Comparison of Two Steinernematid Species for Control of the Root Weevil Diaprepes abbreviatus

Steinernema carpocapsae Weiser All strain was compared to Steinernema riobravis Cabanillas, Poinar, and Raulston for control of the root weevil, Diaprepes abbreviatus (L.), in the laboratory and in potted citrus. In the laboratory bioassay, D. abbreviatus larvae were exposed to 30, 60, and 120 nematodes/cm³ in sand. Insect mortality 1 week after application was greater (P ≤ 0.05) for S. riobravis than for S. carpocapsae in the laboratory bioassay. In the greenhouse bioassay, D. abbreviatus larvae were exposed to 3 and 9 nematodes per cm³ of soil in potted citrus. Again, at each rate, mortality was greater (P ≤ 0.05) in pots treated with S. riobravis than in pots treated with S. carpocapsae. The results of this study suggest that S. riobravis is a better biological control agent against D. abbreviatus larvae in potted plants than S. carpocapsae.     

Survival of Heterodera zeae in Soil in the Field and in the Laboratory

Eggs and (or) second-stage juveniles (J2) inside cysts of Heterodera zeae survived over winter in the field with no detectable mortality at all six depths to 30 cm from which soil samples were collected between corn stubble in the row at 4-8-week intervals. Few or no free J2 were recovered from soil collected in January-April from the top 5 cm, but some were recovered at all samplings from soil collected at greater depths. Emergence of J2 from cysts and numbers of females developing on corn roots in bioassays of cysts increased substantially between January and April. Cyst numbers in a fallow area of the corn field did not decline at any depth to 30 cm during 20 months. Free soil J2, J2 emerged from cysts, and females from the bioassay of cysts were highest at the first soil sampling in July after 10 months of fallow; numbers of nematodes in all three categories declined thereafter, but a few were still detectable after 20 months of fallow. Some cysts were still being recovered after 51 months from naturally infested field soil stored moist in the laboratory at 2 C and 24 C. Females were produced in the bioassays of cysts recovered from soil stored for 38 months at 24 C and for 32 months at 2 C. No free J2 were recovered from soil after 1 month of storage at -18 C, but even after 7 months storage J2 emerged from cysts that were recovered and many females developed in bioassays of those cysts.     

Parasitism of Western Corn Rootworm Larvae and Pupae by Steinernema carpocapsae

Virulence and development of the insect-parasitic nematode, Steinernema carpocapsae (Weiser) (Mexican strain), were evaluated for the immature stages of the western corn rootworm, Diabrotica virgifera virgifera LeConte. Third instar rootworm larvae were five times more susceptible to nematode infection than second instar larvae and 75 times more susceptible than first instar larvae and pupae, based on laboratory bioassays. Rootworm eggs were not susceptible. Nematode development was observed in all susceptible rootworm stages, but a complete life cycle was observed only in second and third instar larvae and pupae. Nematode size was affected by rootworm stage; the smallest infective-stage nematodes were recovered from second instar rootworm larvae. Results of this study suggest that S. carpocapsae should be applied when second and third instar rootworm larvae are predominant in the field.     

Estimating Sample Size and Persistence of Entomogenous Nematodes in Sandy Soils and Their Efficacy Against the Larvae of Diaprepes abbreviatus in Florida

In two studies to estimate sampling requirements for entomogenous nematodes in the field, highest persistence of Heterorhabditis bacteriophora after application occurred beneath the canopies of mature citrus trees. Nematode persistence declined with distance from the center-line of the tree row toward the row-middles. Immediately after nematode application to soil, 32 samples (15 cm deep, 2.5-cm diameter) beneath a single tree were required to derive 95% confidence intervals that were within 40% of mean nematode population density. The estimated probability of measuring the mean density within 40%, using 32 samples, declined to 88% at 2 days post-application and to 76% at 7 days. The persistence in soil of Steinernema carpocapsae, S. riobravis, and two formulations containing H. bacteriophora and their efficacy against the larvae of Diaprepes abbreviatus were compared in a grove of 4-year-old citrus trees. Within 6 days, the recovered population densities of all nematodes declined to <5% of levels on day 0. The recovery of H. bacteriophora during the first 2 weeks was lower than that of the other two species. Steinemema riobravis and both formulations of H. bacteriophora reduced recovery of D. abbreviatus by more than 90% and 50%, respectively. Steinernema carpocapsae did not affect population levels of the insect.     

