The application of large numbers of entomopathogenic nematodes (EPN) to control insect pests of agriculture is likely to
have an impact on the local EPN fauna, yet little is known about the intraspecific relationships between EPN populations, particularly
with regard to phylogeny and outbreeding. Here we assess the fitness, with regards to fecundity, host insect mortality and time taken
to produce progeny, of isolates of Steinernema feltiae from Bull Island, Ireland. Exon-primed, intron-crossing (EPIC) PCR was
used to examine intraspecific phylogenies between S. feltiae isolates, and identified up to three possible colonisation events of Bull
Island. EPIC-PCR grouped two isolates, 33.D.(2) and 59.F.(2), separately from the remaining ten S. feltiae isolates These same two
isolates consistently performed poorly in all fitness assessments. Following the crossbreeding of all isolates in Galleria mellonella,
the number of host cadavers exhibiting emerging infective juveniles was significantly fewer than expected and there were significant
differences between isolates in the number of days until progeny were observed. Host insect mortality varied between 40 and 87%.
Such intraspecific variation may be a result of adaptation to different microhabitats of Bull Island, which in turn may be accentuated by
laboratory culture practices