Genetic and phenotypic variability in Spodoptera exigua nucleopolyhedrovirus isolates from greenhouse soils in southern Spain Rosa Murillo, Sonia Elvira, Delia Mun ˜ oz, Trevor Williams, Primitivo Caballero * Departamento de Produccio ´ n Agraria, Universidad Pu ´ blica de Navarra, Pamplona 31006, Spain Received 3 August 2005; accepted 1 December 2005 Available online 19 January 2006 Abstract Genotypic and phenotypic variation of SeMNPV was examined in seven isolates of SeMNPV originating from occlusion body (OB) populations in the soil of greenhouses in Spain. Semi-quantitative PCR indicated that some of the isolates were composed of a single dominant genotype, whereas other isolates were composed of two or three genotypes in equal proportions. The coexistence of genotypes could be explained by trade-offs among the three phenotypic traits analyzed, namely pathogenicity (LD 50 ), speed of kill (mean time to death), and OB yields, so that increases in one trait were accompanied by decreases in another. Mixed genotype isolates tended to behave differently to single genotype isolates. Two of the genotypic mixtures were significantly more pathogenic (lower LD 50 values) for Spo- doptera exigua (Hu ¨ bner) larvae than the single genotypes that they comprised. OB yield/insect was greater for single genotype compared to mixed genotype isolates, despite genotype-specific differences in mean times to death. Total OB production/insect was positively cor- related with time to death. Two out of three of the mixed genotype isolates had lower OB yield/mg insect weight at death compared to single genotype isolates. Each genotypic combination appeared to interact to produce a unique phenotype. This suggests the existence of trade-offs between traits leading to the coexistence of distinct genotypes and genotypic mixtures with similar transmissibility. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Spodoptera exigua; Nucleopolyhedrovirus; Soil isolates; Genetic variability; Phenotypic variability 1. Introduction Baculoviruses are pathogens of numerous forest and agricultural pest insect species, particularly the larval stages of Lepidoptera. Their high degree of pathogenicity and host specificity permit their use as biological insecticides (Moscardi, 1999). However, successful pest control with these products has not always been achieved for a number of reasons. The lack of effective control observed following trials against certain pests has been attributed to the use of slow-acting virus strains or isolates with low pathogenicity (Ribeiro et al., 1997; Williams et al., 1999). To overcome these problems, it is necessary to identify and select highly insecticidal strains (those that combine high pathogenicity and fast speed of kill) appropriate for inundative control targeted at a particular host. This is likely to be possible for numerous host-crop combinations since, based on the diversity of insect species, the interspecific diversity of bac- uloviruses is expected to be enormous (Miller, 1997). How- ever, what is probably more interesting for baculovirus insecticide development is the diversity existing within populations of a particular baculovirus species (Van Regenmortel, 2000). Restriction endonuclease profiles of field-collected isolates invariably reveal a high degree of genotypic diversity occurring between different isolates (Cooper et al., 2003; Getting and McCarthy, 1982; Hodg- son et al., 2001), while in vitro and in vivo cloning of such isolates demonstrates the presence of a wide array of geno- typically distinct variants within each isolate (Knell and Summers, 1981; Lee and Miller, 1978; Maeda et al., 1990; Smith and Crook, 1988). This genotypic variation often 1049-9644/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2005.12.001 * Corresponding author. Fax: +34 948 169 732. E-mail address: pcm92@unavarra.es (P. Caballero). www.elsevier.com/locate/ybcon Biological Control 38 (2006) 157–165