Distribution of Field Corn Roots and Parasitic Nematodes in Subsoiled and Nonsubsoiled Soil

A field trial was conducted for 2 years in an Arredondo fine sand containing a tillage pan at 15-20 cm deep to determine the influence of subsoiling on the distribution of corn roots and plant-parasitic nematodes. Soil samples were taken at various depths and row positions at 30, 60, and 90 days after planting in field corn subsoiled under the row with two chisels and in non-subsoiled corn. At 30 and 60 days, in-row nematode population densities to 60 cm deep were not affected by subsoiling compared with population densities in nonsubsoiled plots. After 90 days, subsoiling had not affected total root length or root weight at the 20 depth-row position sampling combinations, but population densities of Meloidogyne incognita and Criconemella spp. had increased in subsoiled corn. Numbers of Pratylenchus zeae were not affected. Subsoiling generally resulted in a change in distribution of corn roots and nematodes in the soil profile but caused little total increase in either roots or numbers of nematodes. Corn yield was increased by subsoiling.     

Nonsusceptibility of the Honey Bee,Apis mellifera (Hymenoptera: Apidae), to Steinernematid and Heterorhabditid Nematodes

We exposed honey bee workers and brood to four entomopathogenic nematode species under conditions normally encountered in the hive by spraying nematodes onto combs. Mortality of adult bees exposed to any of the nematode species was less than 10%, and there was no evidence of nematode infection when dead adults were dissected. To assess the impact of nematodes on brood, we used smaller-size honey combs placed in the second story (super) of a hive and large brood combs placed in the main section of the hive. Our results were inconsistent between these two experimental designs. The smaller honey combs sprayed with Steinernema carpocapsae contained the largest number of uncapped ceils, those sprayed with Heterorhabditis baeteriophora or S. riobravis contained an intermediate number of uncapped cells, and control combs and those sprayed with S. glaseri contained the fewest nmnber of uncapped cells. Large combs sprayed with S. riobravis contained more uncapped ceils than controls or those sprayed with S. carpocapsae, although the differences were not significant. Our results do not support the hypothesis that high-temperature-tolerant species of nematodes are necessarily more infective to honey bees.     

Abundance and Vertical Distribution of Longidorus breviannulatus Associated with Corn and Potato

Longidorus breviannulatus was detected in a field planted to corn after 13 years of potato. Nematode populations were maintained in this field in adjacent corn and potato plots for 2 years but did not increase significantly on either crop. Population levels increased until approximately 60 days after planting and then declined until the end of the growing season. Overwinter mortality was negligible. The vertical distribution of the nematode population changed during the course of the season. More nematodes were recovered from depths of 0-15 cm in early season samples and from depths of 15-30 cm in late season samples. Data indicated that this redistribution was due to nematode migration.     

Influence of Soil pH and Oxygen on Persistence of Steinernema spp.

