RESUMEN
A range of crops have been transformed with delta-endotoxin genes from Bacillus thuringiensis (Bt) to produce transgenic plants with high levels of resistance to lepidopteran pests. Parasitoids are important natural enemies of lepidopteran larvae and the effects of Bt plants on these non-target insects have to be investigated to avoid unnecessary disruption of biological control. This study investigated the effects of Cry1Ac-expressing transgenic oilseed rape (Brassica napus) on the solitary braconid endoparasitoid Cotesia plutellae in small-scale laboratory experiments. C. plutellae is an important natural enemy of the diamondback moth (Plutella xylostella), the most important pest of brassica crops world-wide. Bt oilseed rape caused 100% mortality of a Bt-susceptible P. xylostella strain but no mortality of the Bt-resistant P. xylostella strain NO-QA. C. plutellae eggs laid in Bt-susceptible hosts feeding on Bt leaves hatched but premature host mortality did not allow C. plutellae larvae to complete their development. In contrast, C. plutellae developed to maturity in Bt-resistant hosts fed on Bt oilseed rape leaves and there was no effect of Bt plants on percentage parasitism, time to emergence from hosts, time to adult emergence and percentage adult emergence from cocoons. Weights of female progeny after development in Bt-resistant hosts did not differ between plant types but male progeny was significantly heavier on wildtype plants in one of two experiments. The proportion of female progeny was significantly higher on Bt plants in the first experiment with Bt-resistant hosts but this effect was not observed again when the experiment was repeated.
Asunto(s)
Bacillus thuringiensis/genética , Brassica rapa/parasitología , Himenópteros/fisiología , Mariposas Nocturnas/fisiología , Plantas Modificadas Genéticamente/parasitología , Animales , Toxinas Bacterianas/genética , Toxinas Bacterianas/farmacología , Peso Corporal , Brassica rapa/genética , Productos Agrícolas/genética , Productos Agrícolas/parasitología , Femenino , Interacciones Huésped-Parásitos , Himenópteros/efectos de los fármacos , Resistencia a los Insecticidas , Larva/efectos de los fármacos , Larva/crecimiento & desarrollo , Masculino , Metamorfosis Biológica , Mariposas Nocturnas/efectos de los fármacos , Plantas Modificadas Genéticamente/genética , Pupa/efectos de los fármacos , Pupa/crecimiento & desarrollo , Razón de Masculinidad , Factores de TiempoRESUMEN
Crops expressing Bacillus thuringiensis (Bt) insecticidal Cry proteins are grown on millions of hectares. Recommendations to delay resistance are based on a high expression/refugia strategy that aims to kill resistant heterozygotes and enable some susceptible insects to survive. Leaf-dip bioassays on F1 crosses of Malaysian populations of diamondback moth (Plutella xylostella (L)) showed that CrylAc resistance was not fully recessive. The survival of ca 50% of heterozygotes on Bt canola (Brassica napus L) leaves expressing low concentrations of CrylAc agreed with a non-fully-recessive model for resistance. Extrapolations based on log dose-logit mortality regressions for heterozygotes using leaf-dip bioassays showed that a relatively high level of expression, of ca 2000 ng CrylAc mg(-1) total leaf protein, would be required to give 90% mortality to heterozygotes. If high enough levels of expression of Bt toxin to kill heterozygotes cannot be achieved and maintained under field conditions, the effectiveness of the high-dose/refugia strategy would be reduced.
Asunto(s)
Proteínas Bacterianas/genética , Brassica napus/parasitología , Endotoxinas/genética , Mariposas Nocturnas/genética , Plantas Modificadas Genéticamente/parasitología , Animales , Bacillus thuringiensis , Toxinas de Bacillus thuringiensis , Proteínas Bacterianas/análisis , Proteínas Bacterianas/biosíntesis , Toxinas Bacterianas/biosíntesis , Toxinas Bacterianas/genética , Bioensayo , Brassica napus/química , Brassica napus/genética , Endotoxinas/análisis , Endotoxinas/biosíntesis , Femenino , Regulación de la Expresión Génica de las Plantas , Genes de Insecto , Genes Recesivos/genética , Proteínas Hemolisinas , Heterocigoto , Resistencia a los Insecticidas/genética , Larva/genética , Larva/crecimiento & desarrollo , Masculino , Mariposas Nocturnas/crecimiento & desarrollo , Herencia Multifactorial , Control Biológico de Vectores , Hojas de la Planta/química , Plantas Modificadas Genéticamente/genéticaRESUMEN
Parasitoids are important natural enemies of many pest species and are used extensively in biological and integrated control programmes. Crop plants transformed to express toxin genes derived from Bacillus thuringiensis (Bt) provide high levels of resistance to certain pest species, which is likely to have consequent effects on parasitoids specialising on such pests. A better understanding of the interaction between transgenic plants, pests and parasitoids is important to limit disruption of biological control and to provide background knowledge essential for implementing measures for the conservation of parasitoid populations. It is also essential for investigations into the potential role of parasitoids in delaying the build-up of Bt-resistant pest populations. The diamondback moth (Plutella xylostella), a major pest of brassica crops, is normally highly susceptible to a range of Bt toxins. However, extensive use of microbial Bt sprays has led to the selection of resistance to Bt toxins in P. xylostella. Cotesia plutellae is an important endoparasitoid of P. xylostella larvae. Although unable to survive in Bt-susceptible P. xylostella larvae on highly resistant Bt oilseed rape plants due to premature host mortality, C. plutellae is able to complete its larval development in Bt-resistant P. xylostella larvae. Experiments of parasitoid flight and foraging behaviour presented in this paper showed that adult C. plutellae females do not distinguish between Bt and wildtype oilseed rape plants, and are more attracted to Bt plants damaged by Bt-resistant hosts than by susceptible hosts. This stronger attraction to Bt plants damaged by resistant hosts was due to more extensive feeding damage. Population scale experiments with mixtures of Bt and wildtype plants demonstrated that the parasitoid is as effective in controlling Bt-resistant P. xylostella larvae on Bt plants as on wildtype plants. In these experiments equal or higher numbers of parasitoid adults emerged per transgenic as per wildtype plant. The implications for integrated pest management and the evolution of resistance to Bt in P. xylostella are discussed.