Effects of Earthworms on the Dispersal of Steinernema Spp. D

Plant Pathology and Microbiology Publications Plant Pathology and Microbiology

3-1995 Effects of Earthworms on the Dispersal of Steinernema spp. D. I. Shapiro Iowa State University

G. L. Tylka Iowa State University, [email protected]

E. C. Berry United States Department of Agriculture

L. C. Lewis United States Department of Agriculture

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This Article is brought to you for free and open access by the Plant Pathology and Microbiology at Iowa State University Digital Repository. It has been accepted for inclusion in Plant Pathology and Microbiology Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Journal of Nematology 27(1):21-28. 1995. © The Society of Nematologists 1995. Effects of Earthworms on the Dispersal of Steinernema spp.1

D. I. SHAPIRO, 2 G. L. TYLKA, 3 E. C. BERRY, 4 AND L. C. LEWIS5

Abstract: Previous studies indicated that dispersal of S. carpocapsae may be enhanced in soil with earthworms. The objective of this research was to determine and compare the effects of earthworms on dispersal of other Steinernema spp. Vertical dispersal of Steinernema carpocapsae, S. feltiae, and S. glaseri was tested in soil columns in the presence and absence of earthworms (Lumbricus terrestris). Dispersal was evaluated by a bioassay and by direct extraction of nematodes from soil. Upward dispersal of S. carpocapsae and S. feltiae increased in the presence of earthworms, whereas upward dispersal of S. glaseri was not affected by earthworms. No significant differences were detected in downward dispersal of S. carpocapsae and S. feltiae in soil with earthworms compared to soil without earthworms. Downward dispersal of S. gla~eri, however, was greater in soil without earthworms relative to soil with earthworms. In soil void of earthworms, dispersal of S. glaseri was greatest followed by dispersal of S. carpocapsae. The presence of earthworm burrows in soil did not influence nematode dispersal. Nematodes were recovered from the surface, interior, and casts of earthworms. Therefore, nematodes may have a phoretic association with earthworms. Key words: earthworm, entomophyllic nematode, nematode, nematode dispersal, Steinernema spp.