Survival of infective juveniles of Steinernema carpocapsae and Steinernema glaseri gradually declined during 16 weeks of observation as the tested soil pH decreased from pH 8 to pH 4. Survival of both species of Steinernema dropped sharply after 1 week at pH 10. Survival or S. carpocapsae and S. glaseri was similar at pH 4, 6, and 8 during the first 4 weeks, but S. carpocapsae survival was significantly greater than S. glaseri at pH 10 through 16 weeks. Steinernema carpocapsae and S. glaseri that had been stored at pH 4, 6, and 8 for 16 weeks, and at pH 10 for 1 or more weeks were not infective to Galleria mellonella larvae. Steinernema carpocapsae survival was significantly greater than that of S. glaseri at oxygen:nitrogen ratios of 1:99, 5:95, and 10:90 during the first 2 weeks, and survival of both nematode species declined sharply to less than 20% after 4 weeks. Survival of both nematode species significantly decreased after 8 weeks as the tested oxygen concentrations decreased from 20 to 1%, and no nematode survival was recorded after 16 weeks. Steinernema carpocapsae pathogenicity was significantly greater than that of S. glaseri during the first 2 weeks. No nematode pathogenicity was recorded at oxygen concentrations of 1, 5, and 10% after 2 weeks and at 20% after 16 weeks.     

Use of Cellulose Acetate Electrophoresis in the Taxonomy of Steinernematids (Rhabditida,Nematoda)

A steinernematid nematode was isolated from soil samples collected near St. John''s, Newfoundland, Canada. On the basis of its morphometry and RFLPs in ribosomal DNA spacer, it was designated as a new strain, NF, of Steinernema feltiae. Cellulose acetate electrophoresis was used to separate isozymes of eight enzymes in infective juveniles of S. feltiae NF as well as four other isolates: S. feltiae Umeå strain, S. feltiae L1C strain, Steinernema carpocapsae All strain, and Steinernema riobravis TX strain. Based on comparisons of the relative electrophoretic mobilities (μ) of the isozymes, one of the eight enzymes (arginine kinase) yielded zymograms that were distinctive for each of the isolates, except for the Umeå and NF strains of S. feltiae, which had identical banding patterns. Four enzymes (fumarate hydratase, phosphoglucoisomerase, phosphoglucomutase, and 6-phosphogluconate dehydrogenase) yielded isozyme banding patterns that were characteristic for all isolates, except for the L1C and NF strains of S. feltiae, which were identical. Two enzymes (aspartate amino transferase and glycerol-3-phosphate dehydrogenase) yielded zymograms that permitted S. carpocapsae All strain to be discriminated from the other four isolates, while the remaining enzyme (mannose-6-phosphate isomerase) was discriminatory for S. riobravis TX strain. Except for one enzyme, the isozyme banding pattern of the NF isolate of S. feltiae was the same as in the L1C strain, isolated 13 years previously from Newfoundland. Cellulose acetate electrophoresis could prove invaluable for taxonomic identification of isolates of steinernematids, provided that a combination of enzymes is used.     

Multiple Pest Interactions in Soybean: Effects on Heterodera glycines Egg Populations and Crop Yield

Population changes of Heterodera glycines eggs on soybean in small field plots were influenced by the lepidopterous insect pest, Helicoverpa zea; however, few effects on eggs due to the presence of annual weeds were detected. Soybeans defoliated 15-35% by H. zea during August remained green and continued to produce new flowers and pods later into the season than soybeans without H. zea, resulting in higher numbers of H. glycines eggs at harvest on insect-defoliated soybeans. Final H. glycines populations also were influenced by soil population density (Pi) of the nematode at planting. Fecundity of H. glycines was generally greater at the undetected and low Pi than at high Pi levels. Soybean yields were suppressed 12, 22, and 30% by low, moderate, and high H. glycines Pi, respectively. When weed competition and H. zea feeding damage effects were added, yields were suppressed 34, 40, and 57% by the three respective nematode Pi levels. Effects among the three pests on soybean yield were primarily additive.     

