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Keywords:

  • community indices;
  • Heterorhabditis bacteriophora;
  • Heterorhabditis indica;
  • soil fauna;
  • turfgrass ecosystem

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    There is growing awareness that biological control carries risks as well as benefits, but there are few data on below-ground effects of inundative insect pathogens. We addressed this issue using entomopathogenic nematodes and the soil nematode community in a turfgrass ecosystem as a model.
  • 2
    Application of Heterorhabditis bacteriophora strain GPS11, Heterorhabditis bacteriophora strain HP88 and Heterorhabditis indica strain LN2 significantly reduced the abundance, species richness, diversity and maturity of the nematode community by reducing the number of genera and abundance of plant-parasitic, but not free-living, nematodes.
  • 3
    Our results are the first to indicate selective suppression of plant-parasitic nematodes by entomopathogenic nematodes, H . bacteriophora and H. indica , with no adverse effect on free-living nematodes.
  • 4
    In contrast to the entomopathogenic nematode treatments, trichlorfon (a commonly used insecticide in turfgrass) reduced the number of genera, abundance and diversity of the nematode community by adversely affecting both plant-parasitic and free-living nematodes.
  • 5
    The reduction in abundance and diversity of plant-parasitic nematodes without any adverse effect on free-living nematodes that play a role in nutrient cycling, can be considered as a beneficial non-target effect of entomopathogenic nematodes. The mechanisms causing such an effect need to be elucidated in future studies.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Soil faunal biodiversity is an important determinant of key ecosystem functions (Hendrix et al. 1986; Moore & de Ruiter 1991; Lavelle et al. 1992). Control of insect pests can influence the densities of target and non-target organisms and the associated indirect interactions in the soil food web, potentially leading to a reduction in species richness and diversity in native communities. Recent reviews of the ecological consequences of the introduction of biological control agents (Follet & Duan 2000; Strong & Pemberton 2000; Wajnberg, Scott & Quimby 2001) suggest that biological control carries risks as well as benefits. These reviews emphasize the need for understanding any collateral damage to native fauna resulting from biological control introductions. Most studies on non-target effects of biological control introductions have been focused on above-ground flora and fauna with emphasis on direct (parasite–host or predator–prey) relationships. Little is known about the effects of introduction or augmentation of biological control agents on the diversity of native fauna in below-ground food webs. We addressed this issue for the case of entomopathogenic nematodes, considering the response of the naturally occurring nematode community in a turfgrass ecosystem. Further, we compared the impact of entomopathogenic nematodes on the soil nematode community with that of trichlorfon, a commonly used insecticide in turfgrass.

Nematodes are ubiquitous soil fauna that interact in ecosystems directly as herbivores on plants and indirectly as consumers of microflora and fauna, thus playing a significant role in regulating primary production, predation, decomposition of organic matter and nutrient cycling (Coleman, Cole & Elliott 1984; Ingham et al. 1985; Griffiths 1990). Nematodes possess attributes that make them useful ecological indicators (Bernard 1992; Ritz & Trudgill 1999). Analyses to determine the effect of agricultural management practices on nematode community structure and function are generally based on nematode species, generic or trophic group abundance, diversity indices and maturity indices (Heip, Herman & Soetaert 1988; Ludwig & Reynolds 1988; Bongers 1990; Yeates et al. 1993). Nematode community indices have been used for monitoring the changes in both natural ecosystems and agroecosystems induced by a variety of disturbances (Semoiloff 1987; Wasilewska 1989; Bongers, Alkemade & Yeates 1991; Ettema & Bongers 1993; Freckman & Ettema 1993; Ferris, Venette & Lau 1996; Yeates, Wardle & Watson 1999).

