Rose sawfly, Arge ochropus (Gmelin) is one of the most important pests of ornamental plants such as roses and wild rose bushes in northern Iran. We investigated the interactions between the insecticide imidacloprid and the entomopathogenic nematodes Heterorhabditis bacteriophora and Steinernema carpocapsae as control agents of 5th instar larvae in the laboratory. The larvae were very susceptible to S. carpocapsae (LC50: 21 infective juvenile per larva) and H. bacteriophora (LC50: 32). Combinations of two imidacloprid rates (LC30 and LC50) and four rates of each nematode species (LC25 to LC75) were tested. Combinations with the lower imidacloprid rate except for that with the highest H. bacteriophora rate caused higher mortality than both respective single agent treatments. In combination with the higher imidacloprid rate only one combination with H. bacteriophora and two combinations with S. carpocapsae caused higher mortality than both respective single agent treatments. Interactions were generally stronger at the lower imidacloprid rate and were stronger for S. carpocapsae (synergistic in seven combinations, additive in one) than for H. bacteriophora (synergistic in two, additive in six). Synergistic imidacloprid-S. carpocapsae combinations could be a useful tool for the control of A. ochropus larvae that would simultaneously control other common pests susceptible to imidacloprid.

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