The potential of biopesticides for control of Tuta absoluta (Meyrick) (Lepidoptera : Gelechiidae

PhD (Science with Environmental Sciences), North-West University, Potchefstroom Campus

The invasive South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is one of the biggest threats to tomato production. Although other management strategies are used, control of this pest mainly relies on the application of synthetic chemical insecticides. Chemical control is, however, negatively affected by the cryptic feeding behaviour of the larvae and the pests’ ability to rapidly evolve insecticide resistance. It is therefore crucial that the use of synthetic insecticides is integrated with more environmentally friendly management options. Biopesticides and pheromone-based control strategies represent some of the most promising alternatives. Firstly, this study aimed to determine the susceptibility of T. absoluta pupae to Beauveria bassiana and Metarhizium anisopliae administered as soil drench treatments and the possible effects of these entomopathogenic fungi (EPFs) on the fecundity and fertility of moths that emerged from pupae that were previously treated with EPFs. High pupal mortality was caused by the respective EPF products in bioassays and growth tunnel experiments. Fecundity of moths that emerged from surviving pupae, was significantly reduced compared to moths from untreated pupae. This provides evidence of the suppressive potential of EPF conidia administered as a soil drench for control of T. absoluta pupae. Secondly, this study aimed to determine the genetic diversity of South African T. absoluta populations, and whether moths were able to reproduce parthenogenetically. The strain composition and frequency of Wolbachia infection in T. absoluta populations were also investigated. The genetic diversity of T. absoluta in South Africa is low, and consists of only four haplotypes. A novel haplotype was reported from South Africa during this study. Despite high Wolbachia infection rates (≥ 95%) of virgin T. absoluta females with a single Wolbachia strain (ST41), no parthenogenetic reproduction occurred. Control failure of pheromone-based strategies in South Africa can therefore not be attributed to Wolbachia-induced parthenogenesis. Lastly, this study aimed to determine the systemic efficacy of spinosad applied as a soil drench for control of T. absoluta, the persistence of spinosad in the leaves and fruit, and the potential effect of this drench application on tomato plant growth parameters. Although spinosad concentration in leaves decreased gradually over time, high larval mortalities (> 96%) were recorded for the duration of the experiment, which lasted 25 days. The spinosad residues detected in fruit were well below the maximum residual limits mandated for South Africa. This study demonstrated the systemic properties of spinosad for control T. absoluta in tomato. Future research should investigate the compatibility of spinosad and entomopathogenic fungi applied as a soil drench, taking into account the possibility of resistance evolution, while also incorporating pheromone-based strategies as more environmental friendly additions to IPM programmes.

Doctoral