Entomopathogenic nematodes are currently marketed world-wide for the biological control of insect pests (Grewal & Georgis 1998). Wide host range, high efficacy, lack of mammalian toxicity and the availability of techniques for economic mass production have led to the rapid increase in the use of these biological control agents in recent years (Grewal & Georgis 1998). Entomopathogenic nematodes in the genera Steinernema Travassos and Heterorhabditis Poinar (Nematoda: Steinernematidae and Heterorhabditidae) are lethal insect parasites (Kaya & Gaugler 1993). The non-feeding third-stage infective juveniles penetrate into the haemocoel of host insects through natural openings and release symbiotic bacteria (Xenorhabdus spp. Thomas and Poinar for Steinernematidae, and Photorhabdus spp. Boemere et al. for Heterorhabditidae). Toxins produced by the developing nematodes (Burman 1982; Ehlers, Wulff & Peters 1997) and bacteria (Dunphy & Webster 1988; Bowen et al. 1998) cause septicaemia and kill the insect host usually within 48 h of infection. Nematodes complete two to three generations inside the host. When the host cadaver is consumed, the next group of infective juveniles is produced, which leaves the cadaver in search of new hosts in the soil.

The susceptibility of non-target organisms to entomopathogenic nematodes has been tested in several laboratory and glasshouse experiments, but only a few studies have considered their impact on non-target soil fauna under field conditions. No significant adverse effects of entomopathogenic nematodes were observed on populations of collembolans and mites (Ishibashi et al. 1987; Georgis, Kaya & Gaugler 1991) or non-target insects in the families Carabidae, Histiridae, Staphylinidae and Gryllidae (Georgis, Kaya & Gaugler 1991; Koch & Bathon 1993) under field conditions. Some field studies have indicated that inundative applications of entomopathogenic nematodes suppress populations of plant-parasitic nematodes in soil (Smitley, Warner & Bird 1992; Grewal et al. 1997), but little is known about their impact on the structure and function of the rest of the nematode community.

In this study we tested the following hypotheses: (i) inundative application of entomopathogenic nematodes changes the structure of the nematode community in soil; and (ii) entomopathogenic nematodes and chemical insecticides differ in their impact on the soil nematode community. We addressed these hypotheses by using nematode community indices to quantify the changes in the composition of the soil nematode fauna in response to inundative application of entomopathogenic nematodes and a chemical insecticide.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

STUDY SITE AND DESIGN OF EXPERIMENT

Experimental plots were established in autumn 1999 on a golf course rough in Medina County, Ohio, USA, which was managed with minimal fertilizer and pesticide applications. The site was composed of about 40% kentucky blue grass Poa pratensis L., 30% perennial ryegrass Lolium perenne L. and 20% annual blue grass Poa annua L. The remaining 10% of vegetation was a mixture of other annual grasses and broad-leaf weeds. The experimental site was surrounded by golf course fairways composed of a mixture of kentucky blue grass and perennial ryegrass. The fairways were managed intensively with fertilizer and pesticide applications recommended for golf courses. The soil type was sandy-loam with 12% clay, 30% silt, 58% sand, 4·2% organic matter and pH 6·0. The soil moisture and temperature during the sampling period ranged from 15% to 20% and 12 °C to 21 °C, respectively. The experiment was laid out in a randomized block design with four replicates for each treatment. Plot size was 4 m2 and plots were separated from each other by 90-cm wide alleyways. The experiment included three biological control treatments of entomopathogenic nematodes, (i) Heterorhabditis bacteriophora Poinar strain GPS11, a strain native to Ohio (Wang & Grewal 2002), (ii) Heterorhabditis bacteriophora strain HP88, a strain non-native to Ohio but indigenous to the USA (Poinar & Georgis 1990) and (iii) Heterorhabditis indica Poinar, Karunaker & David strain LN2, a non-native species to the USA (Poinar, Karunakar & David 1992), each applied at 4 × 109 infective-juveniles ha−1, and a chemical insecticide treatment of trichlorfon (dimethyl 2,2,2-trichloro-1-hydroxyethyl phosphonate) at 9 kg a.i. ha−1. The dosage used for all the treatments was comparable with their respective commercial application rates. Four plots were maintained without any treatment as untreated controls. The nematode species and strains used in this study were cultured in vivo in the last instar larvae of the greater wax moth Galleria mellonella L. using the methods described by Dutky, Thompson & Cantwell (1964). The infective juveniles that emerged within the first 10 days were collected and stored at 10 °C in tap water in tissue culture flasks for 10 days. All the nematodes were acclimated to room temperature for at least 48 h before application. Nematode and trichlorfon suspensions of 420 infective juveniles ml−1 and 0·01% concentration, respectively, were prepared with tap water. Nematode or trichlorfon suspensions were applied at 3·78 l plot−1 in the evening with a sprinkler can. An equal volume of tap water was applied to the untreated control plots. A 0·5-cm irrigation was given to the plots immediately after application of treatments to rinse the nematodes and insecticide into the soil.

