Entomopathogenicity of hyphomycete fungi to fruit fly bactrocera invadens (Diptera: Tephritidae) and their potential for biological control on mango
Ouna, Elizabeth Awuor
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Sustainable mango production will rely increasingly on alternatives to conventional chemical insecticides that are environmentally friendly for the management of fruit flies. The use of microbial control agents such as fungi in pest suppression is considered suitable since microorganisms usually exert low environmental impact and are target specific. Application methods of bio-pesticides in the environment, which use minimal amount of inoculum is currently under improvement from inundative to auto-inoculatioe methods. Such devices are usually designed to attract insects into focus of the entomopathogen and use the insect as a vector for transmission of pathogen to other members of its population. Virulence of 24 isolates of metarhizium anisopliae (Metschnik.) Sorok. and Beauveria hassiana (Bals.) Vuill. (Ascomycota: Hypocrales) collected from the microbial culture collection at International Center of Insect Physiology and Ecology and isolates collected pathogen survey of the study was evaluated on immature adult Bactrocera invadens Drews, Tsuruta and White in the laboratory, followed by testing sub-lethal effects of one of the most pathogenic isolates on fecundity and fertility. Out of the 24 isolates of M. anisopliae and B. bassiana screened for pathogenicity against B. invadens, 7 isolates (ICIPE 20, 43, 62, 69, 295, 303 and M. anisopliae var. acridium IMI 330189 or ICIPE 21, available as Green Muscle® induced significantly higher mortalities (ranging from 93.7% to 94.8%) than the rest of the isolates. Lethal time to 50% mortality (LT50) ranged from 2.8 to 3.6 days (M. anisopliae) and 2.7 to 2.8 days (B. bassiana); and slope value estimates of 0.96 and 0.88 respectively at 5 days post treatment. Sub-lethal effect of selected isolate of M. anisopliae (ICIPE 20) on B. invadens exhibited significantly lower egg fecundity and hatchability in treated flies than control at levels of 6.8 and 28.4 eggs per fly per day and a gross of 28.8 eggs against 113.2 eggs/female at 4 days post inoculation; and hatchability of 17.2% and 64.8% respectively. Evaluation of three different bait formulations contaminated with M. anisopliae was carried out based on resident time flies spent on the baits, persistence of conidia on the baits exposed in the field under mango canopy and pathogenicity of fungus contaminated baits to adult B. invadens in the laboratory at 0, 7, 14, 21, and 28 days of exposure in the field under mango canopy.This caused an overall reduction in fecundity between 80.2 - 99.8% and 51- 80% in hatchability. Resident time spent by B. invadens on M. anisopliae-contaminated food baits was not significantly different among the 3 baits (2% NuLure, local bait DuduLure® and a diluted 10% DuduLure®); however germination of conidia from 10% DuduLure and 2% NuLure was significantly higher than that DuduLure® at 14, 21 and 28 days of field exposure and were at the levels of 83%, 96%, 78% (10% DuduLure); 74%, 95%, 96% (2% NuLure); 54% 6%, 6% (DuduLure©). Two-way ANOVA demonstrated there was significant effect of time and environmental factors on conidial germination. Mortality of flies treated with fungus contaminated- 10% DuduLure and 2%NuLure baits was significantly higher than in treated DuduLure®. Mortality of flies exposed treated 10% DuduLure (84%) was significantly higher than 2% NuLure (64%) at 21 days but was significantly lower (48%) than 2% NuLure (90%) by 28 days. In conclusion this study has identified 7 new hyphomycete isolates as highly pathogenic to adult stages of B. invadens. Among the isolates M. anisopliae ICIPE 20 can cause reduction in egg fecundity and hatch rate. Wooden sphere soaked in commercial (NuLure) or locally developed bait (DuduLure) and contaminated with spores of M. anisopliae can provide a viable option for the management of B. invadens on mango. The adaptability of this technology will depend in part on its efficacy to suppress populations of B. invadens in the field.