Insecticides influence colonization and toxin accumulation by Aspergillus and Fusarium in orange and white maize flour varying in susceptibility to mycotoxigenic fungal pathogens

Abstract

Mycotoxigenic Aspergillus and Fusarium species that contaminate cereal crops with aflatoxins and fumonisins are a major threat to food security, human and animal health. Cases of mycotoxin contamination have been on the rise globally, with causes of such surges raising research interests. We conducted factorial experiments in the laboratory to test the hypothesis that some recently produced improved maize varieties that are not purposively bred for resistance to storage insect pests and mycotoxigenic fungi, require protection by synthetic insecticides, which in turn serve as nutrient sources to mycotoxigenic fungi, leading to invasive growth and enhanced toxin production by fungi, especially in conditions of high moisture. Flour from the improved white maize variety Western 505 that is not purposively bred for resistance to storage insect pests and mycotoxigenic fungi had the highest level of colonisation by A. flavus (6,633 CFU g-1) and Fusarium sp. (10,870 CFU g-1) with consequent elevated concentrations of aflatoxin (203 ppb) and fumonisin (2.23 ppm); while flour from the local yellow maize variety Spindi exhibited the least colonisation and mycotoxin production by A. flavus (4,133 CFU g-1) and Fusarium sp. (6,145 CFU g-1) with the local white Rachar being intermediate, at high moisture conditions. The synthetic insecticides Super Blue Cross (BC) and Actellic Super (AS) promoted growth and mycotoxin production by A. flavus and Fusarium sp.; while Ash promoted growth of A. flavus and Fusarium sp., but suppressed mycotoxin production by the two mycotoxigenic fungi (Aflatoxin and Fumonisin: control=114 ppb and 1.46 ppm; Ash=114 ppb and 1.46 ppm; AS=270 ppb and 2.31 ppm; BC=320 ppb and 3.55 ppm). It is concluded that flours from the local yellow Spindi and local white Rachar maize varieties are less susceptible to infection and mycotoxin contamination by A. flavus and Fusarium sp. compared to the improved white maize variety WH505, which accumulates higher levels of the mycotoxigenic fungi and their toxins, especially when treated with the synthetic insecticides Super Blue Cross and Actellic® Super that act as fungal nutrient sources under high moisture conditions. The implications of these findings are further discussed in the context of some relevant policies.