Evaluation of effectiveness of different entomopathogenic fungal against Tribolium confusum vector of aflatoxigenic fungi in stored maize

BARRA, P.; NESCI, A; ETCHEVERRY, M.

Mendoza

Conferencia; MYCORED ARGENTINA ISM 2011 Conference, Strategies to reduce the impact of mycotoxins in Latin Amrica in a global context; 2011

MYCORED ARGENTINA

Maize can be colonized by fungal communities at harvest and storage, among these fungi some species, such as Aspergillus section Flavi produce toxins associated with harmful effects on animal and human health. Also insects such as Sitophilus zeamais, Rhyzopertha dominica and Tribolium confusum cause significant damage to stored maize and its constant movement within a granary ecosystem contributes the dispersal of aflatoxigenic fungi. Frequent use and exposure to increasing dosages of insecticides has caused insects resistant. A promising strategy with good potential to minimize the adverse effects of insecticides is the use of entomopathogenic fungi as microbial control agents. The purpose of this study was to determine the capacity of different isolates of entomopathogenic fungi to control insect vectors of aflatoxigenic fungi. Entomopathogenic fungi were grown on PDA (potato dextrose agar) for 7 days at 25°C to obtain heavily sporulating cultures to prepared spores suspension. Subsamples of 0.1 g of Tribolium confusum were treated by immersion for 30 sec with 600 µl of 107 spores/ml suspension of each entomopathogenic fungi, in order to evaluate the adherence of viable conidia to the insects. Then the treated insects were homogenized with 0.1% peptone-water and 0.1 ml of dilution was spread plate on PDA media and incubated at 25°C for 7 days. The fungal count was expressed as log10 per g of insect . The mean lethal time (LT50), the number of days until 50% of insects were dead was determined for each of the fungal suspension. For this, subsamples of maize grains of 500 g were put into plastic jars and then twenty adults of Tribolium confusum treated by immersion for 30 sec with 600 µl of 107 spores/ml suspensions of each entomopathogenic fungi were introduced in each jar. All jars were placed in a chamber with controlled conditions (27 ± 1°C, 70 ± 5% r.h., in the dark). Mortality was analyzed during 20 days and compared with the untreated control samples. All dead insects were placed directly on plates containing water agar, which were incubated at 25°C for 7 days to confirm that the inoculated fungus were the causal agent of the death of insects. The results showed that the viability of conidia of all isolates ranged between 5 and 7 log. LT50 values varied from 3 and more than 15 days. The highest percentage of fungi (59.2 %) showed a LT50 between 3 and 10 days, 18.5 % between 10 and 15 days and 22.2 % over 15 days. That is, most of the isolates demonstrated the shortest LT50. Results of these laboratory assessments are essential steps to consider the fungi evaluated as potential biocontrol agents.