The Effects of Lichen Extracts on Feeding by Gypsy Moths (Lymantria dispar)

Abstract

A bioassay was developed to determine the effects of six lichen extracts on food con- sumption and growth of gypsy moth (Lymantria dispar) larvae. The larvae were allowed to feed on leaves of Quercus rubra which had been painted with either acetone or water extracts of Cladina subtenuis, Flavoparmelia caperata, Hypogymnia physodes, Parmelia sulcata, Parmotrema hy- potropum, or Usnea strigosa. Acetone extracts containing depsides, depsidones, and usnic acid did not affect food consumption or larval growth. All water extracts except Flavoparmelia caperata decreased leaf consumption and growth by 30-80%. Although water extracts do not completely deter feeding, they could play an important ecological role in preventing lichenivory by gypsy moths. The gypsy moth, Lymantria dispar, is a common forest pest in eastern United States and can cause serious damage to hardwood forests during out- break years. Gypsy moth larvae are voracious feed- ers and are reported to utilize 485 species of North American plants as food sources (Gerardi & Grimm 1979). Although their preferred foods are leaves of Quercus or Populus species, they will eat a variety of unusual foodstuffs, including plastic tree flagging and styrofoam soaked in sugar (Zielinski 1986). De- spite the fact that larvae of gypsy moths come in contact with corticolous lichens during all stages of their development, our observations suggest that they never utilize lichens as a food source in nature. Secondary compounds in lichens have been shown to play important ecological roles in influencing food selection by lichenivores (Lawrey 1980, 1983, 1984, 1986, 1989; Reutimann & Scheidegger 1987; Run- del 1978). Such compounds include the acetone- soluble depsides, depsidones, usnic and pulvinic acid derivatives, which are exuded as crystals onto the surface of the lichen. Lichen secondary com- pounds-vulpinic acid and a combination of vul- pinic acid and atranorin--have also been shown to deter feeding by an insect which does not normally come into contact with lichens, the yellow-striped armyworm, Spodoptera ornithogalli (Slansky 1979). In order to provide a possible explanation as to why gypsy moth larvae do not feed on lichens, we investigated the effects of water and acetone lichen extracts on larval consumption and growth.