Lateral movement of the entomopathogenic nematode Heterorhabditis bacteriophora in soil under laboratory conditions

Despite the growing importance of entomopathogenic nematodes as biocontrol agents for soil inhabiting insect pests, little is known of their population and spatial ecology. In a series of three consecutive laboratory experiments, the horizontal movement of Heterorhabditis bacteriophora GPS11 strain was quantified in 5 cm deep autoclaved soil with 24% moisture content placed in three different sized arenas (22.86 cm x 22.86 cm, 61 cm x 61 cm, and 122 cm x 122 cm) at room temperature (21°C). A single 10-day old cadaver of fourth instar Galleria mellonella infected with H. bacteriophora was placed in the center of each arena and the movement of at least one nematode from it to site of the soil sample, collected in small cups with soil core samples (2 cm in diameter) at intervals from 6 to 240 hours and at distances from 7 to 61 cm from the center, was inferred from death of the bait (one uninfected G. mellonella larva per cup) three days later. Each of the three arena sizes were replicated five times and all three experiments were repeated with similar sampling distances and time intervals. The data collected comprised the proportion of dead G. mellonella baits at 96 combinations of distance and time computed from 5 – 10 replicates. A two-dimensional modified Fick diffusion model was fit to the spatio-temporal data by least squares method and descriptive statistics calculated. The average movement of infective juveniles in soil was 6 cm/day. The number of infective juveniles moving a given distance declined with increasing distance from the cadaver with 40% traveling >15 cm and 2.5% traveling >60 cm in up to 240 hours. This study has shown that the dispersal ability of H. bacteriophora in soil with no source of attraction in the form of bait or carbon dioxide from other organisms.G054

Lateral Dispersal and Foraging Behavior of Entomopathogenic Nematodes in the Absence and Presence of Mobile and Non-Mobile Hosts

Entomopathogenic nematodes have been classified into cruisers (active searchers) and ambushers (sit and wait foragers). However, little is known about their dispersal and foraging behavior at population level in soil. We studied lateral dispersal of the ambush foraging Steinernema carpocapsae (ALL strain) and cruise foraging Heterorhabditis bacteriophora (GPS11 strain) from infected host cadavers in microcosms (0.05 m2) containing Wooster silt-loam soil (Oxyaquic fragiudalf) and vegetation in the presence or absence of non-mobile and mobile hosts. Results showed that the presence of a non-mobile host (Galleria mellonella larva in a wire mesh cage) enhanced H. bacteriophora dispersal for up to 24 hr compared with no-host treatment, but had no impact on S. carpocapsae dispersal. In contrast, presence of a mobile host (G. mellonella larvae) increased dispersal of S. carpocapsae compared with no host treatment, but had no effect on H. bacteriophora dispersal. Also H. bacteriophora was better at infecting non-mobile than mobile hosts released into the microcosms and S. carpocapsae was better at infecting mobile than non-mobile hosts, thus affirming the established cruiser-ambusher theory. However, results also revealed that a large proportion of infective juveniles (IJs) of both species stayed near (≤ 3.8 cm) the source cadaver (88-96% S. carpocapsae; 67–79% H. bacteriophora), and the proportion of IJs reaching the farthest distance (11.4 cm) was significantly higher for S. carpocapsae (1.4%) than H. bacteriophora (0.4%) in the presence of mobile hosts. S. carpocapsae also had higher average population displacement than H. bacteriophora in the presence of both the non-mobile (5.07 vs. 3.6 cm/day) and mobile (8.06 vs. 5.3 cm/day) hosts. We conclude that the two species differ in their dispersal and foraging behavior at the population level and this behavior is affected by both the presence and absence of hosts and by their mobility

Lateral Dispersal and Foraging Behavior of Entomopathogenic Nematodes in the Absence and Presence of Mobile and Non-Mobile Hosts.

