Studies on the Role of the Coleopteran Species Callosobrochus Maculatus Fab., Sitophilus Zeamais Mots. And Tribolium Castaneum Herbst. In The Dispersal of Fungi among Stored Grains of Maize (Zea Mays L.) And Rice (Oryza Sativa L.) And Seeds Of Cowpea (Vigna Unguiculata Walp.) And Bambara Groundnut (Vigna Subterranea L.) Verdc]

Abstract

Seven Aspergillus species, A . clavatus, A . flavus, A.fumigatus, A. niger, A . ochraceus, A. sulpl1ureus and A. ustus have been used to investigate the role of three Coleopteran insect pests, namely, Callosobrochus maculatus, sitopl1ilus zeamais and Tribolium castaneum in the persistance and spread of contaminant fungi among grains of maize (Zea mays) and rice (oryza sativa) and seeds of bambara groundnut (vigna subterranea) and cowpea (Vigna unguiculata) . The Aspergillus species were among fungi isolated from the grains and seeds. Bambara groundnut seeds on saIe at Kaneshie , La, Madina, Makola and Mallam Atta markets in Accra district contained species of Absidia, Aspergillus, Cladosporium, Fusarium, Neurospora, Paecilomyces, Penicillium and PIll lularia. The dominant genera were Aspergillus and Penicillium represented by five and [our species, respectively, and the dominant species were Aspergillus flavus and Aspergillus niger . The predominant apecles of rowpea seeds from the same markets were Aspergillus £lnvus <lnu l'ner:Jlomyces puntonii among 19 species, and, the dominant genera were Aspergillus, Paecilomyces and Penicillium . The rest of the genera were Cladosporium, Drecl1s1era, Epicoccum, Fusarium, Mucor, Neocosmospora, Neurospora, Phoma, pullularia, Rl1izopus and verticillium. Aspergillus flavus and Penicillium citrinum were the predominant spec i es on maize grains from the five markets among 16 contaminant fungal species belonging to the genera Aspergillus, Fusarium, Mucor, Neocosmospora, Neurospora, Penicillium, Pullularia and Rl1izopus. Rice grains had the shortest list of genera. The fungi belonged to only four genera, Aspergillus, Cladosporium, Mucor and Penicillium and there were 14 species in all. The most frequently occurring species >Jere Aspergillus oryzae, Cladosporium herbarum, Penicillium cl1rysogenum and Penicillium expansum. Although all the . seven As pergillus species could grow on insect body leachate agar prepared >J i th leacha te of the three insect pests, the conidia of some of them could germinate in the leacha te of only some of the insects. only A. flavus and A. ocl1raceus condidia germinated in all the three leachates. Germination of the conidia of all the species occurred , anY>Jay, in leachates containing extracts of various tissues of seeds (axis of the embryo , cotyledon and testa) and extracts of grains. Conidia of all the species germinated in solution of dissolved faecal pellets of Callosobrochus maculatus, while conidia of 11.. clavatus, A. ocl1raceus and A . sulplJureus only germinated in the solution of dissolved faecal pellets of Sitophilus zeamais and conidia of also three species, 11. . clavatus, A . flavus and A. ochraceus germinated in the solution of faecal pellets of Tribolium castaneum. Aspergillus flavus conidia adhering to the bodies of sitoplJilus zeamais and Tribolium castaneum were transported through maize grains packed in wide glass tubes. The amount of the conidia detached as the insects moved depended on the size of the spaces among the grains and the frequency of contact between the insects and the grains. s. zeamais lost 87.8, 8~.7 and 76.7 percent of the original load of conidia as the insects travelled over 100cm through grains measuring 5.3-8.3 x 4.04-7.3mm, 8.1-10.2 x 6.0-8.2mm and 9.5-12 . 2 x 7.5-9.5mm, respectively. The corresponding figures for conidia on T. castaneulll 'vere, 88.5, 87.9 and 82.8 per cent respectively. Dead insect bodies were invaded by many fungi despite the presence of large populations of sur face bacter ia. The colonyforming- units of bacteria recorded for C. lIlaculatus, S. zeamais and T. castaneum per ml of suspending medium immediately after death were 27 . 5 x 10', 281 x 10' and 104.5 x 10' respectively i and six days later they were 37.8 x 1~, 5.0 x 1~ and zero, respectively. On the sixth day, Aspergillus flavus was isolated from the bodies of all the three insect pests. In addition, Aspergillus niger, Mucor sp., Rhizopus sp. and Trichoderma Viride were isolatC2.cI ·: c_,: from C. lIlaculatus . , Aspergillus niger and Curvularia sp. from S. zeam~ is and Clasdosporiulll sp. from T. castaneulll. The mycelium growing in the bodies after death might have arisen from inoculum either on the surface of the body or in the gut . For, the gut of the insects had extensive mycoflora. Fourteen, thirteen and sixteen fungal species were isolated from the gut of C. maculatus, S. aeolllois and T. castaneum, respectively. The predominant genera were Aspergillus and Penicillium and five species, namely Aspergillus £lavus, Aspergillus fumigatus, Cladosporium herbarum, PellicilliJ.J.m citinum and Penicillium purpurogenum were isolated from the guts of all the three insect pests. Three Aspergillus species red experimentally to the insects persisted for different lengths of time in the guts. A. flavus was isolated 6,8, and 5 days respectively, after feeding the insects, from the guts of C. maculatus, s. zeamais and T. castaneum. The corresponding survival periods for A . fumigatus were 4,6 and 5 days respecti vely, and for A. oC/lraceus, 6,4 and 3 days respectively. It was concluded that products and dead bodies of the insects would contribute to the persistence of the contaminant fungi and living insects would be responsible for both persistence and dispersal of the fungi in stored grains and seeds. Persistence and dispersal of the fungi could be reduced by measures which control the insect pest popUlation, by periodic removal of dead insect bodies especially in comparatively smaller stocks kept i.n the markets and by exposure of the products to light to drive the insects to the dark base and discourage the frequent migration.