Selection and characterization of Mexican strains of Bacillus thuringiensis active against four major lepidopteran maize pests

In order to isolate novel delta-endotoxins from Bacillus thuringiensis Berliner, a total of 426 native isolates (in varying numbers for each pest) were screened against four major maize pests: corn earworm, Helicoverpa tea; fall armyworm, Spodoptera frigiperda; southwestern corn borer, Diatraea graridiosella, and sugarcane borer, Diatraea saccharalis. Spore-crystal complexes from the isolates were integrated into semi-artificial diets of each pest and mortality was assessed 7 days after treatment. A total of 25 isolates were selected on the basis of highest larval mortality against at least one insect species. Then was no correspondence of the most toxic isolates when tested against the four different insect species. Most of the 25 selected isolates caused higher toxicities against all four pests than the standard strain HD-1. regardless of not achieving 100% mortality in any bioassay. H. zea demonstrated the highest level of mortality (96%) and was susceptible to the largest number of isolates (98). None of the other insect species were found susceptible at levels greater than 60%. All the selected active strains were isolated from stored grain dusts (except for LBIT-167), and had bipyramidal crystals with Cry I-like proteins. Most isolates also formed an associated square (cubic) inclusion, with Cry II-like proteins according to SDS-PACE analysis of their parasporal bodies. The most active isolates will be subjected to further studies, in order to identify putative novel genes to be expressed in transgenic maize.