SAMPLING, EXTRACTION AND IDENTIFICATION OF NEMATODES

Soil samples were collected prior to the application of treatments, 30 and 60 days after treatment to monitor changes in the nematode community. At each sampling time, nine soil cores (2-cm diameter, 10-cm deep) were removed randomly from each replicate plot. Soil cores from each replication were mixed thoroughly to form a composite soil sample to reduce the variance associated with the aggregated spatial pattern of nematodes in soil (Barker & Campbell 1981). Soil cores were taken from 10-cm depth because preliminary studies showed greater abundance of nematodes and roots in this zone. All the soil samples were stored at existing field moisture levels at 5 °C to minimize changes in the nematode population prior to the extraction (Barker, Nusbaum & Nelson 1969). Analysis of soil samples collected prior to the application of treatments indicated no significant differences in nematode community indices in experimental plots (data not shown).

Nematodes were extracted using the Baermann funnel technique from 20-g soil subsamples taken from each composite sample (Flegg & Hooper 1970). Nematodes were collected at 24-h intervals for 72 h and preserved in a 5% formalin solution. Nematodes in one-fifth of the extracted sample were identified to the genus level under an inverted compound microscope at 400× using diagnostic keys given by Goodey (1963), Andrassy (1984), Siddiqi (1986), Jairajpuri & Ahmed (1992) and Hunt (1993). The abundance of each genus in each sample was calculated by multiplying the number of nematodes of that genus in the 1/5 of the extracted sample used for identification with the total volume of the sample (Freckman & Ettema 1993; Ferris, Venette & Lau 1996; Wall-Freckman & Huang 1998). The nematode genera were assigned to a trophic group (plant parasites, bacterial feeders, fungal feeders, predators and omnivores) according to Yeates et al. (1993), and were also assigned a colonizer–persister value (cp-value) according to Bongers (1990).

NEMATODE COMMUNITY ANALYSES

The nematode community in each sample was analysed by the following measures. (1) Species/community measures: (i) number of taxa (species richness); (ii) abundance; (iii) trophic structure based on the relative abundance of each trophic group (Yeates et al. 1993). (2) Diversity measures (Heip, Herman & Soetaert 1988; Ludwig & Reynolds 1988): (i) Shannon–Wiener diversity index (H′), i.e. H′ = -ΣPi(ln Pi), where Pi is the proportion of taxa i in the total population; (ii) Hill's N1, i.e. N1 = eH′; (iii) Simpson's diversity index (λ), i.e. λ=Σni(ni − 1)/n (n − 1), where ni is the number of individuals of the taxa i and n is the total number of individuals in the community; (iv) Hill's N2, i.e. N2 = 1/λ. (3) Maturity indices (Bongers 1990; Yeates 1994): (i) maturity index for free-living nematodes (MI); (ii) maturity index of plant-parasitic nematodes; (PPI); (iii) combined maturity of free-living and plant-parasitic nematodes (ΣMI). The maturity index measures the stability and disturbance level of an ecosystem based on the type and abundance of nematodes present. It is a measure based on the life-history characteristics of nematode taxa. In this index, the nematode taxa were classified on a c–p scale of 1–5, with colonizers (short life cycle, high reproduction rates, tolerance to disturbance) = 1 and persisters (long life cycle, low colonization ability, few offspring, sensitive to disturbance) = 5. Free-living nematode taxa were assigned c–p-values 1–5 while plant-parasitic taxa were assigned c–p-values from 2 to 5, because there were no plant-parasitic nematodes designated as c–p1 (Bongers 1990). Maturity indices were calculated as weighted means of the values assigned to the constituent nematode taxa. Weighted means were expressed mathematically as (Σvi × fi)/n, where vi is the c–p-value assigned to genus i, fi is the frequency of genus i in the sample and n is the total number of the individuals in sample. In addition to the diversity indices based on all nematodes, indices for plant-parasitic and free-living nematode groups were computed separately to obtain better resolution of the impact of treatments on the structure and function of the soil nematode community.

