Effects of the southwestern corn borer on Aspergillus flavus kernel infection and aflatoxin accumulation in maize hybrids

Field studies were conducted in 1995 to 1997 to determine the effect of the southwestern corn borer (SWCB) on Aspergillus flavus kernel infection and aflatoxin accumulation in maize hybrids. In 1995, when A. flavus conidia were applied to silks in a spray and SWCB neonate larvae in maize cob grits were placed in the leaf axil at the top ear of commercial hybrids, aflatoxin contamination and A. flavus kernel infection were highest in plants treated with both the fungus and the insect. In 1996, using the same inoculation and infestation techniques, aflatoxin levels and kernel infection were much lower than in 1995 and SWCB had no effect on aflatoxin contamination or kernel infection. In another study in 1996, the effect of SWCB on aflatoxin contamination and A. flavus kernel infection in hybrids resistant and susceptible to A. flavus was determined. The inoculation-infestation technique involved applying maize cob grits containing A. flavus conidia and SWCB larvae to silks. When SWCB was combined with A. flavus, aflatoxin levels and kernel infection were dramatically higher than in hybrids inoculated with A. flavus alone, regardless of whether the hybrids were resistant or susceptible to A. flavus. In 1997, die interaction of A. flavus and SWCB was determined on hybrids resistant and susceptible to A. flavus and on a commercial hybrid with and without the Bacillus thuringiensis (Bt) toxin. Maize cob grits were used to inoculate A. flavus and infest SWCB on the silks 7 or 21 days after midsilk (50% of the plants in a row had silks emerged). All four hybrids had the highest levels of Aspergillus spp. kernel infection and aflatoxin contamination when A. flavus and SWCB were applied at 21 days after midsilk. These studies indicate that SWCB can substantially increase aflatoxin levels when combined with A. flavus. However, inoculation and infestation techniques, placement of the fungus and the insect, and timing of inoculation and infestation are all critical in demonstrating a synergistic relationship between A. flavus and SWCB on aflatoxin contamination of maize.