Plant waxy bloom on peas affects infection of pea aphids by Pandora neoaphidis Patrick S. Duetting, 1 Hongjian Ding, Jeffrey Neufeld, and Sanford D. Eigenbrode * Division of Entomology, Department of Plant, Soil, and Entomological Sciences, College of Agricultural and Life Sciences, University of Idaho, Moscow, ID 83844-2339, USA Received 28 April 2003; accepted 14 October 2003 Abstract This study examined the effects of the surface wax bloom of pea plants, Pisum sativum, on infection of pea aphids, Acyrthosiphon pisum, by the fungal pathogen Pandora neoaphidis. In prior field surveys, a higher proportion of P. neoaphidis-killed pea aphids (cadavers) had been observed on a pea line with reduced wax bloom, as compared with a sister line with normal surface wax bloom. Laboratory bioassays were conducted in order to examine the mechanisms. After plants of each line infested with aphids were exposed to similar densities of conidia, the rate of accumulation of cadavers on the reduced wax line was significantly greater than on the normal wax bloom line; at the end of the experiment (13 d), the proportion of aphid cadavers on the reduced wax line was approximately four times that on the normal wax bloom line. When plants were exposed to conidia first and then infested with aphids, the rate of accumulation of cadavers was slightly but significantly greater on the reduced wax line, and infection at the end of the experiment (16 d) did not differ between the lines. When aphids were exposed first and then released onto the plants, no dif- ferences in the proportion of aphid cadavers were observed between the pea lines. Greater infection of pea aphid on reduced wax peas appears to depend upon plants being exposed to inoculum while aphids are settled in typical feeding positions on the plant. Additional experiments demonstrated increased adhesion and germination by P. neoaphidis conidia to leaf surfaces of the reduced wax line as compared with normal wax line, and this could help explain the higher infection rate by P. neoaphidis on the reduced wax line. In bioassays using surface waxes extracted from the two lines, there was no effect of wax source on germination of P. neoaphidis conidia. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Pisum sativum; Pandora neoaphidis; Acyrthosiphon pisum; Conidia; Plant surface wax; Tritrophic interaction; Adhesion; Germination; Extrinsic resistance 1. Introduction Plant characteristics can influence the effectiveness of natural enemies of insect herbivores. When conducive to natural enemy effectiveness, plant characteristics can confer an ecologically dependent or ÔextrinsicÕ resistance against insect herbivores (Price, 1986). Among the plant characteristics influencing predators and parasitoids are trichomes (Obrycki, 1986; van Lenteren and de Ponti, 1990), gross morphological characteristics such as plant size and architecture (Coll et al., 1997; Frazer and McGregor, 1994; Grevstad and Klepetka, 1992; Kareiva and Perry, 1989; Kareiva and Sahakian, 1990), and sur- face waxes (Eigenbrode and Kabalo, 1999; Eigenbrode et al., 1995, 1996, 1998a, 1999; White and Eigenbrode, 2000a,b). Effects of plant characteristics on insect pathogens have received less attention than have effects on insect carnivores (Elliot et al., 2000). Surface waxes are a potentially important plant characteristic in this con- text. In a notable series of papers, Inyang and others showed that cuticular wax extracts with associated phy- tochemicals from cruciferous plants influenced the germination behavior and pathogenicity of the entomo- pathogenic fungus, Metarhizium anisopliae (Metsch.) Journal of Invertebrate Pathology 84 (2003) 149–158 Journal of INVERTEBRATE PATHOLOGY www.elsevier.com/locate/yjipa * Corresponding author. Fax: 1-208-885-7760. E-mail address: sanforde@uidaho.edu (S.D. Eigenbrode). 1 Present address: University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, 2199 S. Rock Rd., Ft. Pierce, FL 34945-3138, USA. 0022-2011/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jip.2003.10.001