Population outbreaks of gypsy moth (Lymantria dispar) larvae have resulted in major defoliation of North American forests since the late 19th century. These outbreaks are controlled by a nuclear polyhedrosis virus that kills the larvae, phenolic compounds in the foliage consumed by them, and predators. These compounds may damage the larval gut, allowing the virus to infect its host more effectively, and/or alter the behavior of the larvae to make them more likely to encounter the virus. Ultimately, understanding how the phenolics alter virus transmission may elucidate why induced responses play a role in controlling gypsy moth population cycles. A 70% acetone extract of red oak (Quercus rubra) foliage was chromatographed against a series of decreasingly polar solvents against a solid phase of Sephadex LH-20. In multiple experiments, neonate gypsy moth larvae were fed artificial diet infused with 5 or 10 mg/mL aqueous solutions of the separated red oak phenolics for two weeks, after which the larvae were frozen and weighed.
Patterns were largely consistent among the several experiments. For example, in the initial experiment with 5 mg/ mL of the chromatographically obtained phenolics added to the artificial diet, larvae that consumed the control (non-infused) diet had the greatest masses (mean = 65.5 mg FW), whereas larval mass decreased as the polarity of the infused solids decreased (e.g. mean = 52.4 mg FW for larvae that consumed the 50% acetone fraction: significant fraction effect in a general linear model, p < 0.001). Average larval consumption, measured near the end of the feeding period, was reduced by the addition of the isolated phenolics (p = 0.008 in a general linear model), with larvae feeding on the control artificial diet consuming 18.7 mg DW/day versus larvae feeding on artificial diet infused with the 30% acetone fraction of isolated red oak phenolics consuming 10.9 mg DW/day. This pattern is consistent with a pre-ingestive effect of the phenolics, and thus future experiments will explore if increased concentrations of red oak phenolics lead to more frequent larval movement which could increase their potential exposure to virus in the wild.