Characterization and Evaluation of Entomopathogenic Nematodes for the Management of Tomato Leafminer (Tuta Absoluta Meyrick)

Abstract

Tomato (Solanum lycopersicum L.) is one of most known and grown vegetable globally. However, its production is constrained mainly by insect pests among them Tomato leafminer (Tuta absoluta Meyrick.) that causes yield losses of upto 100%. The objective of the study was to characterize and evaluate indigenous entomopathogenic nematodes (EPNs) isolates for the management of T. absoluta on tomato. Five indigenous EPN isolates (TK1, S86, 97, 75 and R52) at different concentrations (100, 150, 200, and 250) of infective juveniles (IJs)/ ml) were evaluated for infectivity on 2nd and 3rd larval stages of T. absoluta under laboratory conditions. Larval mortality (infectivity) was recorded at 24th h interval up to 120 h. The most infective isolate was subjected to morphological and molecular identification and associated symbiotic bacteria isolated and identified. The bacteria larvicidal effect on T. absoluta was evaluated and bacterial crude protein extracted and analysed through SDS-PAGE. All the tested EPN isolates infected and reproduced within T. absoluta. The EPN isolate TK1 was the most infective causing mortality100%; 92±5%), on 2nd and 3rd larval instars, respectively, at a concentration of 100 IJs/ml in the 48th h. The EPN isolate TK1 infective juveniles body was 834.54±87.36 μm (658.60-986.89 μm), hyaline 20.47±3.33 μm (13.45-24.95 μm) and tail 53.22±8.35 μm (40.30-71.51) μm in length. The 1st generation males were J shaped but strongly curved posteriorly almost spiral; spicule 82.05±13.94 μm (57.86-128.30 μm) and gubernaculum 45.28±5.89μm (34.48-54.87 μm) in length. Sequence analysis of ITS region of rDNA of EPN isolate TK1 revealed similarity (83% to 92%) with best BLASTNn hits with closest relative being Steinernema spp. (AY230186.1). The low similarity index result indicates EPN isolate TK1 could be a new species. Sequence analysis of 16S of rDNA symbiotic bacteria TK1, revealed similarity of 97 to 98.93% with best BLASTNn hits with Xenorhabdus sp. My8NJ (AB507811.1 being closest relative (98.93%). Mortality of 68% to 100% at 24 h at different concentrations against T. absoluta was achieved from bacteria isolate TK1 outside the host. The study concludes that EPN isolate TK1 is a Steinernema sp. while its symbiont bacteria TK1 is a Xenorhabdus sp. and both are pathogenic to T. absoluta larvae. From this study, the EPN isolate TK1, its symbiotic bacteria TK1 and bacterial crude protein are recommended for use in the management of T. absoluta. Further research is recommended to determine insecticidal compounds in the bacterial crude protein. Key Words: Entomopathogenic nematodes; Infectivity; Tuta absoluta, Steinernema sp.; Xenorhabdus sp.; larvicidal activity