Laboratory and field comparisons of strains of Bacillus thuringiensis for activity against noctuid larvae using granular formulations (Lepidoptera)

The toxicity of 6 strains of Bacillus thuringiensis representing 4 serovars, aizawai, galleriae, kurstaki, and kumamotoensis, were evaluated in laboratory and field studies for activity against 3 lepidopteran crop pests, Heliothis virescens (F.), Spodoptera exigua (Hubner), and Trichoplusia ni (Hubner). Results of laboratory tests showed that some B. thuringiensis strains isolated from Mexico (GM-7 and GM-10), were sufficiently toxic to warrant further investigation as potential bioinsecticides for lepidopteran control. Nixtamalized corn flour was used to create a B. thuringiensis granule formulation that remained toxic for at least 2 yr. Field tests were conducted in 1994 and 1995 on corn plots in a semiarid region in central Mexico. Granular formulations were made with 2% (wt:wt) or 4% (wt:wt) of a spore-crystal B. thuringiensis concentrate and tested with carbaryl and Dipel 2X. In 1994, the corn yields obtained from plots treated with 2 strains from the Howard Dulmage collection (HD-187 and HD-193--kurstaki and galleriae, respectively) and 1 Mexican strain (GM-10, aizawai) at 2% were significantly higher than from plots treated with Dipel 2X or the untreated control. Formulations of 4% containing strains GM-7, GM-10, HD-187, HD-193, and HD-263 provided better insect control than formulations made with C-4. These tests indicated that B. thuringiensis survived in the nixtamalized flour matrix complex and was effective in causing pest reduction. In the 1995 field test, the 3 most toxic strains were evaluated individually or by mixing 2 in the same granular formulation at a total dose of 3%. The highest yields of corn were obtained from plots treated with strain HD-263 (kurstaki), which had originally shown the highest efficacy in laboratory bioassays, and with strain GM-10 (aizawai), formulated 2 yr earlier. These data suggest that the product can be produced and retain shelf life suitable for commercialization.