STATISTICAL ANALYSES

All the analyses were performed using the general linear model procedure of Statistica, release 5·5 (Statsoft 1999). Data on nematode community indices were analysed by analysis of variance (anova) and treatments were compared using linear contrasts. An alpha level ≤ 0·05 was considered significant.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

ABUNDANCE

Nematode abundance was significantly lower in all the treatments relative to the control 30 and 60 days after the treatment [F = 21·96, d.f. = 1,12; P = 5·2 × 10−4 (30 days); F = 58·78, d.f. = 1,12; P = 6 × 10−6 (60 days)] (Table 1a,b). Entomopathogenic nematode treatments yielded significantly lower abundances of plant-parasitic nematodes [F = 43·81, d.f. = 1,12; P = 2·5 × 10−5 (30 days); F = 72·58, d.f. = 1,12; P = 2 × 10−6 (60 days)] but not of free-living nematodes (including bacterial feeders, fungivores, predators and omnivores) relative to the untreated control 30 and 60 days after the treatment (P > 0·05) (Table 1a,b). In contrast, the insecticide, trichlorfon yielded significantly lower abundances of both plant-parasitic [F = 8·87, d.f. = 1,12; P = 0·012 (30 days); F = 12·09, d.f. = 1,12; P = 0·005 (60 days)] and free-living nematodes [F = 34·68, d.f. = 1,12; P = 7·4 × 10−5 (30 days); F = 11·63, d.f. = 1,12; P = 0·005 (60 days)] relative to the control 30 and 60 days after the treatment (Table 1a,b).

