Bioactivity of plant extracts against tomato leaf miner, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

Abstract

Tomato (Solanum lycopersicum L.) is economically and nutritionally important in Rwanda, but its production is challenged by the tomato leaf miner (Tuta absoluta Meyrick), an invasive pest. Synthetic insecticides which are primarily used for its control, have various drawbacks. Bioactivity of Tephrosia vogelii, Tithonia diversifolia, Vernonia amygdalina and Phytolacca dodecandra aqueous extracts was evaluated against T. absoluta in laboratory. Leaflets with third instar larvae (3.85 5.65 mm) of T. absoluta in mines were collected from established tomato field. Aqueous plants extracts were evaluated at a dose of 10 % weight/volume. Sterile tap water and azadirachtin 0.03 % EC were used as negative and positive controls, respectively. Petri-dishes of 9 cm diameter (n=10) were used as bioassay arenas in a completely randomized design with four replications. Data on larval mortality were collected every 24 h for 5 days. Three bioassays were conducted on different dates. Results indicated that tested plant extracts exhibited a capacity to kill T. absoluta larvae in tomato leaf galleries with significant difference among them (P <0.0001). The killing capacity increased with exposure time. At 24 h of exposure, T. absoluta larvae mortality was in a range of 35.0 37.5 % for azadirachtin and 5.0 10.0 % for T. vogelii while all other aqueous extracts had 0.0 % mortality, except V. amygdalina which recorded 2.5 % in bioassay one. In all bioassays, the lowest mortality recorded 5 days after treatments with T. vogelii, T. diversifolia, V. amygdalina, P. dodecandra and azadirachtin was 32.2, 2.8. 2.5, 20.5 and 97.5 % while the highest mortality at this time was 35.1, 10.6, 13.3, 24.9 and 100 %, respectively. Tephrosia vogelii and P. dodecandra, which recorded higher efficacy compared to the other local plants, should be advanced to field evaluation. The observed higher efficacy of azadirachtin to Rwandan population of T. absoluta should also be confirmed under field conditions.