Combination effects of entomopathogenic nematodes, Heterorhabditis bacteriophora and Steinernema feltiae, with Abamectin on developmental stages of Phthorimaea operculella (Lepidoptera, Gelechiidae)

Abstract

The potato tuber moth (PTM) Phthorimaea operculella (Zeller, 1873) is a major pest of Solanaceae crops globally. PTM is mainly controlled using chemical pesticides. The objectives of this study were to compare the efficacy of two species of entomopathogenic nematode (EPNs), Heterorhabditis bacteriophora (Poinar, 1976) and Steinernema feltiae (Filipjev, 1934) Wouts et al. (1982), against PTM pre-pupa and pupa, while also assessing their interaction in combination with abamectin in a series of laboratory and greenhouse assays. The results showed that S. feltiae was considerably more virulent than H. bacteriophora against both PTM life stages tested. PTM pupae were less susceptible to EPN than pre-pupae: LC50 values for pupae were 721 and 570 IJs/insect for H. bacteriophora and S. feltiae, respectively; corresponding values for pre-pupae were 98 and 6 IJs/insect, respectively. While both species of EPN were negatively affected by abamectin, the effect on H. bacteriophora was less pronounced. This present study shows that the extent to which the nematodes interact with chemical pesticides is dependent on a number of factors, including nematode species, concentration of combined agents, developmental stage of the targeted pest, exposure method and the complexity of the environment in which the interaction takes place. In the glasshouse study, for S. feltiae, synergistic effects were observed only in the combinations which included lower rates of abamectin and higher S. feltiae concentrations. In the case of H. bacteriophora, other than at the lower concentrations of both the insecticide and the nematode (where the interaction was additive), all other combinations were synergistic. That there were more synergistic interactions in the H. bacteriophora + insecticide combinations is most likely attributable to the lower susceptibility of H. bacteriophora to abamectin compared to S. feltiae. This study demonstrates that combined applications of chemical insecticides and entomopathogenic nematodes allow for a reduction of both inputs. For end users, reducing nematode and abamectin inputs make their combined use an extremely attractive proposition, in both economic terms and as a resistance management strategy.