Host plant suitability and baseline susceptibility of Tuta absoluta (Lepidoptera : Gelechiidae) to insecticides in South Africa

MSc (Environmental Sciences), North-West University, Potchefstroom CampusThe devastating tomato pest, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) (tomato leafminer/tomato pinworm), originates from South America. It invaded South Africa in 2016. It not only causes severe damage to tomato, but also to other cultivated crops, which mainly belong to the Solanaceous family. Knowledge on the expansion of the host range of T. absoluta is important, since movement between crops and weeds can have an influence on the management of this pest. In this study, the aims were to evaluate selected weed species to determine which of these can act as hosts, and to determine baseline toxicity of selected insecticides to T. absoluta. Laboratory choice tests were conducted to determine the suitability of five weed species, from four plant families, for reproduction and development of T. absoluta. These weeds were Amaranthus hybridus L. (Cape pigweed) (Amaranthaceae), Solanum scabrum (Garden huckleberry/African nightshade) (Solanaceae) and S. retroflexum (Wonderberry) (Solanaceae), Malva parviflora (Malvaceae) and Galinsoga parviflora (Gallant soldier) (Asteraceae). Amaranthus hybridus, M. parviflora and G. parviflora did not sustain larval development, and will therefore not contribute to the establishment of T. absoluta in invaded areas. In contrast, the two solanaceous weeds, S. scabrum and S. reteroflexum, are potentially as suitable as S. lycopersicum L. (tomato) for reproduction and development of T. absoluta, and may therefore contribute to the establishment of T. absoluta. A number of insecticides are registered in South Africa for control of T. absoluta, but its susceptibility to these insecticides is unknown, and as a result, no baseline toxicity data exist in South Africa. The leaf dip bioassay, method 022, approved by the international Insecticide Resistance Action Committee was used to estimate baseline toxicity of indoxacarb, emamectin benzoate, spinetoram and lufenuron for T. absoluta populations from Mareetsane (North-West province), Polokwane (Limpopo province) and Swartwater (Limpopo province), in South Africa. The probability for control failure of insecticides was also determined. All three T. absoluta populations were highly susceptible to indoxacarb, emamectin benzoate and spinetoram, resulting in low variability in the responses of T. absoluta from all three localities, to these active ingredients. The differences in LC50 values were below 2.5 fold. No control failure to these active ingredients is therefore currently expected. The T. absoluta population from Polokwane was significantly less susceptible to lufenuron, compared to the populations from Swartwater and Mareetsane. All three populations were, however, susceptible to lufenuron despite the Polokwane population being 47 times and 18 times less susceptible than the populations from Swartwater and Mareetsane, respectively. Although not registered for control of T. absoluta in South Africa, effective control will be achieved when lufenuron is applied for control of potato tuber moth, at the registered dosage rate.Master