Mortality and nematode production in Spodoptera littoralis larvae in relation to dual infection with Steinernema riobrave, Heterorhabditis bacteriophora, and Beauveria bassiana, and the host plant

•Interactions between S. riobrave and H. bacteriophora and B. bassiana were synergistic.•S. littoralis larvae reared on cabbage leaves were highly resistant to infection with S. riobrave and H. bacteriophora.•The highest nematode progeny production was attained in cadavers fed on castor leaves.•The highest virulence of B. bassiana was obtained for larvae fed on mallow leaves.

The present study aimed to evaluate the interactive applications of the promising biocontrol agents, the entomopathogenic nematodes (EPNs) Steinernema riobrave and Heterorhabditis bacteriophora and the entomopathogenic fungus (EPF) Beauveria bassiana, each at the LC25 level, to the last instars of the Egyptian cotton leafworm, Spodoptera littoralis, in either sequential or simultaneous fashion to determine whether interactions (synergistic, additive, or antagonistic) among these entomopathogens were present. Moreover, the indirect impact of two cultivated host plants (cabbage and cowpea) and three wild host plants (castor, Jew’s mallow, and mallow) or their primary metabolites (total protein, carbohydrate, and lipid content) on the virulence of these entomopathogens and on nematode production also was assessed. Synergisms were observed among the different entomopathogen pairings. In case where sequential applications were made, applying the EPF first was best for infective juvenile (IJ) production; applying the EPN prior the EPF significantly reduced IJ production. Larvae raised on cabbage (the most lipid–rich diet) were most resistant for both EPN species tested. In general, not many IJs were produced from hosts that fed on mallow, but many were produced from hosts that fed on castor. In the case of EPF, B. bassiana was most effective at controlling larvae that fed on mallow, which was considered the least nutritive of the host plants. The results obtained are suggestive of an efficient control to S. littoralis. This would be achieved through the implementation of an integrated program including combining the entomopathogens studied with each other, or intercropping castor (in the case of the nematode application), or mallow (in the case of the fungus application) with cotton for enhancing the control of this insect pest in Egypt.