Entomopathogenic nematodes in the ge- host presence, temperature, soil texture, nus Steinernema have many attributes as bi- and soil moisture (14). ological control agents, including wide Dispersal characteristics vary among host ranges, safety to nontarget organisms, Steinernema spp. Steinernema carpocapsae and vectoring a bacterium, Xenorhabdus (Weiser) moves little from the site of appli- sp., which rapidly kills insect hosts (15). cation, whereas S. glaseri (Steiner) is a rel- Despite their great potential as biological atively active disperser (18). Steinernema control agents, results of nematode appli- carpocapsae applied to the surface of verti- cations have been inconsistent (11). Biotic cal soil columns remained within the up- and abiotic factors that influence the effi- per 15 cm after 30 days, but S. glaseri was cacy of nematode applications must be in- recovered 90 cm below the application vestigated (8). point (18). Because of these differences in Increased dispersal may increase the ef- behavior, S. carpocapsae is classified as an ficacy of nematodes against insect pests (9). ambusher and S. glaseri is classified as a Adverse environmental conditions can de- cruise forager (15). The foraging behavior crease nematode mobility, thereby de- of S. feltiae (Filipjev) is considered interme- creasing their effectiveness (8). Factors diate between S. carpocapsae and S. glaseri that may affect nematode dispersal include (1). Differences in dispersal behavior may be important in selecting nematode species for use against pests occupying different Received for publication 8 August 1994. niches in soil habitats (16). I Journal Paper No. 15954 of the Iowa Agricultural and Home Economics Experiment Station, Ames; Project No. Interactions with soil invertebrates may 3130. increase nematode dispersal (14). For ex- 2 Department of Entomology, Iowa State University, Ames, IA 50011. ample, a phoretic relationship was ob- Department of Plant Pathology, Iowa State University, served between nematophagous mites and Ames, IA 50011. 4 USDA ARS, National Soil Tilth Laboratory, Ames, IA S. carpocapsae (6). Shapiro et al. (19) re- 50011. ported increased dispersal of S. carpocapsae USDA ARS, Corn Insects Research Unit, Ames, IA 50011. in the presence of earthworms. Earth- We thank M. Abbas, R. D. Gunnarson, D. Soh, and S. Sou- worms are not adversely affected by ento- hrada for assistance in the laboratory and Drs, I. Glazer, E. E. Lewis, and J. J. Obrycki for reviewing an earlier draft of this mopathogenic nematodes (4,17). There- manuscript. We also thank Dr. G. O. Poinar for nematode fore, it may be possible to exploit the asso- identification. This research was supported in part by the Leopold Center of Sustainable Agriculture, Ames, IA, Grant ciation between S. carpocapsae and No. 92-18. earthworms in the soil to increase the effi- 21 22 Journal of Nematology, Volume 27, No. 1, March 1995 cacy of the nematode as a biological con- earthworms, Lumbricus terrestris (L.), ob- trol agent. The influence of earthworms tained from the USDA ARS National Soil on the dispersal of other Steinernema spe- Tilth Laboratory (Ames, IA), were added cies and the mechanism(s) of how earth- to the selected columns and allowed to worms affect nematode dispersal have not burrow for 1 week before nematode appli- been examined. Because of differences in cation. Subsequently, 10,000 infective ju- dispersal behavior, the effects of earth- veniles of each species were applied in 0.3- worms on dispersal may vary among 0.4 ml of distilled water to the bottom sur- nematode species. Shapiro et al. (19) sug- face of the columns (section 6) to test gested two hypotheses relating to the upward movement, and to the top surface earthworm-nematode relationship: the of the columns (section 1) to evaluate nematodes are dispersed by direct contact downward movement. Soil columns were with earthworms and (or) nematodes dis- then incubated for 14 days at approxi- perse at different rates in soil with earth- mately 21 C and 54% relative humidity, worm burrows than in soil without bur- after which they were dismantled to eval- rows. The objectives of this research were uate nematode dispersal. to determine and compare the effects of Nematode dispersal was evaluated using earthworms on dispersal of S. carpocapsae, two methods: a bioassay against G. mel- S. glaseri, and S. feltiae and to investigate lonella larvae, and direct extraction and the mechanisms involved in how earth- counting of nematodes. Four replicate soil worms influence nematode dispersal. columns per treatment were used for each method of evaluation. For the bioassay, MATERIALS AND METHODS soil from each column section was removed and placed in plastic petri dishes The effects of earthworms on dispersal (150 x 25 ram) with 20 last instar G. mel- of three nematode species were deter- loneUa (19). The petri dishes were incu- mined in vertical soil columns. Steinernema bated at approximately 27 C and 80% rel- carpocapsae, All strain, S. glaseri, NC strain, ative humidity for 72 hours, at which time and S. feltiae, strain 27, were originally ob- nematode-induced larval mortality was re- tained from biosys (Palo Alto, CA) and corded. Mortality was considered to be were reared in last instars of the greater nematode induced if cadavers exhibited wax moth, Galleria meUonella (L.). Experi- flaccidity and coloration characteristic of mental units were polyvinyl chloride Steinernema