Entomol. exp. appL 68: 265-267, 1993. 9 1993 Kluwer Academic Publishers. Printed in Belgium. 265 Incorporation of Quercus rubra foliage into artificial diet alters development of a fungal pathogen of Lymantria dispar Ann E. Hajek & J. Alan A. Renwick Boyce Thompson Institute, Tower Road, Ithaca, NY 14853-1801, USA Accepted: February 2, 1993" Key words: bioassay, plant phenolics, host plant, Entomophaga maimaiga, fungi Introduction Our knowledge of the effects of foliage and/or phytochemicals on insects has often been based on results from studies in which these materials were incorporated into artificial diet (AD) (Ber- enbaum, 1986). The validity of this technique has been questioned; results from studies using incor- poration of plant materials into AD usually dem- onstrate digestibility reduction but when alle- lochemicals have been administered externally on leaf material, no digestibility reduction is re- ported (see Berenbaum, 1986). This argument suggests that although effects can be shown with AD + plant materials, this effect may be ar- tificial. In this study, gypsy moth, Lymantria dispar (L.), larvae were fed AD, red oak (RO = Quercus rubra L.) foliage, or AD + RO foliage; then larvae were infected with a fungal pathogen, Entomophaga maimaiga Humber, Shimazu & Soper and dis- ease development was evaluated. We report in- hibition of E. maimaiga development in larvae eating foliage-incorporated AD, although E. mai- maiga developed normally in larvae eating AD or RO alone. Materials and methods Laboratory colony gypsy moth larvae were ob- tained as eggs from the USDA, APHIS, Methods Development Center at Otis Air National Guard Base, Massachusetts, U.S.A. Larvae were reared on high wheat germ diet (Bell et al., 1981) with a wheat germ-casein base in 236.6 ml plastic cups with paper lids. 'Wild' larvae were field-collected as eggs, surface-sterilized in 3.7?/0 formalin for 1 h to kill surface contaminants, and then main- tained at 4 ~ until needed. RO foliage was col- lected in Allegheny hardwood forests near Ithaca, New York. Larvae fed foliage alone were pro- vided bouquets of RO foliage in ventilated plas- tic boxes after conidial inoculation (see Hajek, 1989a). Foliage incorporation into artificial diet. AD was made using standard procedures but 10 g finely ground, lyophilized leaves minus petioles were added before AD gelled. Laboratory colony lar- vae were fed either AD or AD + RO. This experi- ment was replicated three times for AD + 6-7 June foliage, and twice for AD + 29-30 June fo- liage. When foliage was collected 6-7 June, third instars were abundant and 29-30 June, fifth in- stars were predominant. Wild larvae were reared on sleeved branches of RO in the field. Three parallel bioassays, each using larvae from three sleeves on three RO trees, were conducted in the laboratory between 8-28 June. Solvent extracts of RO foliage were made and, after removal of solvent by drying, the extracted material was added to AD. Acetone and metha- nol were specifically used as solvents based on