Table 1.  Mean (± SE) abundance (number per 20 g soil) and number of genera of plant-parasitic, free-living and total nematodes, and relative abundance of nematode trophic groups in soil samples from four replicates collected (a) 30 days, and (b) 60 days after treatment. Treatments were: Hb-gps, Heterorhabditis bacteriophora strain GPS11; Hb-hp,  Heterorhabditis bacteriophora strain HP88; H-ind,  Heterorhabditis indica strain LN2; TF, trichlorfon; UC, untreated control. NS, not significant
 TreatmentF -ratio 12 d.f. P
Hb-gpsHb-hpH-indTFUC
(a) 30 days after treatment
Abundance
 Plant parasites 75·00 ± 12·60 75·00 + 5·00 70·00 ± 5·80105·00 ± 12·90155·00 ± 9·6011·45  0·001
 Free-living200·00 ± 14·10175·00 + 9·60160·00 ± 8·20 95·00 ± 12·90175·00 ± 9·6016·68< 0·001
 Total nematodes275·00 ± 17·10250·00 ± 12·9230·00 ± 12·90200·00 ± 21·60330·00 ± 17·3 7·98  0·002
Number of genera
 Plant parasites  3·50 ± 0·50  3·25 ± 0·25  3·25 ± 0·25  4·00 ± 0·25  5·00 ± 0·41 5·80  0·008
 Free-living  7·00 ± 0·40  5·25 ± 0·25  4·25 ± 0·48  4·00 ± 0·40  5·50 ± 0·29 8·84  0·001
 Total nematodes 10·50 ± 0·86  8·50 ± 0·28  7·50 ± 0·50  8·00 ± 0·40 10·50 ± 0·28 6·15  0·006
Relative abundance of trophic groups
 Plant parasites  0·25 ± 0·03  0·29 ± 0·02  0·30 ± 0·01  0·53 ± 0·05  0·45 ± 0·0323·67< 0·001
 Bacterivores  0·56 ± 0·04  0·61 ± 0·04  0·57 ± 0·04  0·40 ± 0·05  0·39 ± 0·03 5·40  0·01
 Fungivores  0·02 ± 0·02  0·02 ± 0·02  0·00 ± 0·00  0·02 ± 0·02  0·02 ± 0·02 0·25NS
 Predators  0·04 ± 0·02  0·02 ± 0·02  0·04 ± 0·04  0·00 ± 0·00  0·04 ± 0·03 0·55NS
 Omnivores  0·12 ± 0·03  0·06 ± 0·02  0·09 ± 0·05  0·05 ± 0·05  0·09 ± 0·02 0·92NS
(b) 60 days after treatment
Abundance
 Plant parasites 75·00 ± 9·60 55·00 + 5·00 60·00 ± 5·00115·00 ± 12·60165·00 ± 17·1019·84< 0·001
 Free-living175·00 ± 17·10170·00 + 9·60170·00 ± 9·60 95·00 ± 5·00155·00 ± 12·90 8·63  0·002
 Total nematodes250·00 ± 17·30225·00 ± 5·80230·00 ± 10·00210·00 ± 10·00320·00 ± 9·6016·73< 0·001
Number of genera
 Plant parasites  3·50 ± 0·29  3·00 ± 0·25  2·75 ± 0·25  4·00 ± 0·40  5·50 ± 0·50 9·83< 0·001
 Free-living  5·75 ± 0·75  5·25 ± 0·25  5·50 ± 0·29  4·00 ± 0·25  5·50 ± 0·65 8·84  0·001
 Total nematodes  9·25 ± 0·75  8·25 ± 0·25  8·25 ± 0·48  8·00 ± 0·40 11·00 ± 0·40 5·74  0·008
Relative abundance of trophic groups
 Plant parasites  0·30 ± 0·03  0·24 ± 0·02  0·26 ± 0·01  0·55 ± 0·04  0·50 ± 0·0520·504< 0·001
 Bacterivores  0·54 ± 0·03  0·61 ± 0·03  0·57 ± 0·06  0·33 ± 0·04  0·32 ± 0·08 9·05  0·001
 Fungivores  0·00 ± 0·00  0·00 ± 0·00  0·02 ± 0·02  0·00 ± 0·00  0·02 ± 0·02 0·70NS
 Predators  0·04 ± 0·03  0·07 ± 0·04  0·07 ± 0·04  0·05 ± 0·02  0·08 ± 0·02 0·24NS
 Omnivores  0·12 ± 0·02  0·08 ± 0·02  0·08 ± 0·04  0·07 ± 0·02  0·08 ± 0·03 0·20NS

TROPHIC STRUCTURE

The trophic structure in all the treatments was dominated by bacterial-feeding and plant-parasitic nematodes (Table 1a,b). The relative abundance of plant-parasitic nematodes was significantly lower in entomopathogenic nematode treatments considered as a group compared with the trichlorfon treatment [F = 78·19, d.f. = 1,12; P = 1 × 10−6 (30 days); F = 58·69, d.f. = 1,12; P = 6 × 10−6 (60 days)] and the untreated control [F = 36·84, d.f. = 1,12; P = 5·6 × 10−5 (30 days); F = 44·07, d.f. = 1,12; P = 2·4 × 10−5 (60 days)] 30 and 60 days after the treatment (Table 1a,b). The percentage of plant-parasitic nematodes never exceeded 30% in the entomopathogenic nematode treatments. In contrast, the relative abundance of bacterial feeders was significantly more in the entomopathogenic nematode treatments compared with trichlorfon [F = 12·72, d.f. = 1,12; P = 0·004 (30 days); F = 19·54, d.f. = 1,12; P = 8·3 × 10−4 (60 days)] and the untreated control [F = 14·33, d.f. = 1,12; P = 0·003 (30 days); F = 22·11, d.f. = 1,12; P = 5·1 × 10−4 (60 days)], and accounted for more than 50% of the community 30 and 60 days after the treatment (Table 1a,b). The relative abundance of fungivores, predators and omnivores in the nematode community was not affected by the treatments (P > 0·05) and these three groups collectively accounted for less than 20% of the nematode community in all the treatments (Table 1a,b).

