentrations increased from polar to non-polar fractions, and these compounds reduced larval weight gain (linear regression of hydrolyzable tannin concentration vs. mean larval fresh mass: R2 = 0.85, p = 0.001). Clearly, hydrolyzable tannins reduce the quality of red oak foliage for gypsy moth larvae separate from any effects of condensed tannins. Future work will concentrate on isolation and quantification of individual tannins as well as understanding their biological effects and interactions with the viral pathogen of Lymantria dispar.
