COMPARISON OF BIOASSAYS TO MEASURE VIRULENCE OF DIFFERENT ENTOMOPATHOGENIC NEMATODES

Five bioassays were compared for their usefulness to determine the vir ulence of four nematode strains. The objective of this study was to de velop standard assays for particular nematode species. In all assays, the nematodes Steinernema feltiae (strain UK), S, riobravis, S. scapte risci Argentina and Heterorhabditis bacteriophora HP88 were exposed to Galleria mellonella larvae. All bioassays except the sand column assa y were conducted in multi-well plastic dishes. In the penetration rate assay, the number of individual nematodes invading the insect was det ermined after a 48-h exposure to 200 infective juveniles (IJs). In the one-on-one assay, each lawn was exposed to nn individual nematode for 72 h before insect mortality was recorded. In the exposure time assay , insect mortality was recorded after exposure to 200 IJs for variable time periods. The dose-response assay involved exposing larvae to dif ferent nematode concentrations over the range 1-200 IJs/insect and rec ording mortality every 24 h for a 96-h period. In the sand columns ass ay, insects were placed in the bottom of a plastic cylinder filled wit h sand. Nematodes were applied on top of the sand and insect mortality was determined after IJs had migrated through the cylinder. The highe st mortality level in the sand column assay was obtained with IJs of S , feltiae followed by H. bacteriophora; treatments with S. riobravis a nd S. scapterisci produced low levels of insect mortality. In the othe r four assays, S riobravis was the most virulent, followed by S. felti ae, H. bacteriophora and S. scapterisci. In the exposure time assay, r apid mortality was achieved when the insects were exposed to S. feltia e and S. riobravis. For these nematode species, a gradual increase in the number of individuals which penetrated into cadavers was recorded Conversely, the number of nematodes in the cadavers of insects infecte d by H. bacteriophora and S, scapterisci remained low during the entir e exposure period. In this assay, exposing the insects to these nemato des resulted in a gradual increase in mortality. In the dose-response assay, complete separation among nematode species was obtained only af ter 48 h of incubation at a concentration of 15 IJs/insect. LD(50) and LD(90) values were calculated from dose-response assay data. However, these values did not indicate differences among the different nematod e species. The present study demonstrated rite variation in entomopath ogenic nematode performance in different bioassays and supports the no tion that one common bioassay cannot be used as a universal measure of virulence for all species and strains because nematodes differ in the ir behavior. Furthermore, particular assays should be used for differe nt purposes. To select a specific population for use against a particu lar insect, assays that are more laborious but which simulate natural environmental conditions (e.g. the sand column assay) or invasion by t he nematode (e.g. the penetration rate assay) should be considered. In cases where commercial production batches of the same nematode strain s are compared, simple and fast assays are needed (e.g. the one-on-one and exposure time assays). Further studies are needed to determine th e relationships between data obtained in each assay and nematode effic acy in the field.