© 2006 The Authors Entomologia Experimentalis et Applicata 121: 145–157, 2006 Journal compilation © 2006 The Netherlands Entomological Society 145 Blackwell Publishing Ltd Location of resistance factors in the leaves of potato and wild tuber-bearing Solanum species to the aphid Myzus persicae A. E. Alvarez 1,4 *, W. F. Tjallingii 1 , E. Garzo 2 , V. Vleeshouwers 3 , M. Dicke 1 & B. Vosman 4 1 Wageningen University, Laboratory of Entomology, 6700 EH Wageningen, The Netherlands, 2 Centro de Ciencias Medioambientales, CSIC, Departamento de Protección Vegetal, Serrano 115 dpdo, 28006 Madrid, Spain, 3 Wageningen University, Laboratory of Plant Breeding, 6700 AA, Wageningen, The Netherlands, 4 Plant Research International, Department of Biodiversity and Breeding, 6700 AA, Wageningen, The Netherlands Accepted: 4 July 2006 Key words: Solanaceae, Solanum tuberosum, wild potatoes, Homoptera: Aphididae, green peach aphid, resistance mechanisms, EPG, probing, feeding behaviour, phloem Abstract Analysis of electrically recorded feeding behaviour of aphids was combined with colony-development tests to search for sources of resistance to Myzus persicae (Sulzer) (Homoptera: Aphididae) in tuber- bearing Solanum species (Solanaceae), aiming at a reduction of potato leaf roll virus (PLRV) transmission. Twenty genotypes, originating from 14 gene bank accessions, representing 13 wild tuber-bearing Solanum spp., three Solanum tuberosum L. (potato) cultivars, and one S. tuberosum breeding line, were selected. Colony-development tests were carried out in no-choice experiments by placing adult aphids on plants of each genotype and counting numbers of nymphs and adults on young plants after 8 and 15 days, and on flowering plants after 14 and 30 days. Large differences were observed among genotypes: some developed small colonies and others developed large ones. Also, in a few genotypes, resistance in mature plants was different for leaves of different ages; young leaves were resistant to aphids whereas old senescent leaves were susceptible. The electrical penetration graph (DC-EPG system) technique was used to study aphid feeding behaviour on each Solanum genotype for 6 h. Electrical penetration graph (EPG) results also showed large differences among the genotypes, indicating resistance at the leaf surface and at three different levels of plant tissue (epidermis, mesophyll, and phloem). Therefore, it was concluded that different mechanisms of resistance to M. persicae exist among the genotypes analysed. EPGs recorded from aphids on Solanum berthaultii Hawkes and Solanum tarijense Hawkes with and without glandular trichomes showed that strong surface resistance can bias EPG parameters associated with resistance located in deeper tissues. Experimental evidence is presented that the resistance to aphids in the genotypes with glandular trichomes strongly depends on these morphological structures. Introduction With 235 recognized species, wild Solanum spp. (Solana- ceae) represent a rich biological diversity, although only a few species gave rise to the cultivated potato (Hawkes, 1990). Until now, genes from at least 18 wild Solanum species have been incorporated into North American and European potato varieties. So far, no commercial potato varieties have been developed specifically for resistance to insects (Flanders et al., 1999). The green peach aphid, Myzus persicae (Sulzer) (Homoptera, Aphididae), has a worldwide distribution and causes damage principally indirectly by its ability to transmit plant viruses (Salazar, 1996). At least 54 viruses are known to infect potatoes (Brunt et al., 1996) and aphids are their most important vectors (Peters, 1987; Raman & Radcliffe, 1992; Jeffries, 1998). In potato crops, the percentage of plants infected with potato leaf roll virus (PLRV) strongly depends on the number of aphids that have previously visited PLRV- infected plants (Beekman, 1987). A low degree of PLRV * Correspondence: A. E. Alvarez, Wageningen University, Laboratory of Entomology, 6700 EH Wageningen, The Netherlands. E-mail: adriana.alvarez@wur.nl