Abstract: A biocontrol product based on the Heterorhabditis bacteriophora nematode (Rhabditida: Heterorhabditidae) and its symbiotic Photorhabdus bacterium (Enterobacterales: Morganellaceae) had been developed against the larvae of the maize pest, western corn rootworm, Diabrotica v. virgifera (Coleoptera: Chrysomelidae). The product, called DianemTM, is available in a number of countries. Novel hybrid inbred lines of this nematode and its symbiotic bacteria have been successfully selected for improved traits for better mass production, higher virulence, and longer persistence. Sometimes breeding can have negative trade-off effects such as decreased control of pests under field conditions; something we tested here. We compared two of the most promising novel H. bacteriophora strains, i. e., the H2L3 and D2D6 hybrids combined with a novel symbiotic bacterial strain Photorhabdus laumondii DE2.2 in their effects on larval populations under field conditions. We compared these to the commercial H. bacteriophora EN01 (DianemTM) and to a standard granular soil insecticide, the pyrethroid cypermethrin. Plant scale field experiments in Hungary revealed that all nematode-bacteria combinations were capable to reduce pest levels indicating that the breeding and selection programs have not led to any trade-off effects with regard to field efficacy. Three out of the six nematode-bacteria combinations reached a higher-than-usual efficacy of up to 70 or 80 %. Novel Photorhabdus strains did not lead to a largely improved insecticidal efficacy of nematodes under field conditions. Subsequently, we tested into-seeding furrow applications of the H. bacteriophora H2L3 with its original P. laumondii Hb1.3, the commercial H. bacteriophora EN01, and cypermethrin in two heavily infested maize fields using farmer machinery. Both strains were comparably able to reduce heavy D.v.virgifera populations of up to 50 % averaged across fields. This is a high level of efficacy considering the high pest infestation. Both were more effective than cypermethrin (16 %). In conclusion, extensive breeding and selection programmes of nematodes targeting prolonged field persistence and increase of virulence appear to be worth the investment to improve the biological control potential of the nematode H. bacteriophora.