Susceptibility of Spodoptera exigua to 9 toxins from Bacillus thuringiensis Patricia Herna ´ndez-Martı ´nez, Juan Ferre ´, Baltasar Escriche * Departament de Gene `tica, Universitat de Vale `ncia, Dr. Moliner 50, 46100 Burjassot (Valencia), Spain Received 17 April 2007; accepted 5 November 2007 Available online 12 November 2007 Abstract Nine of the most common lepidopteran active Cry proteins from Bacillus thuringiensis have been tested for activity against Spodoptera exigua. Because of possible intraspecific variability, three laboratory strains (FRA, HOL, and MUR) have been used. Mortality assays were performed with the three strains. LC 50 values for the active toxins were determined to the FRA and the HOL strains, whereas sus- ceptibility of the MUR strain was assessed using only two concentrations. The results showed that Cry1Ca, Cry1Da, and Cry1Fa were the most effective toxins with all strains. Cry1Ab was found effective for the HOL strain, but very little effective against FRA (6.5-fold) and MUR strains. Cry1Aa and Cry1Ac were marginally toxic to all strains, whereas the rest of the toxins tested (Cry1Ba, Cry2Aa, and Cry2Ab) were non toxic. Significant differences in susceptibility among strains were also found for Cry1Da, being the FRA strain 25-fold more susceptible than the HOL strain. Growth inhibition, as an additional susceptibility parameter, was determined in the FRA strain with the 9 toxins. The toxicity profile obtained differed from that observed in mortality assays. Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, Cry1Da, and Cry1Fa toxins produced a similar larval growth inhibition. Cry2Aa had a lower but clear effect on larval growth inhibition, whereas Cry1Ba and Cry2Ab did not have any effect. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Cry proteins; Insect control; Intraspecific variability; LC 50 ; EC 50 1. Introduction Spodoptera exigua (Hu ¨bner), the beet armyworm, is an important insect pest with a world wide distribution. The beet armyworm has a wide host range, and it is considered a serious pest of vegetables and flower crops. Nowadays chemical insecticides such us methomyl, cypermethrin, and carbaryl are being used for controlling this insect pest (Liburd et al., 2000). Chemical insecticides are not the saf- est alternative, since they cause damage to the environment and they are harmful to animals and humans. One of the best alternatives to chemical insecticides is the use of bioin- secticides based on Bacillus thuringiensis. This is a gram- positive bacterium, which produces insecticidal crystal pro- teins (called Cry proteins, Cry toxins, or Bt toxins), which are highly specific against their target insects (Crickmore et al., 1998; Schnepf et al., 1998). They are produced as full-length proteins (called protoxins) which form a crystal- line inclusion during the sporulation phase. After the parasporal crystals are ingested by the insect larvae, they are solubilized and finally processed by midgut proteases to render the active toxins. These bind to specific receptors in the brush border membrane of midgut epithelial cells, and then it is believed that the toxin inserts into the mem- brane producing pores which lead to an osmotic imbal- ance, cell lysis and insect death. Recently, some variations regarding to the previous model has been pro- posed by different authors (Bravo et al., 2004; Jurat-Fuen- tes and Adang, 2006; Zhang et al., 2005; Leonardi et al., 2007). There are a number of studies which have determined the relative toxicity of the individual Cry toxins for differ- ent insect pests and a very useful data base exists (http:// www.glfc.forestry.ca/bacillus/). However, differences in 0022-2011/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jip.2007.11.001 * Corresponding author. Fax: +34 96 354 3029. E-mail address: baltasar.escriche@uv.es (B. Escriche). www.elsevier.com/locate/yjipa Available online at www.sciencedirect.com Journal of Invertebrate Pathology 97 (2008) 245–250 Journal of INVERTEBRATE PATHOLOGY