Evaluation of entomopathogenic nematodes and their combined application against Curculio elephas and Polyphylla fullo larvae

Abstract

The larvae of Curculio elephas (Coleoptera: Curculionidae) and Polyphylla fullo (Coleoptera: Scarabaeidae) are major agricultural pests of chestnut and roots of cultivated crops, respectively. Previous research showed that they are relatively resistant to nematode infection. Accordingly, we evaluated the efficacy of Steinernema glaseri, S. weiseri or Heterorhabditis bacteriophora alone or in combination against these two insect pests. A nematode concentration of 50 or 100 infective juveniles (IJs)/larva for C. elephas or 50 or 100 IJs/cm2 for P. fullo at 25 °C was used. The highest (81 %) and the lowest (21 %) larval mortalities for C. elephas were obtained with S. weiseri+H. bacteriophora combined application and S. glaseri alone, respectively. The nature of the interactions (antagonism, additive, or synergy) for the larval mortality was evaluated. For C. elephas, S. weiseri combined with S. glaseri or H. bacteriophora was additive, whereas the combination of S. glaseri+H. bacteriophora was antagonistic. For P. fullo, the efficiency of nematodes used alone or combinations was very low, and there were no significant differences among the treatments at 50 or 100 IJs/cm2. The interaction was additive with the combinations of S. glaseri+H. bacteriophora against P. fullo larvae. No synergistic effect was observed for any combination against C. elephas and P. fullo larvae. Our results show that the EPN species tested, either alone or in combination, are not economically feasible for use against C. elephas or P. fullo, but that further research with other combinations of EPN species or EPNs with other entomopathogens is warranted.