Efficacy of entomopathogenic nematodes for control of Tuta absoluta in South Africa
The South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is one of the most devastating pests of tomato (Solanum lycopersicon L.) in South America, Europe, the Middle East and Africa. Current management tactics of T. absoluta consist mainly of monitoring with sex pheromone traps and application of insecticides. Resistance to various insecticide groups has, however, been reported in Asia, Europe and South America. Development of integrated pest management (IPM) strategies for this pest is therefore important. There is currently no tomato cultivar commercially available which is resistant to T. absoluta, and parasitoids from only four families are known as biological control agents of T. absoluta in Africa. A variety of insect pests are controlled with entomopathogens such as fungi, bacteria and nematodes, which are used as biopesticides. Although entomopathogenic nematodes (EPNs) were initially applied as soil applications against pests, investigations to use EPNs as foliar applications also received renewed interest. In Europe, Heterorhabditis bacteriophora Poinar, 1976, Steinernema feltiae (Filipjev, 1934) Wouts, MráÏcek, Gerdin and Bedding, 1982, and Steinernema carpocapsae (Weiser, 1955) Wouts, MráÏcek, Gerdin and Bedding, 1982 have been reported to effectively control T. absoluta as foliar applications. The Agricultural Pest Act 36 of South Africa, prohibits importation of exotic species without a full impact study and permit. A search for native biological control agents for T. absoluta is therefore warranted. The aim of this study was to evaluate the efficacy of four native EPN species, viz. Steinernema jeffreyense Malan, Knoetze and Tiedt, 2015, Steinernema yirgalemense Nguyen, Tesfamariam, Gözel, Gaugler and Adams, 2005, Heterorhabditis baujardi Phan, Subbotin, Nguyen and Moens, 2003 and Heterorhabditis noenieputensis Malan, Knoetze and Tiedt et al., 2014 against T. absoluta in South Africa. Fourth instar T. absoluta larvae and pupae were exposed to IJs of the four EPN species in vitro. All four EPNs were found to be highly effective in controlling the larvae, with 100% larval morality caused, but pupae were less susceptible. Following the successful in vitro assays using the EPNs against T. absoluta larvae, greenhouse trials were conducted. Efficacy of S. jeffreyense and S. yirgalemense applied to the foliage of tomato seedlings for control of third and fourth instar T. absoluta larvae, was evaluated at four concentrations, viz. 250, 500, 1 000 and 2 000 IJS. mL⁻¹ distilled water containing 0.05% adjuvant (Nu-Film-P). High mortality rates of T. absoluta larvae in tomato leaves were recorded with both species at application rates of 1 000 and 2 000 IJs mL⁻¹. Results from this study identified S. jeffreyense and S. yirgalemense as promising biocontrol agents of T. absoluta under greenhouse tomato production in South Africa, which could be included in IPM of this pest. By applying an IPM system and not relying on chemical control only, resistance to insecticides in South Africa, may be prevented or delayed.