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Achieving predictability (i.e., consistency of control) is a key challenge in the use of entomopathogenic nematodes for pest control. As nematode species/strain plays a key role in the success of white grub control, two strains, Heterorhabditis zealandica X1 (Hz-X1) and Heterorhabditis bacteriophora GPS11 (Hb-GPS11), identified through laboratory assays to be the most virulent, were tested in a series of field trials against the second and third instar white grubs Popillia japonica and Cyclocephala borealis at different locations over a 2-year period. The HP88 strain of H. bacteriophora (Hb-HP88), the NJ and MB strains of Steinernema glaseri (Sg-NJ and Sg-MB) and a UK strain of Steinernema kraussei (Sk-UK) were also included for comparison in some tests. Overall, Hz-X1 and Hb-GPS11 strains were the most effective nematodes resulting in 73–98 and 34–97% control of P. japonica and 72–96 and 47–83% control of C. borealis, respectively. The Hz-X1 and Hb-GPS11 strains did not differ significantly between each other against either grub species in any of the tests. The Hb-HP88 provided 52 and 36% control of P. japonica and C. borealis, respectively. The Sg-MB provided 6–58% control of P. japonica, no control of C. borealis, and 54–74% control of mixed populations of the two species. The Sg-NJ provided 20% control of P. japonica. Doubling the nematode application rate from 2.5 × 109 to 5 × 109 infective juveniles/ha did not significantly increase control of a mixed population of P. japonica and C. borealis. Halofenozide (Mach 2 2SC) and imidacloprid (Merit 75WP) applied as preventative treatments in July provided 97 and 98% control, respectively, of P. japonica. Imidacloprid applied as a curative treatment in September provided 47% control of C. borealis but 0% control of P. japonica. Trichlorfon (Dylox 6.2G) applied as a curative treatment in September provided 29–92% control of P. japonica, 49% control of C. borealis, and 0–77% control of mixed populations of the two grub species. Grub control provided by the Hz-X1 strain was always equal to or better than trichlorfon and that by the Hb-GPS11 strain was equal to trichlorfon except for one occasion. The amount of post-application irrigation plus rainfall had a strong effect on nematode efficacy. A total of 10 cm of post-application irrigation plus rainfall was found to be optimum at which the Hb-GPS11 and Hz-X1 strains produced 83–97 and 96–98% control of the two white grub species, respectively. Combined analyses of the results from all 8 trials containing 46 treatments indicate that the consistency in the control of P. japonica and C. borealis by the new Hz-X1 and the Hb-GPS11 strains is equal to or better than the curative application of chemical insecticide standard, trichlorfon.