LABORATORY AND FIELD COMPARISONS OF STRAINS OF BACILLUS-THURINGIENSISFOR ACTIVITY AGAINST NOCTUID LARVAE USING GRANULAR FORMULATIONS (LEPIDOPTERA)

The toxicity of 6 strains of Bacillus thuringiensis representing 4 ser ovars, aizawal, galleriae, kurstaki, and kumamotoensis, were evaluated in laboratory and field studies for activity against 3 lepidopteran c rop pests, Heliothis virescens (F.), Spodoptera exigua (Hubner), and T richoplusia ni (Hubner). Results of laboratory tests showed that some B. thuringiensis strains isolated from Mexico (GM-7 and GM-10), were s ufficiently toxic to warrant further investigation as potential bioins ecticides for lepidopteran control. Nixtamalized corn flour was used t o create a B. thuringiensis granule formulation that remained toxic Fo r at least 2 yr. Field tests were conducted in 1994 and 1995 on corn p lots in a semiarid region in central Mexico. Granular formulations wer e made with 2% (wt:wt) or 4% (wt:wt) of a spore-crystal B. thuringiens is concentrate and tested with carbaryl and Dipel 2X. In 1994, the cor n yields obtained from plots treated with 2 strains from the Howard Du lmage collection (HD-187 and HD-199-kurstaki and galleriae, respective ly) and 1 Mexican strain (GM-10, aizawai) at 2% were significantly hig her than From plots treated with Dipel 2X or the untreated control. Fo rmulations of 4% containing strains GM-7, GM-10, HD-187, HD-193, and H D-263 provided better insect control than formulations made with C-4. These tests indicated that B. thuringiensis survived in the nixtamaliz ed flour matrix complex and was effective in causing pest reduction. I n the 1995 field test, the 3 most toxic strains were evaluated individ ually 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 wit h strain HD-263 (kurstaki), which had originally shown the highest eff icacy in laboratory bioassays, and with strain GM-10 (aizawai), formul ated 2 yr earlier. These data suggest that the product can be produced and retain shelf life suitable for commercialization.