Efficacy of selected biopesticides against the tomato leaf miner, tuta absoluta (meyrick) (lepidoptera: gelechiidae) in Zambia.

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Mazimba, Emma
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The University of Zambia
The tomato leafminer, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is a major invasive insect pest attacking tomatoes in Zambia. Tuta absoluta was recently introduced into Zambia and was first reported in 2016. The pest is known to cause losses of about 80-100% if not managed. Control strategies of T. absoluta include the use of biopesticides. The efficacy of four commercially available biopesticide formulations, Azadirachtin indica (Nimbecidine®), Beauveria bassiana (Bio-power®), Metarhizium anisopliae (Bio-magic®) and Verticillium lecanii (Bio-catch®) were compared with an untreated control for the management of T. absoluta under laboratory and field conditions. The selected biopesticides are fungal entomopathogens with the exception of A. indica, which is a botanical insecticide. This study assessed the most effective biopesticide on larvae mortality and oviposition and assessed effects of the selected biopesticides on leaf damage caused by T. absoluta. In laboratory studies, the efficacy of different concentrations of the selected biopesticides was tested on second instar larvae of T. absoluta. In the field studies, tomato plants were grown at two locations (Lusaka and Chongwe Districts), and the fields were laid out in a randomised block design with three replications. The selected biopesticides were applied to tomato plants on a weekly basis and efficacy was determined by the density of T. absoluta eggs on leaves and from leaf damage using a scoring scale of 1-5. The efficacy of the treatments were analysed using analysis of variance (ANOVA) and means that were statistically different were separated by Fishers least significant difference (LSD) test with α= 0.05. Probit analysis was used to determine the estimated median lethal dose (LC50). All the analyses were performed using Genstat statistical software. The selected biopesticides significantly (P<0001) reduced T. absoluta egg count compared with the untreated control. All the biopesticides exhibited a higher performance than the control in the following ascending order; A. indica (2.8±1.1 and 10.7±1.2), B. bassiana (3.0±0.6 and 10.7±1.9) V. lecanii (4.3±1.2 and 11.7±1.4) and M. anisopliae (6.8±1.4 and 13.0±1.6) for the first and second egg count, respectively. All the biopesticides significantly (P<0001) reduced leaf damage compared to the control. The severity of the leaf damage for all the biopesticides was 25-50% and 50-75% damage in the control. The estimated LC50 for A. indica was 30.4 ± 0.4 μL, while LC50 for B. bassiana, M. anisopliae, V. lecanii were 107.1 ± 0.4μL, 193± 0.4 μL and 118.7 ± 0.4μL, respectively. Thus, A. indica was the most effective biopesticide followed by B. bassiana then V. lecanii, while M. anisopliae was the least effective on larval mortality. The greatest percentage corrected mortality was obtained from A. indica (69.8±8.1% to 88.4±41.9%), followed by B. bassiana (32.6±4.0% to 60.5±16.3%) and V. lecanii (30.2±1.7 to 55.8%±14.6%) and lastly M. anisopliae (32.6±4.0% to 53.5±5.4%). The findings of this study showed that selected biopesticides were effective and should be used by farmers as an important component of Integrated Pest management (IPM) in the control of T. absoluta in Zambia. Further research should evaluate the effectiveness of the selected biopesticides in other agroecological zones of Zambia.
Thesis of Master of Science in Entomology.