Vertical Distribution of Plant-parasitic Nematodes in Sandy Soil under Soybean

Vertical distribution of five plant-parasitic nematodes was examined in two north Florida soybean fields in 1987 and 1988. Soil samples were collected from 0-15 cm, 15-30 cm, and 30-45 cm deep at each site. Soil at the three depths consisted of approximately 96% sand. More than 50% of Belonolaimus longicaudatus population densities occurred in the upper 15-cm soil layer at planting, but the species became more evenly distributed through the other depths as the season progressed. Criconemella sphaerocephala was evenly distributed among the three depths in one field but was low (< 20% of the total density) in the upper 15 cm at a second site. Maximum population densities of Pratylenchus brachyurus were observed at 15-30 cm on most sampling dates. Vertical distributions of Meloidogyne incognita and Paratrichodorus minor were erratic and showed seasonal variation. A diagnostic sample from the upper 0-15 cm of these soybean fields revealed only a minority of the populations of most of the phytoparasitic species present.     

Effects of Culture Method and Formulation on the Virulence of Steinernema riobrave (Rhabditida: Steinernematidae) to Diaprepes abbreviatus (Coleoptera: Curculionidae)

The Diaprepes root weevil, Diaprepes abbreviatus, is a pest of vegetables, ornamental plants, sugarcane, and citrus in Florida and the Caribbean. The entomopathogenic nematode, Steinernema riobrave, can reduce larval populations of D. abbreviatus substantially. Efficacy of entomopathogenic nematodes, however, may be affected by culture method and formulation. Using D. abbreviatus as the host, we compared the efficacy of two commercial S. riobrave formulations, a liquid and a waterdispersible granule (WDG), with each other and with in vivo produced S. riobrave. Nematodes in the commercial formulations were produced in vitro through liquid fermentation; the in vivo nematodes were cultured in Galleria mellonella and applied in aqueous suspension. Laboratory experiments measured nematode virulence in plastic cups containing soil and seventh-eighth instar D. abbreviatus. One laboratory experiment was conducted using only fresh nematodes (less than 5 days old); another experiment included WDG nematodes that were stored for 25 days at 10 °C. Two field experiments were conducted in which nematodes were applied either to potted citrus (containing D. abbreviatus larvae) placed beneath mature citrus trees or to soil directly beneath the tree. In the latter experiment, efficacy was determined by measuring mortality of caged D. abbreviatus larvae that were buried beneath the soil surface prior to application. Mortality of D. abbreviatus treated with nematodes ranged from 80-98% and 50-75% in laboratory and field experiments, respectively. In all experiments, we did not detect any significant effects of culture method or formulation.     

Pathogenicity of Steinernema scapterisci to Selected Invertebrates

Steinernema scapterisci was more pathogenic to insects tested in the order Orthoptera than to those in the orders Lepidoptera or Hymenoptera; it was not pathogenic to earthworms. The nematode also infected and killed the mole crickets Scapteriscus acletus and S. vicinus when released four successive times at 10-day intervals in containers of soil infested with the nematode.     

Simple In Vivo Production and Storage Methods for Steinernema carpocapsae Infective Juveniles

Methods are described for standardized in vivo production, rapid harvest, and storage, in a concentrated form, of infective juveniles of the entomopathogenic nematode, Steinernema carpocapsae Mexican strain Kapow selection. Nematodes were stored in nematode wool configurations, consisting of mats of intertwined infective juveniles. Freshly harvested nematodes are readily available in adequate quantities for laboratory and small-scale field evaluations as well as cottage industry production.     

Effects of Earthworms on the Dispersal of Steinernema spp.

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. glaseri, 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.     

Vertical Dispersal of Steinernema scapterisci

When infective juveniles ofSteinernema scapterisci Nguyen &Smart were released on the soil surface in the field and in the laboratory, some of them moved downward through the soil at least 10 cm in 5 days and infected and killed mole crickets. When released 2 cm below the soil surface, most of the juveniles moved into the upper 2 cm layer of soil, but some moved downward 10 cm. When placed at the center of a 16-cm soil column, infective juveniles moved in both directions with three times more moving downward than upward. Infective juveniles were more efficient in killing mole crickets in the field than in the laboratory.     

