M. Ricci et al., COMPARISON OF BIOASSAYS TO MEASURE VIRULENCE OF DIFFERENT ENTOMOPATHOGENIC NEMATODES, Biocontrol science and technology, 6(2), 1996, pp. 235-245
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.