infections. In the second (PVC) pipe consisting of six 4-cm-long sec- method, nematodes were extracted from tions and 5-cm-d pipe joined with duct the soil in each column section using su- tape and filled with soil (34% sand, 28% crose centrifugation (13), and Steinernema silt, and 38% clay, pH 7.0). Soil moisture in spp. were quantified by observation with columns was adjusted to approximately an inverted compound microscope at a field capacity. The column sections were magnification of x 40. numbered vertically; the top section was Analysis of nematode movement was ac- one and the bottom section was six. complished by determining the number of The experiments were organized as ran- nematodes reaching the half of the column domized block designs with depth as a opposite where the nematodes were placed repeated measure. Experiments testing (sections 1-3 for upward movement and upward and downward movement of 4-6 for downward movement). Bioassay nematodes were run simultaneously. Six data were analyzed through ANOVA of treatments (three nematode species, with mean numbers of dead G. meUonella larvae and without earthworms) were each ap- obtained from soil in the three sections op- plied to eight soil columns for upward posite to where the nematodes were movement and eight columns for down- placed. Because differences in virulence ward movement (96 columns total). Two against G. melloneUa among the three Dispersal of Steinernema spp.: Shapiro et al. 23 nematode species were not determined, in- with S. carpocapsae) were dissected and ex- terspecific comparisons of dispersal could posed to G. mellonella as previously de- not be made using the bioassay data. For scribed. the direct extraction method, ANOVA was An additional experiment was con- performed on mean proportions of nema- ducted to determine whether nematode todes recovered. Proportions were calcu- dispersal is greater in soil with earthworm lated based on the number of nematodes burrows than in soil without burrows. Up- recovered from the half of the column op- ward dispersal of S. carpocapsae was deter- posite to where the nematodes were placed mined in vertical soil columns in an exper- relative to the total number of nematodes iment with a design similar to previous ex- recovered in that column. Using these pro- periments. Soil columns and experimental portions, extraction efficiency for each conditions were as previously described, species was not a factor; hence compari- but treatments were columns with earth- sons of dispersal among Steinernema spp. worms and burrows, columns with bur- could be made. If significant differences rows only (earthworms removed), and col- were found with the ANOVA, then multiple umns with no earthworms or burrows. range tests (LSD) were used to distinguish There were five replications per treatment treatment effects. (15 columns total). Two earthworms (L. An experiment was conducted to inves- terrestris) were added to ten randomly se- tigate whether nematodes were moved lected columns and allowed to burrow for through soil via a direct contact relation- 1 week, after which five columns were dis- ship with earthworms. Approximately 1.5 mantled. Earthworms were manually re- z 106 S. carpocapsae were placed in 3 liters moved from the dismantled columns, of soil with 10 L. terrestris and incubated at which were then reconstructed with the 27 C for 7 days. Subsequently, the earth- earthworm burrows realigned. Realign- worms were removed and washed vigor- ment of column sections was accomplished ously using distilled water. The wash from by aligning straight vertical lines, which each earthworm was poured onto filter pa- were drawn on the columns prior to sepa- per in 100 x 15 mm petri dishes. The ration of the column sections. Ten thou- earthworms were rinsed further in run- sand nematodes were then applied in 0.4 ning tap water, placed in separate petri ml distilled water to each column, and col- dishes with filter paper, and allowed to cast umns were incubated for 2 weeks as de- for approximately 1 hour. Five earth- scribed herein. Following incubation, worms were then placed at -10 C for 7 nematode dispersal was estimated and an- minutes to immobilize them and were sub- alyzed using the bioassay method de- sequently cut into 1-cm-long sections; scribed herein. This experiment was re- nematodes were extracted from the dis- peated with no changes in procedure ex- sected material through a Baermann fun- cept that all columns were dismantled and nel (2). The extracted nematode suspen- then realigned, and nematode dispersal sions were poured onto filter paper in five was evaluated using the direct extraction petri dishes. Three last instar G. meUoneUa method. were placed in each petri dish (from the wash, casts, and dissections), and the petri RESULTS dishes were incubated at 27 C and 80% relative humidity. If larval mortality oc- The G. melloneUa bioassays indicated in- curred, the cadavers were placed on White creased upward dispersal of S. carpocapsae traps (20) to verify nematode infection. To and S. feltiae in columns with earthworms test whether a natural population of Stein- relative to columns without earthworms ernema spp. may have been associated with (Table 1). No effects of earthworm pres- earthworms, five additional L. terrestris ence were detected using the bioassay (that were not incubated in soil infested method to determine upward dispersal of 24 Journal of Nematology, Volume 27, No. I, March 1995

