Diversity and potential of nematodes associated with bacteria for bio-control of insect pests in Uganda
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A significant number of nematodes are associated with bacteria. These nematodes live mainly in the soil where they feed on bacteria and other soil organisms. They play commensal, obligate parasitism and pathogenesis associations with insects infecting many different types of soil insects and have been found ecologically diverse habitats, from cultivated fields to deserts. In Uganda, information on the diversity, distribution and potential of nematodes associated with bacteria for control of insect pests is currently non-existent. Until now, no systematic survey has been conducted to document the presence of these nematodes in the country. For current study, 900 soil samples were collected from nine agro-ecological zones of Uganda (viz central wooden savanna, west nile farmlands, northern moist farmlands, Mt. Elgon farmlands, Lake Victoria crescent and Mbale farmlands, southern and eastern Lake Kyoga basin, southwestern highlands, southwestern grass farmlands, western mid-altitude farmlands and the Semliki flats) with the objectives of isolating and identifying nematodes associated with bacteria, and characterize their infectivity against some important insect crop pests. Nematodes were recovered by a baiting technique, which involve the use of trap insects that are known to be preferred hosts for nematodes. A total of 54 samples were found to contain nematodes associated with bacteria that were identified as bacterivores based on morphological characterisation and on appearance of Galleria mellonella cadaver. Highest population of bacterivores was recovered from southern western highlands and lowest in western mid-altitude farmlands and Semliki flats and northern moist farmlands. Bacterivores were not found in Mt. Elgon farmlands and Lake Victoria crescent and Mbale farmlands. On cropped fields, highest bacterivores populations were recovered in soils from sorghum, sweetpotatoes and a combination of maize, cassava and groundnuts fields and lowest in soils from egg plants and sunflowers fields. Bacterivores were not recovered from soyabeans and sugarcane fields. Loam soil had the highest number of isolates while sand soil and clay soil had the lowest nematode population. Fallow fields, bare soil, soil rich in organic matter and stony soils did not favour nematode populations. Among the isolates recovered, isolates PC4, PS17, IC15, KP8, Kab20, Mas68 and Mas57 were considered for nematode characterized and tested on insect pests because they exhibited higher survival ability under laboratory conditions. The characterized isolates were tested on larvae and adults of sweetpotato weevils and banana weevils to determine their infectivity in the laboratory using sand barrier bioassays. In all experiments, sand was washed, heat sterilized at 110oC for 3 days and passed through a 2 mm sieve. Results indicated that sweetpotato weevil mortality increased with the increase in nematode concentration but isolate Mas68 maintained constant insect mortality at all concentrations. The weevils were most susceptible to isolate Mas57 and isolate Kab20. The pathogenicity of nematode isolates to banana weevils varied greatly and generally mortality of banana weevils were low compared to sweet potato weevils. The increase in mortality with increase in nematode concentration was not observed. The highest mean weevil susceptibility were observed with isolates Mas57 and Mas68, while isolate Kab20 performed moderately and the lowest susceptibility was observed with isolates PS17, KP8 and IC15. The influence of nematode exposure period on banana weevils indicates that isolate PS17 had the highest penetrating ability while with isolates PC4, IC15 and Mas57 no penetrations was recorded. Though percentage recoveries were generally low, the numbers of nematodes recovered increased linearly with exposure period. For sweetpotato weevils highest penetration ability was observed with Kab20, Mas68, IC15, and lowest with KP8. No nematode penetration was observed with isolates PC4, PS17, and Mas57. The susceptibility of sweetpotato and banana weevil larval stages to isolates was determined. The larvae of both sweetpotato weevil and banana weevil were susceptible to all isolates. Sweetpotato larvae were highly susceptible to isolates KP8 and Mas57 while the least susceptibility was recorded with isolate PC4 and Kab20. All isolates had a pronounced homogeneity in infecting the banana larvae though isolate KP8 was the most pathogenic and isolate PC4 had the least pathogenic effect. Overall comparision of the effectivity of nematodes to the adult weevils and their larvae indicated that larvae were highly susceptible to nematode attack.