Entomopathogenic nematodes have been classified into cruisers (active searchers) and ambushers (sit and wait foragers). However, little is known about their dispersal and foraging behavior at population level in soil. We studied lateral dispersal of the ambush foraging Steinernema carpocapsae (ALL strain) and cruise foraging Heterorhabditis bacteriophora (GPS11 strain) from infected host cadavers in microcosms (0.05 m2) containing Wooster silt-loam soil (Oxyaquic fragiudalf) and vegetation in the presence or absence of non-mobile and mobile hosts. Results showed that the presence of a non-mobile host (Galleria mellonella larva in a wire mesh cage) enhanced H. bacteriophora dispersal for up to 24 hr compared with no-host treatment, but had no impact on S. carpocapsae dispersal. In contrast, presence of a mobile host (G. mellonella larvae) increased dispersal of S. carpocapsae compared with no host treatment, but had no effect on H. bacteriophora dispersal. Also H. bacteriophora was better at infecting non-mobile than mobile hosts released into the microcosms and S. carpocapsae was better at infecting mobile than non-mobile hosts, thus affirming the established cruiser-ambusher theory. However, results also revealed that a large proportion of infective juveniles (IJs) of both species stayed near (≤ 3.8 cm) the source cadaver (88-96% S. carpocapsae; 67-79% H. bacteriophora), and the proportion of IJs reaching the farthest distance (11.4 cm) was significantly higher for S. carpocapsae (1.4%) than H. bacteriophora (0.4%) in the presence of mobile hosts. S. carpocapsae also had higher average population displacement than H. bacteriophora in the presence of both the non-mobile (5.07 vs. 3.6 cm/day) and mobile (8.06 vs. 5.3 cm/day) hosts. We conclude that the two species differ in their dispersal and foraging behavior at the population level and this behavior is affected by both the presence and absence of hosts and by their mobility

Comparison of average population displacement of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence and absence of mobile and non-mobile hosts.

<p>Average (±SE) population displacement (cm/day) of infective juveniles (IJs) of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> dispersed to 2 cm wide annuli at all three distances, 3.8, 7.6 and 11.4 cm over a period of 12 to 72 hr in the presence and absence of mobile and non-mobile hosts in the microcosms containing autoclaved field soil with vegetation. Quadratic regression lines fitted for the average population displacement of IJs of each of the two species dispersed in the presence and absence of mobile and non-mobile hosts are bracketed. Double asterisk (**) indicates the significant difference in the estimated linear and quadratic coefficients between the two species, horizontally and the type of host (mobile <i>vs</i> non-mobile) within a species, vertically (from regression analyses, <i>P</i> ≤ 0.05). Analysis of variance indicates significant difference in the average population displacement of a species in the presence and absence of mobile and non-mobile hosts at <i>P</i> ≤ 0.05.</p><p>Comparison of average population displacement of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence and absence of mobile and non-mobile hosts.</p

Lateral dispersal of <i>Steinernema carpocapsae</i> in the presence and absence of mobile hosts.

<p>Mean (±SE) number of infected <i>Galleria mellonella</i> baits from the collected soil core samples and mean (±SE) percentage of infective juveniles (IJs) of <i>Steinernema carpocapsae</i> dispersed to a 2 cm wide annulus at distances, 3.8, 7.6 and 11.4 cm over a period of 12 to 72 hr in the presence of 2 mobile hosts, <i>G</i>. <i>mellonella</i> larvae placed outside the wire mesh cylinder (3.8 cm dia) enclosing the source cadaver in the microcosms containing autoclaved field soil with vegetation as opposed to similar microcosms containing no hosts. Double asterisk (**) indicates significant difference between distances (D), presence or absence of the host (H) and interaction between the two (D*H) at a time point at <i>P</i> ≤ 0.05. Capital and small letters indicate Tukey’s comparison for means separation between distances averaged over presence or absence of the host, and presence and absence of host averaged over distances, respectively at a time point. NS = <i>P</i> > 0.05.</p><p>Lateral dispersal of <i>Steinernema carpocapsae</i> in the presence and absence of mobile hosts.</p

Experimental design.

<p>A pictorial representation of the experimental design showing 0.05 m² sized microcosm used for studying dispersal of infective juveniles (IJs) emerging from the source nematode-infected cadaver (black rectangle) placed in the center in the presence of a non-mobile host (<i>Galleria mellonella</i> larva in a wire mesh cage) (<b>A</b>) and mobile hosts (<i>G</i>. <i>mellonella</i> larvae) (<b>B</b>). Each microcosm was divided into four quadrants. Host insects released in the microcosm are represented by empty circles. Soil cores samples were collected from 2 cm wide arcs (dotted lines) in the microcosms containing non-mobile host at different distances marked by black spots and at different time intervals depicted by the transects running through the microcosms at 12, 24, 48 and 72 hr, and then baited with live <i>G</i>. <i>mellonella</i> larvae in plastic cups (30 ml) to recover the dispersed IJs. Similarly, soil core samples were collected from 2 cm wide annuli (dotted circles) at different distances starting from the outer edge of the wire-mesh cylinder (bold circle) and at different time intervals in the microcosms with and without mobile hosts.</p

Quadratic regression lines fitted for <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence of hosts.

