Efficacy of entomopathogenic nematodes (Rhabditida : Heterorhabditidae and Steinernematidae) against codling moth, Cydia pomonella (Lepidoptera: Tortricidae) in temperate regions
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The biocontrol potential of South African isolates of Heterorhabditis zealandica, Steinernema citrae, S. khoisanae, S. yirgalemense, and Steinernema sp., was evaluated against codling moth, Cydia pomonella. Codling moth was susceptible to all six nematode isolates at a concentration of 50 infective juveniles/insect (78-100% mortality). Low temperatures (10 h at 17ºC; 14 h at 12ºC) negatively affected larvicidal activity (≤3%) for all isolates. All tested isolates were most effective at higher levels of water activity (aw=1). The average aw50-values for all isolates tested was 0.94 (0.93-0.95), except S. khoisanae 0.97 (0.97-0.98). Regarding host-seeking ability, no positive attraction to host cues could be detected amongst isolates, except for H. zealandica. Three of the isolates, H. zealandica, S. khoisanae, and the undescribed Steinernema sp., were selected for field-testing and proven to be effective (mortality >50%). Insect containment methods used during field experimentation was shown to influence larvacidal activity, as different levels of mortality were obtained using various containment methods (wooden planks vs. pear tree logs vs. mesh cages). Pear tree logs were impractical. Predictive equations were subsequently developed, enabling future trials to be conducted using either planks or cages, enabling the prediction of the expected level of control on tree logs. All tested isolates therefore showed a certain degree of biological control potential, however, none of the experiments showed clear efficacy-differences amongst isolates. The study highlighted the importance of environmental factors to ensure the successful application of these nematodes for the control of diapausing codling moth larvae in temperate regions.