SPECIES RICHNESS (NUMBER OF GENERA)

The mean numbers of total nematode genera were significantly lower in entomopathogenic nematode treatments considered as a group relative to the untreated control 30 and 60 days after the treatment [F = 6·41, d.f. = 1,12; P = 0·026 (30 days); F = 16·29, d.f. = 1,12; P = 0·002 (60 days)] (Table 1a,b). The trichlorfon treatment also yielded significantly lower numbers of total nematode genera relative to the control 30 and 60 days after the treatment [F = 9·61, d.f. = 1,12; P = 0·009 (30 days); F = 16·74, d.f. = 1,12; P = 0·001 (60 days)] (Table 1a,b). Mean numbers of plant-parasitic genera were significantly lower in entomopathogenic nematode treatments considered as a group relative to the untreated control [F = 22·22, d.f. = 1,12; P = 0·0005 (30 days); F = 36·25, d.f. = 1,12; P = 6 × 10−5 (60 days)] 30 and 60 days after the treatment. However, mean numbers of free-living genera were not affected by entomopathogenic nematode treatments considered as a group 30 and 60 days after the treatment (P > 0·05) (Table 1a,b). In contrast, trichlorfon yielded significantly lower numbers of both plant-parasitic [F = 5·33, d.f. = 1,12; P = 0·03 (30 days); F = 9·31, d.f. = 1,12; P = 0·01 (60 days)] and free-living [F = 7·01, d.f. = 1,12; P = 0·02 (30 days); F = 5·68, d.f. = 1,12; P = 0·03 (60 days)] nematode genera compared with the untreated control 30 and 60 days after the treatment (Table 1a,b).

Overall, 28 genera in 21 families were identified in the five treatments. The total number of genera within the treatments ranged from 17 in the H. indica treatment to 24 in the untreated control. In all the treatments, three to four families comprised more than 50% of the total. Nematode families Rhabditidae and Cephalobidae showed the highest mean abundance among free-living nematodes in all the treatments, while Tylenchidae or Hoplolaimidae showed the highest mean abundance among plant-parasitic nematodes 30 and 60 days after the treatment. Rhabditis and Acrobeloides were the dominant bacterial feeders and Hoplolaimus was the dominant plant parasite in all the treatments. Mononchus and Aphelenchoides were the only predatory and fungivore genera, respectively, observed in the samples. Aporcelaimellus and Eudorylaimus were the dominant omnivore genera in all the treatments.

DIVERSITY INDICES

The Shannon–Weiner diversity index, Simpson's diversity index and their transformed forms Hill's N1 and Hill's N2, respectively, showed similar differences in nematode diversity among treatments. Consequently, only N1 data are presented to represent the diversity of the nematode community (Fig. 1a,b). Diversity across all the trophic groups was significantly lower when the entomopathogenic nematodes were considered as a group [F = 5·85, d.f. = 1,12; P = 0·03 (30 days); F = 14·84, d.f. = 1,12, P = 0·002 (60 days)] and in the trichlorfon [F = 8·56, d.f. = 1,12; P = 0·012 (30 days); F = 14·23, d.f. = 1,12; P = 0·003 (60 days)] treatments compared with the untreated control 30 and 60 days after the treatment (Fig. 1a,b). Entomopathogenic nematodes considered as a group yielded significantly lower diversity within the plant-parasitic nematodes compared with the untreated control 30 (F = 12·75, d.f. = 1,12; P = 0·004) and 60 (F = 32·70, d.f. = 1,12; P = 9·6 × 10−5) days after the treatment, but did not affect the diversity within the free-living nematodes (P > 0·05). In contrast, in the trichlorfon treatment, the diversity within the free-living [(F = 5·76, d.f. = 1,12; P = 0·03 (30 days)] and plant-parasitic [F = 9·03, d.f. = 1,12; P = 0·01 (60 days)] nematodes was significantly lower compared with the untreated control (Fig. 1a,b).

image

Figure 1. Mean (+ SE) diversity indices (Hill's N1) of plant-parasitic, free-living and total nematodes in soil samples from four replicates collected (a) 30 days and (b) 60 days after treatment. Treatments were: Hb-gps, Heterorhabditis bacteriophora strain GPS11; Hb-hp,  Heterorhabditis bacteriophora strain HP88; H-ind,  Heterorhabditis indica strain LN2; TF, trichlorfon; UC, untreated control.

