Potential control of the seed bug Elasmolomus pallens by entomopathogenic fungi and assessment of its risks for the transmission of aflatoxins / Umaru Fredrick Fidelis

Umaru Fredrick, Fidelis (2021) Potential control of the seed bug Elasmolomus pallens by entomopathogenic fungi and assessment of its risks for the transmission of aflatoxins / Umaru Fredrick Fidelis. PhD thesis, Universiti Malaya.

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Abstract

Elasmolomus pallens is a post-harvest insect pest which causes yield loss of peanuts and sesame in Africa and Asia. In view of mitigating the use of synthetic insecticides on the environment, this study evaluated the potential of entomopathogenic fungi for the biological control of E. pallens and assessment of the bug rostrum for its risks of aflatoxins transmission. Adults bugs were dipped briefly in different conidial concentrations of the isolates and observed at 25 ± 2 ℃, 80 ± 10 RH and 14: 10 L: D for 10 days. Dose-response bioassay employed dipping and filter paper methods for both 0.05% Tween 80 and 5% peanut oil formulations. In the dipping bioassay (Tween 80 formulations), A. flavus caused significant mortalities (F5, 60 =5.644, p ˂0.001) with LC50 of 6.75×106 conidia/ml and LT50 of 3.3 days as well as M. anisopliae (F5, 60 =6.493, p ˂0.001) with LC50 of 8.0×106 conidia/ml and 3.6 days. However, significant differences in virulence was obtained between A. flavus (M =44.56) and M. anisopliae (M =65.09) t (10) =4.830, p < 0.001, when their mortality rates under this bioassay was compared. Using same formulations for filter paper bioassay, significant mortalities of E. pallens were obtained for A. flavus (F5, 60 =5.318, p < 0.001) with an LC50 values of 9.36×107 conidial/ml and LT50 of 5.0 days while M. anisopliae (F5, 60 =8.825, p < 0.001) had an LC50 of 6.85×106 conidia/ml and LT50 of 3.8 days. Comparing their mortality rates, significant difference was obtained between A. flavus (M =54.55) and M. anisopliae (M =71.46) t (10) =5.311, p < 0.001). For dipping bioassay (oil formulations), mortalities were significant for A. flavus (F5, 60 =5.589, p < 0.001) with LC50 of 4.75×106 conidia/ml and LT50 of 3.2 days; and M. anisopliae (F5, 59 =4.314, p < 0.002) had LC50 of 2.14×106 conidia/ml and LT50 of 3.0 days. No difference in virulence was observed between A. flavus (M =60.00) and M. anisopliae (M =62.18) and, t (10) =1.284, p > 0.228, when compared. In the filter paper bioassay, mortalities of E. pallens were significant; A. flavus (F5, 60 =4.394, p < 0.002) with LC50 of 1.95×106 conidia/ml and LT50 of 3.0 days and M. anisopliae (F5, 60 =4.363, p < 0.002) with LC50 of 3.92×106 conidia/ml and LT50 of 3.2 days. No difference in virulence was observed between A. flavus (M =52.64) and M. anisopliae (M =58.91), and t (10) =1.796, p > 0.103. Hydrolytic enzymes activity for A. flavus were 0.587 ± 0.202 U/ml (protease), 0.873 ± 0.028 U/ml (chitinase), 2.513 ± 0.017 U/ml (lipase) while M. anisopliae had 0.739 ± 0.155 U/ml (protease), 0.879 ± 0.023 U/ml (chitinase) and 2.592 ± 0.019 U/ml after 4 days of culture at 25 ℃. A strong positive (r = 0.999; p < 0.03) correlation was obtained for atoxigenic A. flavus while toxigenic strains correlated inversely (r = -0.999; p < 0.03) with E. pallens rostrums between the sampling locations. This study demonstrates that the isolates possess potential against E. pallens and should be considered ideal candidates for field applications.

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