Potential control of woolly apple aphid (Eriosoma lanigerum) using entomopathogenic nematodes

The aim of this project was to control woolly apple aphid (WAA) in South African apple orchards, using entomopathogenic nematodes (EPNs) and fungi (EPF). The first objective was to evaluate the pathogenicity of EPN and EPF isolates against WAA in the laboratory, and to identify the most virulent isolates, while the second objective was to determine the efficacy of the best performing isolates for control of arboreal and subterranean WAA populations in pot trials. The third objective was to investigate the effect of mulching on the efficacy of EPNs for the control of soil populations of WAA in pot trials. The last objective was to investigate the efficacy of the selected EPN species and two EPF isolates for the control of arboreal and subterranean WAA populations in field trials

Apple roots infested with WAA collected from an apple orchard were kept in plastic containers to maintain a steady supply of insects. Seven EPN isolates were evaluated for their potential as biological control agents for WAA in the laboratory, using a 24-well bioassay protocol screening method.

A dipping bioassay was used to test the virulence of five EPF isolates against WAA at a concentration of 1 × 10⁸ conidia.ml^-1. The lethal dose and time of the most promising isolates were also tested in the laboratory. Interaction between EPNs and EPF for the control of WAA was evaluated in pot trials. Treatments included EPNs and EPF alone and in combination.

The potential of using Steinernema yirgalamense and Heterorhabditis zealandica together with two different mulches (pinewood shavings and apple wood chips) to control WAA was also evaluated in pot trials. The efficacy of S. yirgalamense, Beauveria bassiana and Metarhizium anisopliae against WAA in the field was evaluated in a naturally infested apple orchard in Grabouw.

Laboratory bioassays identified S. yirgalemense and H. zealandica as the most virulent against WAA and both nematode species were found to be compatible with imidacloprid. However, the haemolymph of WAA showed an inhibitory effect on the development of the symbiotic bacteria associated with these EPN isolates, which means that the EPN cannot develop and reproduce in the WAA.

All five EPF isolates caused significantly higher mortality of WAA compared to the control treatment. The commercial isolates M. anisopliae (ICIPE 69) (74.2% ± 3.7%) and B. bassiana (Eco-Bb strain R444) (70% ± 6.9%) were the most pathogenic. The LT₅₀ for Eco-Bb strain R444 and ICIPE 69 were found to be 57.7 and 45.4 h, respectively.

Combining nematodes with fungi and with different mulches did not enhance their efficacy.
In the field trial the highest rate of mortality was obtained with B. bassiana (Eco-Bb^® strain R444) and M. anisopliae (ICIPE 69). Aboveground, WAA colony size was reduced significantly compared to the control.

This is the first study undertaken on the use of EPNs to control WAA in South Africa. The EPNs tested did not provide effective WAA control because they are unable to reproduce in the WAA. Increasing EPN concentration and exposure time, and applying different mulches did not improve EPN efficacy. This led to the conclusion that EPNs are not good biocontrol agents against WAA. The EPF isolates tested, provided some level of WAA control, with even better control being recorded for the commercial isolates compared to the indigenous isolates. The results obtained in this study suggest that EPF research is a worthwhile investment and that such organisms are capable of performing effectively against WAA.