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MATURITY INDICES

The maturity indices of free-living (MI) and plant-parasitic nematodes (PPI) were not affected by any treatment 30 and 60 days after the treatment (P > 0·05) (Fig. 2a,b). Entomopathogenic nematode treatments considered as a group yielded significantly lower ΣMI relative to the untreated control 30 (F = 6·61, d.f. = 1,12; P = 0·02) and 60 (F = 11·65, d.f. = 1,12; P = 0·005) days after the treatment, but the trichlorfon showed no effect on ΣMI (Fig. 2a,b). The free-living nematode taxa in the MI were represented in only three of the five c–p groups, 1, 2 and 4. The plant-parasitic nematode taxa in the PPI were represented in all four c–p groups, 2, 3, 4 and 5.

image

Figure 2. Mean (+ SE) maturity indices of plant-parasitic (PPI), free-living (MI) and total nematodes (ΣMI) in soil samples from four replicates collected (a) 30 days and (b) 60 days after treatment. Treatments were: Hb-gps, Heterorhabditis bacteriophora strain GPS11; Hb-hp,  Heterorhabditis bacteriophora strain HP88; H-ind,  Heterorhabditis indica strain LN2; TF, trichlorfon; UC, untreated control.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Our results indicate that inundative application of entomopathogenic nematodes changes the structure of the nematode community in a turfgrass ecosystem. Total nematode abundance significantly decreased in all the treatments relative to the untreated control, reflecting differences that could be attributed to the disturbance induced by pest control treatments. The abundance of plant-parasitic nematodes was significantly reduced in all the entomopathogenic nematode treatments while the abundance of free-living nematodes was not affected. In contrast with the nematode treatments, trichlorfon reduced the abundance of both plant-parasitic and free-living nematodes (including bacterial feeders, fungal feeders, predators and omnivores). These results agree with earlier observations that free-living nematodes were relatively more sensitive to chemical pesticides compared with plant-parasitic nematodes (Ishibashi, Kondo & Ito 1983; Smolik 1983; Yardim & Edwards 1998).

The entomopathogenic nematodes considered as a group significantly reduced the species richness and diversity, and maturity of the nematode community, in soil by reducing the number and abundance of plant-parasitic nematode genera. However, the free-living nematode groups in soil remained unaffected by the entomopathogenic nematode treatments. In contrast to the entomopathogenic nematodes, trichlorfon reduced the species richness and diversity of the nematode community by adversely affecting both plant-parasitic and free-living nematode genera, indicating the differential response of nematode trophic groups to biological and chemical pest control treatments. None of the experimentally applied species were recovered 30 or 60 days after the treatment. This may be because of the extremely low population densities compared with the other nematodes present in the soil (Wasilewska 1979). Previous studies have also shown that populations of entomopathogenic nematodes usually decline to below detectable levels within 4 weeks of inundative application to the soil (Georgis 1992; Smits 1996). Therefore, it is less likely that these species will be recovered in the soil samples collected 30 or 60 days after the application.

Nematode diversity indices based on all the nematodes are considered to be indicators of ecosystem status and are commonly used for comparing nematode communities in ecosystems of varying human intervention (Freckman & Ettema 1993; McSorley 1997; Yeates, Wardle & Watson 1999). The nematode community in the soil is composed of plant-parasitic and free-living nematode trophic groups that play mutually contrasting roles in ecosystem functioning and often differ in their response to disturbances in the ecosystem (Bongers 1990). Therefore, diversity indices based on total nematode taxa alone may not be sufficient to assess the impact of disturbance on the nematode community. For instance, in this study both insecticide and non-native entomopathogenic nematode treatments had lower values of total nematode abundance and diversity indices but they differed in abundance and diversity within plant-parasitic and free-living nematode groups. We suggest that measuring changes in trophic group composition and diversity within plant-parasitic and free-living nematode groups in addition to the indices based on total nematode taxa would yield a better indication of how management practices impact upon the structure and function of the soil nematode community.

