Differences in susceptibility of introduced and native white grub species to entomopathogenic nematodes from various geographic localities

Invasive, non-native, white grubs (Coleoptera: Scarabaeidae) cause significant damage in urban landscapes. Although the lack of natural enemies in their new home is often suggested as an important factor in the establishment and spread of invasive species, the potential of incumbent generalist parasites and pathogens to delay their establishment and spread has not been explored. We compared the susceptibility of the introduced Popillia japonica and the native Cyclocephala borealis to 16 species and strains of entomopathogenic nematodes isolated from within or outside the geographic ranges of the two scarabs. We found large variation in the virulence of the species/strains of nematodes with over 50% mortality of P. japonica produced by Heterorhabditis zealandica strain X1 and H. bacteriophora strain GPS11 and of C. borealis by H. zealandica and H. bacteriophora strains KMD10 and NC1. Heterorhabditis indica and H. marelatus caused less than 20% mortality of both scarab species. When considered as a group the nematode species and strains from within and outside the geographic ranges of either P. japonica or C. borealis did not differ in virulence towards either scarab species. Dose response studies with selected nematode species and strains against P. japonica and two additional non-native species Anomala (Exomala) orientalis and Rhizotrogus majalis and the native C. borealis indicated that R. majalis was the least susceptible and P. japonica and A. orientalis were as susceptible as the native C. borealis. Heterorhabditis zealandica was significantly more virulent than any other species or strain against P. japonica with a LC₅₀ of 272 IJs/grub. The LC₃₀ and LC₅₀ values for H. zealandica were also the lowest among the four nematode species/strains tested against A. orientalis and C. borealis. The LC₅₀ values for H. zealandica and H. megidis (UK strain) were significantly lower for the native C. borealis than the introduced A. orientalis. H. zealandica also showed the highest penetration efficiency and the lowest encapsulation in P. japonica and C. borealis grubs. Results suggest that the introduction of the exotic H. zealandica into the front-line states with respect to the movement of P. japonica and A. orientalis should be explored as a tactic to delay their establishment and spread. The results also suggest that the manipulation of the indigenous H. bacteriophora populations may help in delaying spread and mitigating losses caused by the invasive grub species.