TABLE 1. Number of dead Galleria mellonella larvae from soil columns testing upward dispersal of three Steinernema spp. in the presence and absence of the earthworm, Lumbricus terrestris.

S. carpocapsae S. feltiae S. glaseri

Section depth (cm) L + ? L - L + L - L + L -

1. 0-4 13.5 0.8 7.3 2.8 12.5 17.3 2. 4-8 13.8 6.3 7.0 1.3 13.3 19.0 3. 8-12 16.0 12.3 9.3 1.0 16.0 18.3 4. 12-16 13.8 14.0 10.8 2.5 19.5 19.8 5. 16-20 15.5 17.0 19.0 14.0 19.5 19.3 6. 20-24 19.4 20.0 19.8 19.3 19.0 20.0 m:~ = 14.6 a 6.4 b 7.8 b 1.7 c 13.9 a 18.2 a

Numbers represent means of four replications. Nematodes were introduced 24 cm below the soil surface. Twenty G. mellonella larvae were exposed to soil removed from column sections 2 weeks after nematodes were introduced. Galleria melloneUa mortality indicates relative nematode densities. q" L+ = presence and L- = absence of the earthworm, L. terrestris. ~: m = average mortality of G. melloneUa larvae in the top three sections (1-3). Means from within-species comparisons followed by different letters are significantly different at P ~< 0.05.

S. glaseri (Table 1). Results from the direct seri in the absence of earthworms than in extraction method evaluating upward the presence of earthworms (Table 4). In- nematode dispersal followed the same terspecific comparisons indicated down- trends as bioassay data, but differences be- ward dispersal abilities in the absence of tween earthworm presence and absence earthworms to be greatest in S. glaseri fol- were not significant (Table 2). Interspe- lowed by S. carpocapsae. In the presence of cific comparisons made from direct extrac- earthworms, dispersal of S. glaseri and S. tion data indicated that in the absence of carpocapsae were comparable and were sig- earthworms, upward dispersal of S. glaseri nificantly different from dispersal of S. fel- was greater than dispersal of S. carpocapsae tiae (Table 4). and S. feltiae (Table 2). In the presence of Nematodes were isolated from the sur- earthworms, dispersal of S. carpocapsae was face, casts, and interior of earthworms. not different than that of S. glaseri. Emerged nematodes were observed in 1 of Bioassay results indicated no significant 10, 6 of 10, and 5 of 5 White traps that effect of earthworms on downward dis- contained G. meUoneUa exposed to nema- persal for the three nematode species (Ta- todes from the wash of earthworm sur- ble 3). Results of direct extraction indi- faces, earthworm casts, and dissected cated greater downward dispersal of S. gla- earthw6rms, respectively. The emerged

TABLE 2. Number of nematodes recovered from soil columns testing upward dispersal of three Stein- ernema spp. in the presence and absence of the earthworm, Lumbricus terrestris.

S. carpocapsae S. feltiae S. glaseri

Section depth (cm) L + ~ L - L + L - L + L -

1. 0-4 28.0 0 3.5 0.5 74.0 222.0 2. 4-8 22.8 0.5 6.0 0 213.5 525.0 3. 8-12 27.0 9.0 10.5 5.0 427.5 614.5 4. 12-16 9.0 4.5 28.5 26.0 465.0 530.5 5. 16-20 34.5 52.5 123.0 48.0 904.5 550.5 6. 20-24 515.0 77.5 807.5 478.0 304.0 803.0 p~ = 18.0 ab 7.1 b ~ 2.7 b 0.7 b 29.5 a 37.5 a

Numbers represent raeans of four replications. NematodEs were introduced 24 cm below the soil surface. t L + = presence and L- = absence of the earthwoi:ha, L. terrestris. ~- p = mean percentage of nematodes recovered from the top three sections (1-3) relative to the total number of nematodes recovered. Means followed by the same letter are not significantly different at P ~< 0.05. Dispersal of Steinernema spp.: Shapiro et al. 25

TABLe 3. Number of dead Galleria mellonella larvae from soil columns testing downward dispersal of three Steinernema spp. in the presence and absence of the earthworm, Lumbricus terrestris.

