Evaluation of entomopathogens and plant extracts as options for integrated pest management of tuta absoluta meyrick (lepidoptera: gelechiidae) for enhanced tomato productivity in Rwanda

Abstract

Tomato (Solanum lycopersicum L.) is an economically and nutritionally important crop in Rwanda. However, its production is threatened by the invasive tomato leaf miner (Tuta absoluta Meyrick) since 2015. Options for integrated pest management (IPM) for its control under Rwandan conditions have not been developed. The main objective of this study was to contribute to enhanced tomato productivity and fruit quality in Rwanda through evaluation of entomopathogens and plant extracts as options for integrated management of T. absoluta. Bioassay experiments were conducted at the Rwanda Agriculture and Animal Resources Development Board to determine: the potential of entomopathogenic nematode (EPN) isolates from Rwanda to control T. absoluta, the pathogenicity of selected commercial formulations of entomopathogenic fungi (EPFs) and the bioactivity of local plant extracts (PEs) against the pest. The two most effective EPNs, EPFs, and PEs were further evaluated in two field trials to determine their efficacy against T. absoluta infestation and their effects on tomato growth, yield, and fruit quality. The results indicated that all evaluated local EPN isolates caused high T. absoluta larval mortality (53.3% - 96.7%) only 24 h after inoculation and the mortality reached 100% after three days. The outstanding isolates were Steinernema sp. RW14-M-C2a-3 and Steinernema sp. RW14-M-C2b-1. The evaluated EPFs were also pathogenic against T. absoluta with Metatech® WP (Metarhizium anisopliae, Strain FCM Ar 23B3) and Beauvitech® WP (Beauveria bassiana, Strain J25) recording the highest mortality rates (82.8% and 60.8%) and LT50 values of 3.9 and 5.2 days, respectively. The highest T. absoluta larval mortality rates recorded five days after treatment with plant extracts were 35.1% and 24.9% for Tephrosia vogelii and Phytolacca dodecandra, respectively while azadirachtin caused more than 64% after only two days and 100% after five days. Under field conditions, the entomopathogens (EPNs and EPFs) and azadirachtin exhibited higher efficacy than the plant extracts and the controls with the maximum leaflet damage obtained 10 weeks after transplanting varying between 59.7% and 74.7% while plots treated with the synthetic insecticide, imidacloprid (positive control) recorded 80.0% - 92.1% damage. The entomopathogens and azadirachtin also increased the yield of healthy fruits per plant (average of two trials) 4.8, 4.5, 4.2, 4.1 and 5.0 folds for Steinernema sp. RW14-M-C2a-3, Steinernema sp. RW14-M-C2a-3, Metatech® WP, Beauvitech WP, and azadirachtin,nrespectively, as compared to the positive control. These entomopathogens and azadirachtin are effective against the pest without compromising fruit quality and should be included in IPM of T. absoluta in Rwanda. Further studies are recommended on their possible combinations and efficacy under greenhouse conditions.