Integrated management of Plutella xylostella (Lepidoptera: Plutellidae) in vegetable growth tunnels
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Plutella xylostella (L.) (Lepidoptera: Plutellidae) is a major pest on all Brassicaceae in South Africa. Insecticide application is the main control strategy worldwide and concerns are raised about the susceptibility of P. xylostella populations to insecticides in South Africa. This concern sparked the investigation into other feasible control strategies and measures mainly focused cultural control with trap crops. The objectives of this study were to assess the following: feeding damage and percentage growth of P. xylostella on selected host plants in no choice tests, the feeding preference of P. xylostella larvae on selected host plants in two choice tests, the development and oviposition of P. xylostella on selected host plants for possible use in a Push–Pull strategy and the susceptibility of P. xylostella to selected insecticides. Five known P. xylostella host plant species were selected, viz. pak–choi (Brassica rapa var. chinensis), cabbage (Brassica oleracea), tatsoi (Brassica narinosa), kale (Brassica oleracea) and mustard (Brassica juncea). In the two–choice tests conducted mustard was preferred for oviposition as well as feeding preference by P. xylostella larvae. The fact that mustard was the preferred choice over the other selected crops for oviposition and feeding preference makes it a viable candidate for a trap crop for P. xylostella. Plutella xylostella populations were sampled from two localities, Nelspruit (Mpumalanga province) and Bapsfontein (Gauteng province). The toxicity of three insecticides used for control of P. xylostella in South Africa, viz. lambda–cyhalothrin, flubendiamide and spinosad was estimated for both these populations using an IRAC approved bioassay method. When comparing the susceptibility of the two populations (Nelspruit (N2018–01) and Bapsfontein (B2018–01)) for lambda–cyhalothrin the LC80 values are higher (N2018–01: 9.35 and B2018–01 24.87) than the recommended dosage of 8 ppm. Conversely, the selected populations (Nelspruit and Bapsfontein) are highly susceptible to flubendiamide and spinosad with high mortality levels > 80% with low dosages of 3 and 2 ppm respectively compared to the recommended dosages of 48 and 216 ppm. The integration of trap crops and application of insecticides with high efficacy of control of P. xylostella can contribute to reduce the number of insecticide applications per season. It can also prolong the effective use of these insecticides for control of P. xylostella in South Africa.