S. carpocapsae S. feltiae S. glaseri

Section depth (cm) L + t L - L + L - L + L -

1. 0-4 20.0 20.0 20.0 19.8 10.8 16.0 2. 4-8 19.8 19.8 19.8 20.0 20.0 19.8 3. 8-12 19.3 20.0 19.0 18.8 19.8 20.0 4. 12-16 17.8 19.8 12.0 12.8 19.3 19.8 5. 16-20 18.3 19.3 10.8 1.5 16.3 19.8 6. 20-24 19.5 16.3 4.5 0.8 16.3 19.5 mS = 18.5 a 18.4 a 9.1 b 5.0 b 17.3 a 19.7 a

Numbers represent means of four replications. Nematodes were introduced on the soil surface. Twenty G. mellonella larvae were exposed to soil removed from column sections 2 weeks after nematodes were introduced. Galleria mellonella mortality indicates relative nematode densities. t L+ = presence and L- = absence of the earthworm, L. terrestris. ~: m = average mortality of G. mellonella larvae in the bottom three sections (4--6). Means from within-species comparisons followed by different letters are significantly different at P ~< 0.05.

nematodes were identified as Steinernema tris. In previous research, downward dis- sp. No Steinernema nematodes were recov- persal of S. carpocapsae was increased in the ered from the five earthworms that were presence of L. terrestris (19). The reasons not exposed to soil infected with nema- for discrepancy between previous results todes. and results reported herein are unknown. Dispersal of S. carpocapsae was greater in Upward dispersal of S. glaseri was not af- columns with earthworms than in columns fected by earthworm presence, but down- with earthworm burrows (earthworms re- ward dispersal was increased when earth- moved) and columns without earthworms worms were absent. Reasons for different (Table 5). Dispersal of S. carpocapsae was effects of earthworms on upward and not different in columns with earthworm downward dispersal of these nematode burrows compared to columns void of species are unclear. Perhaps consistent dif- earthworms (Table 5). ferences would have been found in upward and downward nematode dispersal DISCUSSION experiments if more replications had been used. Indeed, the trends observed in all Upward, but not downward, dispersal of experiments were consistent, regardless of S. carpocapsae and S. feltiae was enhanced statistical significance. Alternatively, if a by the presence of the earthworm L. terres- phoretic relationship exists between the L.

TABLE 4. Number of nematodes recovered from soil columns testing downward dispersal of three Stein- ernema spp. in the presence and absence of the earthworm, Lumbricus terrestris.

S. carpocapsae S. feltiae S. glaseri

Section depth (cm) L + t L - L + L - L + L -

1. 0-4 83.5 249.0 886.5 1,115.0 47.0 207.0 2. 4--8 58.0 75.0 659.5 737.0 1,171.5 416.0 3. 8-12 36.5 46.5 109.5 198.0 1,082.0 970.5 4. 12-16 15.0 49.0 76.5 36.5 590.0 621.0 5. 16-20 33.0 43.0 30.5 3.0 170.0 325.0 6. 20-24 30.0 10.5 28.5 6.5 93.5 167.5 pS = 31.5 ab 22.0 b 7.3 c 2.2 c 26.5 b 42.3 a

Numbers represent means of four replications. Nematodes were introduced to the soil surface. t L+ = presence and L- = absence of the earthworm, L. terrestris. :~ p = mean percentage of nematodes recovered from the bottom three sections (4-6) relative to the total number of nematodes recovered. Means followed by the same letter are not significantly different at P ~< 0.05. 26 Journal of Nematology, Volume 27, No. 1, March 1995

TABLE 5. GaUeria mellonella larval mortality from bioassays, and numbers of S. carpocapsae recovered by direct extraction, from soil with earthworms, earthworm burrows, and no earthworms.