<p>Quadratic regression lines fitted for the mean number of <i>Galleria mellonella</i> baits infected by <i>Heterorhabditis bacteriophora</i> (Hb) and <i>Steinernema carpocapsae</i> (Sc) and mean percentage of IJs of the two species at a particular distance from the source cadaver in a microcosm over a period of 12 to 72 hr in the presence of non-mobile and mobile hosts.</p><p>Quadratic regression lines fitted for <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence of hosts.</p

Lateral dispersal of <i>Steinernema carpocapsae</i> in the presence and absence of non-mobile hosts.

<p>Mean (±SE) number of infected <i>Galleria mellonella</i> baits from the collected soil core samples and mean (±SE) percentage of infective juveniles (IJs) of <i>Steinernema carpocapsae</i> dispersed to a 2 cm wide arc at distances, 3.8, 7.6 and 11.4 cm over a period of 12 to 72 hr in the presence of a live non-mobile host, <i>G</i>. <i>mellonella</i> contained in a wire mesh cage buried in one of the quadrants in the microcosms containing autoclaved field soil with vegetation in comparison with the opposite quadrant of the same microcosm containing no host. Double asterisk (**) indicates significant difference between distances (D), presence or absence of the host (H) and interaction between the two (D*H) at a time point at <i>P</i> ≤ 0.05. Capital and small letters indicate Tukey’s comparison for means separation between distances averaged over presence or absence of the host, and presence and absence of host averaged over distances, respectively at a time point. NS = <i>P</i> > 0.05.</p><p>Lateral dispersal of <i>Steinernema carpocapsae</i> in the presence and absence of non-mobile hosts.</p

Lateral dispersal of <i>Heterorhabditis bacteriophora</i> in the presence and absence of mobile hosts.

<p>Mean (±SE) number of infected <i>Galleria mellonella</i> baits from the collected soil core samples and mean (±SE) percentage of infective juveniles (IJs) of <i>Heterorhabditis bacteriophora</i> dispersed to a 2 cm wide annulus at distances, 3.8, 7.6 and 11.4 cm over a period of 12 to 72 hr in the presence of 2 mobile hosts, <i>G</i>. <i>mellonella</i> larvae placed outside the wire mesh cylinder (3.8 cm dia) enclosing the source cadaver in the microcosms containing autoclaved field soil with vegetation as opposed to similar microcosms containing no hosts. Double asterisk (**) indicates significant difference between distances (D), presence or absence of the host (H) and interaction between the two (D*H) at a time point at <i>P</i> ≤ 0.05. Capital and small letters indicate Tukey’s comparison for means separation between distances averaged over presence or absence of the host, and presence and absence of host averaged over distances, respectively at a time point. NS = <i>P</i> > 0.05.</p><p>Lateral dispersal of <i>Heterorhabditis bacteriophora</i> in the presence and absence of mobile hosts.</p

Comparison of lateral dispersal of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence of mobile and non-mobile hosts.

<p>Mean (±SE) number of dead <i>Galleria mellonella</i> baits from the collected soil core samples and mean (±SE) percentage of infective juveniles (IJs) of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> dispersed to 2 cm wide annuli at all three distances, 3.8, 7.6 and 11.4 cm over a period of 12 to 72 hr in the presence of mobile and non-mobile host in the microcosms containing autoclaved field soil with vegetation. Quadratic regression lines fitted for the mean number of dead <i>G</i>. <i>mellonella</i> baits and mean percentage of IJs of each of the two species dispersed up to 11.4 cm distance from the source cadaver over a period of 72 hr in the presence of mobile and non-mobile hosts are bracketed. Double asterisk (**) indicates the significant difference in the estimated linear and quadratic coefficients between the two species, horizontally and the type of host within a species, vertically (from regression analyses, <i>P</i> ≤ 0.05). NS = <i>P</i> > 0.05.</p><p>Comparison of lateral dispersal of <i>Heterorhabditis bacteriophora</i> and <i>Steinernema carpocapsae</i> in the presence of mobile and non-mobile hosts.</p