The leopard moth, Zeuzera pyrina L. (Lepidoptera: Cossidae) is a serious pest of walnut trees in Iran. Due to special habitat of this cossid, the application of entomopathogenic nematodes (EPNs) may offer an efficient strategy in suppressing populations of this pest. The efficacy of two commercially available EPNs Steinernema carpocapsae and Heterorhabditis bacteriophora, as well as a local insect-killing nematode (Acrobeloides K29) were investigated against Z. pyrina larvae in laboratory, semi-field, and field conditions. Laboratory experiments included pathogenicity assessment of aforementioned nematodes in plates using a range of concentrations [5, 10, 20, 50 and 100 infective juveniles (IJs) per larva], as well as evaluation of their reproduction potential on Z. pyrina larvae with concentrations of 5 and 20 IJs per larva. Application of increasing concentrations of nematodes induced higher mortality on Z. pyrina larvae with mean mortality ranging from 28.3% to 100% after 72 h. All three nematodes successfully reproduced and recovered from pest’s cadavers. The highest reproduction was recorded for H. bacteriophora at 5 IJs larva−1 in Z. pyrina. Semi–field experiments using potted walnut seedlings and field trials in a heavily infested walnut orchard during two consecutive years revealed the ability of the aforementioned nematodes to infect and kill Z. pyrina larvae inside the galleries of walnut branches. Overall, this research highlights the ability of all three examined nematodes to find, invade, recycle and effectively infect Z. pyrina larvae; however, S. carpocapsae is comparatively more effective than H. bacteriophora and Acrobeloides K29 under laboratory, semi–field and field conditions. Of significance, the free-living nematode causes high to moderate levels of mortality in Z. pyrina larvae in laboratory and natural conditions.

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