G. mellonella mortality S. carpocapsae recovered

Section depth (cm) Lt LO 0 L LO 0

1. 0-4 3.4 0 0 66.8 0.4 0 2. 4-8 5.2 0.2 0 18.4 2.8 0.8 3. 8-12 6.6 0 0 28.8 3.6 0 4. 12--16 4.6 3.4 2.2 6.0 3.6 19.2 5. 16--20 9.4 8.6 6.2 21.2 48.0 45.5 6. 20-24 11.6 16.6 14.4 952.4 812.4 300.4 mS = 5.1a 0.7b 0b p§ = 15.0 a 3.1 b 0.3 b

Numbers represent means of five replications. Steinernema carpocapsae were introduced 24 cm below the soil surface. For bioassay, 20 G. meUonella larvae were exposed to soil removed from column sections 2 weeks after nematodes were introduced. GaUeria meUonella moratlity indicates relative nematode densities. 1" L = presence of two L. terrestris, L0 = burrows present with earthworms removed, 0 = no earthworms or burrows. :~ m = average mortality of G. melloneUa larvae in the top three sections (1-3). § p = mean percentage of nematodes recovered from the top three sections (1-3) relative to the total number of nematodes recovered. Means followed by the same letter are not significantly differentat P ~< 0.05. terrestris and S. carpocapsae and S. feltiae, termediate between S. glaseri and S. carpoc- then perhaps the nematodes tend to asso- apsae. The disparity may be due to differ- ciate more with earthworms moving up- ences in soil texture or nematode strain. ward than with earthworms moving down- Nematode dispersal may be affected by ward. Steinernema carpocapsae has a natural soil texture; for example, S. carpocapsae tendency to disperse upwards (10,18). In- dispersal increases with increased soil par- creased upward, but not downward, dis- ticle size (10). The study of Alatorre-Rosas persal has been reported previously when and Kaya (1) was conducted in sand, S. carpocapsae was in the presence of Heter- whereas this research was conducted in soil orhabditis bacteriophora Poinar, a nematode (34% sand, 28% silt, and 38% clay). In both species with greater dispersal ability than studies, S. carpocapsae, All strain, was used, S. carpocapsae (1). but the strain of S. feltiae was not given in This research found the natural dis- the report of Alatorre-Rosas and Kaya (1). persal ability of S. glaseri to be greater than This study has provided evidence that that of S. carpocapsae and S. feltiae, which is earthworms may serve as phoretic hosts of consistent with previous studies confirm- Steinernema spp. Nematode dispersal was ing the dispersal behavior of S. glaseri as a not different in soil with earthworm bur- cruiser. Because of the high natural dis- rows compared to soil without burrows, persal ability of S. glaseri, it is not unex- and results indicated that the relationship pected that earthworms would not en- between nematodes and earthworms is one hance dispersal of this species. Perhaps the of direct contact. Steinernema carpocapsae increased downward dispersal of S. glaseri may be dispersed on the surface of earth- observed in soil without earthworms rela- worms, or may be passed through the tive to soil with earthworms may have been earthworm digestive system and remain vi- caused by earthworm burrows or earth- able. Evidence strongly suggests that the worms obstructing the paths of these nematodes that were recovered from nematodes. earthworms and reared in G. mellonella This study found the dispersal of S. car- were the S. carpocapsae that were placed in pocapsae to be greater than that of S. feltiae, the infested soil, because the nematodes which is contrary to the study of Alatorre- recovered were identified as belonging to Rosas and Kaya (1), in which the authors the genus Steinernema, and because no found dispersal ability of S. feltiae to be in- Steinernema were found in earthworms that Dispersal of Steinernema spp.: Shapiro et al. 27 were not incubated in soil infested with S. nematodes in genera Heterorhabditis and Steinernema carpocapsae. Because a high nematode con- for an insect host in sand. Journal of Invertebrate Pathology 55:179-188. centration was used to bait the soil, addi- 2. Ayoub, S. M. 1980. Plant nematology: An agri- tional research will be required to deter- cultural training manual. Sacramento, CA: Nemaid mine to what extent S. carpocapsae is asso- Publications. ciated with earthworms in soil with lower 3. Berry, E. C., and D. L. Karlen. 