are infectious at the same time (Hominick and Reid, 1990). Bohan and
Hominick (1995) developed an infection model based on this hypothesis
and Hay and Fenlan (1995) proposed that there are primary and second-
ary invaders in a population. Campbellet al. (1999) found that, for three
Steinernemaspecies, the cause of the low infection was not that some
infective juveniles were not infectious, but that the presence of acceptable
hosts was the limiting factor. As the number of insects available to infect
increased, so did the number of infective juveniles that infected. The
distribution of nematodes among hosts was clumped, suggesting that
some hosts are more likely to be infected than others. The reasons for this
are not clear, but may be due in part to a preference to infect hosts already
infected. In contrast, results of similar experiments withH. bacteriophora
suggest that a large portion of the population may not be infectious at a
given time (Campbellet al., 1999).Entomopathogenic Nematode Host-search Strategies 29
6040200
Uninfected host
+
uninfected hostInfected host
+
infected hostInfected host
+
uninfected hostNumber of infecting nematodesTreatment (pairs of Galleria mellonella larvae)
Fig. 2.2. Differences inSteinernema feltiaeinfection of twoGalleria mellonella
larvae paired in a 1.5 cm diameter arena. Infected hosts (black bars) had been
exposed to 100S. feltiaeinfective juveniles 24 h prior to the start of the experiment
and uninfected hosts (grey bars) had not been previously exposed to nematodes.
Control insects exposed to 100 nematodes 24 h prior to the start of the experiment
were dissected and the number of nematodes present was determined. Treatments
of two uninfected hosts, two infected hosts, and one infected host and one
uninfected host were tested. After exposure, insects were dissected and the number
of nematodes that infected was determined. The number of nematodes in the
previously infected host treatments was corrected using the average number of
nematodes present in the control hosts.