Our results are the first to indicate selective suppression of plant-parasitic nematodes by the entomopathogenic nematodes H. bacteriophora and H. indica, with no adverse effect on free-living nematodes under field conditions. Suppression of plant-parasitic nematodes in response to the application of entomopathogenic nematodes Steinernema spp. has been documented in several glasshouse and field studies (Bird & Bird 1986; Ishibashi & Kondo 1986; Ishibashi & Choi 1991; Smitley, Warner & Bird 1992; Grewal et al. 1997; Perry et al. 1998; Lewis, Grewal & Sardanelli 2000). Contrary to conclusions based on diversity and maturity indices, it could be assumed that plant production would be enhanced by the entomopathogenic nematodes due to the lower number, abundance and c-p-values of plant-parasitic nematode taxa in these treatments.

Various mechanisms have been proposed to explain the suppression of plant-parasitic nematodes by entomopathogenic nematodes: (i) crowding of entomopathogenic nematodes along the plant roots forces plant-parasitic nematodes away (Bird & Bird 1986); (ii) massive doses of entomopathogenic nematodes lead to the build-up of nematode antagonistic organisms in the soil resulting in nematode suppression (Ishibashi & Kondo 1986; Ishibashi & Choi 1991); and (iii) allelochemicals like ammonium and indole produced by entomopathogenic nematodes and/or their symbiotic bacteria (Xenorhabdus spp. and Photorhabdus spp.) inhibit egg hatching and repel or intoxicate plant-parasitic nematodes (Grewal, Lewis & Venkatachari 1999; Hu, Li & Webster 1999).

In this study, the impact of entomopathogenic nematodes on the soil nematode community can be interpreted as a beneficial non-target effect based on the importance of plant-parasitic nematodes as agricultural pests. However, we cannot predict a similar effect on other soil fauna or flora. Furthermore, the metabolic products of symbiotic bacteria (Xenorhabdus spp. and Photorhabdus spp.) of entomopathogenic nematodes were reported to possess a broad spectrum of biological activity including insecticidal, nematicidal anti-mycotic, anti-carcinogenic and antibiotic properties (Webster, Chen & Li 1998). Therefore, the exact mechanisms that result in this effect need to be resolved in order to ascertain the safety of entomopathogenic nematodes to other organisms in the soil food webs.

Introduced biological control agents may have a short-term and/or long-term impact on native communities over a smaller or larger geographical area (Simberloff & Stiling 1996). Entomopathogenic nematodes do not have the ability to disperse long distances as they can only move up to a few centimetres per day through moist soil. (Strong et al. 1996). Furthermore, field releases of entomopathogenic nematodes quickly come to reflect natural densities and distributions (Georgis 1992; Ehlers & Hokkanen 1996; Smits 1996; Campbell et al. 1998), thus long-term negative effects on the local environment of entomopathogenic nematodes are likely to be negligible (Gaugler, Lewis & Stuart 1997). Therefore, the impact of entomopathogenic nematodes on the soil nematode community structure observed in this study could be a short-term effect confined to the application sites. However, the impact of entomopathogenic nematodes, even if it is a short-term effect, is significant because entomopathogenic nematodes are applied often as inundative biological control agents, and repeated applications of these nematodes to control recurring pest populations may sustain the effect. Therefore, the long-term ecological consequences of inundative applications of entomopathogenic nematodes need to be investigated in future studies.

Our results indicate that introduced biological control agents have the potential to affect the diversity of native fauna in soil ecosystems even though they do not have any direct parasite/host or predator/prey relationship. Therefore, the risks of biological control programmes need to be assessed in the broader context of their impact on all native fauna in the target ecosystems. Furthermore, our results emphasize the need for research to understand collateral damage to native fauna resulting from introduced species to ensure the safety and public confidence in biological control programmes.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This work was supported by a grant to P. S. Grewal from the United States Department of Agriculture's Fund for Rural America program. Elizabeth A. B. De Nardo was supported by funds from EMBRAPA – CNPMA, Brazil. We thank Dr D. A. Neher, University of Toledo, for suggestions on nematode diversity indices.

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  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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