1993. Compari- son of alternative farming systems. II. Earthworm nematode densities. Future studies may population density and species diversity. American also determine if other entomopathogenic Journal of Alternative Agriculture 8:21-26. species are capable of being dispersed by 4. Capinera, J. L., S. L. Blue, and G. S. Wheeler. earthworms through direct contact. 1982. Survival of earthworms exposed to Neoaplectana Two methods were used to evaluate carpocapsae nematodes. Journal of Invertebrate Pa- thology 39:419-421. nematode dispersal in this study. In gen- 5. Edwards, C. A. 1983. Earthworm ecology in cul- eral, the bioassay method was more sensi- tivated soils. Pp. 123-138 inJ. E. Satchell, ed. Earth- tive in finding differences and less labor worm ecology from Darwin to vermiculture. London: intensive than the direct extraction method. Chapman and Hall. However, data from direct extraction al- 6. Epsky, N. D., D. E. Walter, and J. L. Capinera. 1988. Potential role of nematophagous microarthro- lowed interspecific comparisons. Recent pods as biotic mortality factors of entomogenous studies have found that LT50 (time re- nematodes (Rhabiditida: Steinernematidae, Heter- quired to cause 50% host mortality) and orhabditidae). Journal of Economic Entomology 81: efficiency of invasion (number of nema- 821-825. 7. Epsky, N. D., andJ. L. Capinera. 1993. Quanti- todes entering the host per unit time) to be fication of invasion of two strains of Stein~rnema car- very sensitive methods of evaluating nema- pocapsae (Weiser) into three lepidopteran larvae. tode activity (7,12). Perhaps more differ- Journal of Nematology 25:173-180. ences among treatments would have been 8. Gaugler, R. 1988. Ecological considerations in detected in this study if LTs0s or invasion the biological control of soil-inhabiting insects with entomopathogenic nematodes. Agriculture, Ecosys- efficiencies had been determined. tems and Environment 24:351-360. Enhanced dispersal of S. carpocapsae and 9. Gaugler, R., T. McGuire, andJ. Campbell. 1989. S. feltiae in the presence of earthworms Genetic variability among strains of the ento- may improve the efficacy of these nema- mopathogenic nematode Steinernema feltiae. Journal of Nematology 21:247-253. todes as biological control agents. The dis- 10. Georgis, R., and G. O. Poinar, Jr. 1983. Effect persal ability of S. carpocapsae has been re- of soil texture on the distribution and infectivity of ported to be limited (1,10,18), but our re- Neoaplectana carpocapsae (Nematoda: Steinernema- search has established that dispersal of S. tidae). Journal of Nematology 15:308-311. carpocapsae is comparable to that of S. gla- 11. Georgis, R., and. R. Gaugler. 1991. Predictabil- ity in biological control using entomopathogenic seri in the presence of earthworms. Be- nematodes. Journal of Economic Entomology 84: cause host finding is necessary for success- 713-720. ful biological control with nematodes and 12. Glazer, I. 1992. Invasion rate as a measure of increased dispersal will improve host find- infectivity of Steinernematid and Heterorhabditid ing abilities (9), it is important to deter- nematodes to insects. Journal of Inverterbrate Pa- thology 59:90-94. mine if increased earthworm population 13. Jenkins, W.R. 1964. A rapid centrifugal- densities will increase the efficacy of bio- flotation technique for separating nematodes from logical control of insects with nematodes. soil. Plant Disease Reporter 48:692. Additionally, earthworms improve soil 14. Kaya, H. K. 1990. Soil ecology. Pp 93-115 in R. quality through aeration and by breaking Gaugler and H. K. Kaya, eds. Entomopathogenic nematodes in biological control. Boca Raton, FL" down and distributing organic matter (5). CRC Press. Earthworm population densities can be in- 15. Kaya, H. K., and R. Gaugler. 1993. Ento- creased through cultural practices (3). mopathogenic nematodes. Annual Review of Ento- mology 38:181-206. LITERATURE CITED 16. Kaya, H. K., T.M. Burlando, and G. Thur- ston. 1993. Two entomopathogenic nematode species 1. Alatorre-Rosas, R., and H. K. Kaya. 1990. Inter- with different search strategies for insect suppres- specific competition between entomopathogenic sion. Environmental Entomology 22:859-864. 28 Journal of Nematology, Volume 27, No. 1, March 1995

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