|Jurat-fuentes, Juan Luis|
Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2009
Publication Date: 8/1/2009
Citation: Blanco, C.A., Gould, F., Vega-Aquino, P., Jurat-Fuentes, J., Abel, C.A., Perera, O.P. 2009. Response of Heliothis virescens (Lepidoptera: Noctuidae) strains to Bacillus thuringiensis Cry1Ac incorporated into different insect artificial diets. Journal of Economic Entomology. 102(4):1599-1606. Interpretive Summary: The development of resistance of the tobacco budworm to Bt cotton is a possibility that can occur due to the constant exposure of these pest to this types of plants. To be proactive in the detection of this resistance, and properly implement corrective measures, bioassays should be implemented as a frequent routine for early detection of resistance. This bioassay work is usually performed utilizing insect artificial diets and different laboratories employ a wide range of commercially-available and locally-produced diets. Also, since the response of this insect has been measured over time, significant discrepancies in lethal concentrations measured at different time points, can give us an idea of the development of resistance. There is a possibility that these discrepancies might be due to the intrinsic response of different insect artificial diets, and that is what we tested in this study. We compared three locally-produced diets and a commercially-available one, finding that the concentration of Bacillus thuringiensis Cry1Ac to kill the tobacco budworm can be lower in the locally-produced diet that in the commercial one. These findings are a first step in documenting what variation can be expected if two or more studies are compared in which different insect artificial diets were utilized.
Technical Abstract: Susceptibility to the Cry1Ac toxin from Bacillus thuringiensis in Heliothis virescens is usually measured by performing bioassays under laboratory conditions. Currently there is great interest and research devoted to this insect because it is one of the main targets of B. thuringiensis-expressing transgenic cotton varieties and a potential insect to acquire resistance to this biological insecticide. Current resistance monitoring programs use bioassays with field collected insects to test for changes in Cry1Ac susceptibility over time. An accurate comparison of Cry1Ac susceptibility among H. virescens samples is challenged by several important methodological aspects, especially if different insect artificial diets are used to perform bioassays. In this study, we compared Cry1Ac susceptibility of four different-origin H. virescens colonies when challenged with this toxin incorporated into four different insect artificial diets. Our data clearly suggests that at least for one of the commercial diets (Bio-Serv), susceptibility to Cry1Ac was decreased in all the H. virescens colonies tested. Bio-Serv diet was one of the least significantly consumed diets by the larvae of the four different colonies, which might indicate that lower consumption might have allowed the insects to encounter less Cry1Ac toxicity. The opposite pattern was observed with a wheat germ - soybean diet (ARS), that was one of the most consumed diet and produced one of the highest Cry1Ac susceptibilities in almost all the four H. virescens colonies. Additionally, larvae fed Bio-Serv diet seem to display slower Cry1Ac toxin activation compared to larvae fed any of the other diets tested. Our data highlight the importance of using common diets when comparing Cry1Ac susceptibility between diverse H. virescens strains or at different times.