Field and bin application of entomopathogenic nematodes for the control of codling moth (Cydia pomonella, Lepidoptera: Tortricidae)

The aim of this project is to control codling moth (CM) in the South African pome fruit, in an environmentally friendly way, using entomopathogenic nematodes (EPNs). To this end, environmental factors influencing EPN efficacy were investigated in the field and laboratory. The first objective was to identify which nematode isolates effectively control diapausing CM larvae under field conditions, while the second objective was to assess the effect of spray application, and the last objective was to investigate the potential of two imported, and one local, EPN species to disinfest wooden fruit bins of CM larvae.

Apple orchard field trials were conducted at the Welgevallen Experimental Farm in Stellenbosch. Cages that were filled with apple bark and CM larvae were used as an evaluation method. After having been kept moist by spraying with water every 2 h for the first 6 h, they were removed from the field after 24 h. In laboratory trials, after the cages were pre-wet, they were either dipped in, or sprayed with, inoculum. They were then placed in a growth chamber following a cycle of 22°C for 4h, 10°C for 12h, and 22°C for 8h at 100% humidity. After the pre-wetting of miniature wooden fruit bins and planks, made from old bin wood and infested with CM larvae, they were dipped into the nematode suspension, and placed in separate 2-L plastic containers lined with moist paper towels, which were then closed with a lid. The containers were then placed in a growth chamber at 25°C for 3 days.

In vivo produced Steinernema yirgalemense resulted in a higher CM control (>90%) than did H. bacteriophora, while S. feltiae gave better control than did S. yirgalemense and Heterorhabditis bacteriophora in both field and laboratory trials. Humidity seemed to be the most important factor affecting EPN efficacy against the CM. It can, therefore, be concluded that H. bacteriophora will not be suitable for the field control of CM. Steinernema feltiae were found to perform better at lower temperatures. This study has highlighted the biocontrol potential of S. jeffreyense in a field trial. In the bin trials, using low nematode concentrations, the highest percentage of control was obtained using S. feltiae (75%) followed by S. yirgalemense (57%), and then by H. bacteriophora (28%). The addition of adjuvants significantly increased S. feltiae infectivity to >95%, whereas it did not result in a significant increase in levels of H. bacteriophora or S. yirgalemense infectivity.

Relative humidity and temperature are the main factors influencing the effectiveness of EPNs in the Western Cape province. Steinernema feltiae proved to be the most effective candidate for the control of diapausing CM, as it is climatically better adapted to lower temperature regimes. Steinernema yirgalemense and S. jeffreyense are the most promising local candidates, and future studies should compare their efficacy with that of S. feltiae. H. bacteriophora is not recommended for use in the Western Cape, as it resulted in poor control of CM larvae. Further research should aim at identifying a suitable EPN application technique for use in orchards, which will not only be compatible with the EPN isolate used, but which will also ensure good coverage of fruit trees.