Control of Larval Northern Corn Rootworm. (Diabrotica barberi) with Two Steinernematid Nematode Species

The entomogenous nematodes Steinerema feltiae and S. bibionis did not penetrate the roots of corn, Zea mays, to infect larval northern corn rootworm (NCR), Diabrotica barberi, feeding within. Laboratory bioassays against first instar NCR indicated that S. feltiae, Mexican strain (LD₅₀ = 49 nematodes/insect) is more virulent than S. bibionis (LD₅₀ = 100). Numbers of NCR larvae in a grain corn crop were reduced by both nematode species applied at corn seeding time at the rate of 10,000 infective-stage juveniles per linear meter of corn row. The chemical insecticide fonofos provided significantly better control than either nematode species.     

Distribution and Downward Movement of Pasteuria penetrans in Field Soil

Endospores of Pasteuria penetrans were evaluated for their vertical distribution in field soil and their downward movement through soil in the laboratory. In the field trial, the number of endospores attached to second-stage juveniles (J2) of Meloidogyne arenaria race 1 varied greatly in different soil depths. There were higher percentages of J2 with endospores attached in former weed fallow plots during the first 3 years of growing peanut than in former bahiagrass and rhizomal peanut plots (P ≤ 0.05). In weed fallow plots a higher average number of endospores per J2 were maintained in all depths, upper three depths, and upper four depths in 1999, 2000, and 2001, respectively (P ≤ 0.05). However, in 2002, there were no differences in the percentages of J2 with endospores attached and in the average of the numbers of endospores per J2 among the treatments (P > 0.05). In laboratory trials, P. penetrans endospores were observed to move throughout the soil through the percolation of water. After one application of water, some endospores were detected 25 to 37.5 cm deep. Endospores were present at the greatest depth, 37.5 to 50 cm, after the third application of water. These results indicate that rain or water applications by irrigation are likely to move endospores to deeper levels of the soil, but the majority of endospores remain in the upper 0-to-30-cm depth.     

Effects of Crop Residue on the Persistence of Steinernema carpocapsae

We determined the effects of crop residue on the persistence of an entomopathogenic nematode, Steinernema carpocapsae. During 2 consecutive years, nematodes were applied at rates of 2.5 × 10₄ and 1.0 × 10⁵ infective juveniles/m² to small field plots planted with corn. Nematode persistence was monitored by exposing Galleria mellonella larvae to soil samples from plots with and without crop residue (approximately 75% coverage of soybean stubble). Persistence of S. carpocapsae was significantly greater in crop residue plots than in plots without residue. In crop residue plots that received the higher rate of nematode application, larval mortality did not significantly decrease during the study period (3 to 5 days) and remained above 85%. In nematode-treated plots without crop residue, however, larval mortality fell from over 96% to below 11% and 35% in the first and second trials, respectively. The increased crop residue may have benefited nematode persistence through protection from desiccation or ultraviolet light. We conclude that increased ground cover in cropping systems (e.g., due to reduced tillage) may lead to increased insect pest suppression with entomopathogenic nematodes.     

Effect of Entomopathogenic Nematodes on Mesocriconema xenoplax Populations in Peach and Pecan

The effect of Steinernema riobrave and Heterorhabditis bacteriophora on population density of Mesocriconema xenoplax in peach was studied in the greenhouse. Twenty-one days after adding 112 M. xenoplax adults and juveniles/1,500 cm³ soil to the soil surface of each pot, 50 infective juveniles/cm² soil surface of either S. riobrave or H. bacteriophora were applied. Another entomopathogenic nematode application of the same density was administered 3 months later. The experiment was repeated once. Mesocriconema xenoplax populations were not suppressed (P ≤ 0.05) in the presence of either S. riobrave or H. bacteriophora 180 days following ring nematode inoculation. On pecan, 200 S. riobrave infective-stage juveniles/cm² were applied to the soil surface of 2-year-old established M. xenoplax populations in field microplots. Additional applications of S. riobrave were administered 2 and 4 months later. This study was terminated 150 days following the initial application of S. riobrave. Populations of M. xenoplax were not suppressed in the presence of S. riobrave.