Mermithid Parasitism of Black Flies (Diptera: Simuliidae)

Mermithid nematodes are common parasites of black flies and play a significant role in the natural regulation of these medically important insects. Infection levels tend to he moderate and perennial, with epizootics rare and highly localized. Mermithid parasitism almost invariably results in the death of the black fly, and thus considerable attention has focused on the potential of these nematodes as biocontrol agents. Early instar black fly larvae appear most susceptible to infection, and integumental penetration hy mermithid preparasites is the only known mode of entry. Postparasitic nematodes typically emerge before host pupation. However, carryover of parasitism into adult simuliids is an important mechanism for local dispersal and recolonization of upstream areas. Following emergence, the mermithids molt to the adult stage. Copulation ensues, the females then laying eggs which eventually give rise to the next generation of infective preparasites. The number of described species is conservatively estimated at 35-40, with most species within the genera Mesomermis, Gastromermis, and Isomermis. The taxonomy of this group of mermithids is a challenging and little explored area. Host-specificity statements, therefore, must be made cautiously because of these systematic problems and others within the Simuliidae. In most instances, temporal and spatial factors limit the host range of these mermithids among simuliid species. Differential susceptibilities anmng larvae concurrently present within the same microhabitat probably reflect varying degrees of host attractiveness and behavioral-physiological resistance. Effects of parasitism on the host may include prevention of metamorphosis, sterility, intersexual development, and behavior modification. Evaluation of the technical feasibility of mermithid control of black flies has been stymied by the limitations of current inoculum-production technology. Continued advances in in vivo and in vitro culture methods are required to accelerate the research process.     

Potentialities of mermithid nematodes for the biocontrol of blackflies (Diptera: Simuliidae)--a review

Mermithids comprise a family of nematodes which invariably kill/sterilize their insect host(s). These nematodes have considerable potential as biocontrol agents of agricultural insect pests and medically important insect vectors. More specifically, mermithid nematodes appear to regulate natural population of blackflies. The taxonomy of the Mermithidae has been only partially evaluated and the taxonomic status of many representatives is uncertain. At least three mermithid genera and species parasitize North American blackflies, although a more varied mermithid fauna probably exists. The host specificity of mermithid parasites of simuliids is variable, but these nematodes do not appear to infect other stream fauna. The sporadic distribution of mermithid parasites of simuliids among potential biotopes may be associated with a relatively inefficient mode of dispersal for such nematodes. Detailed information is lacking concerning stages in the life cycles of these pathogens and their synchronization with the simuliid host. Mermithids cause pathogenic effects upon several blackfly tissues, although no information is available concerning physiological manifestations of mermithid parasitism in blackflies. A brief review of the present state of knowledge of simuliid taxonomy and bionomics is presented. The physiology of blackflies and their mermithid parasites has been largely ignored. The potentialities of mermithid nematodes for the biocontrol of blackflies are assessed and a feasible research programme presented, in relation to the present state of knowledge of mermithid-simuliid interrelationships and related areas of insect nematology.     

Mermithid Nematodes: In Vitro Culture Attempts

Few attempts at in vitro culture of mermithids have been undertaken. The various methods used to initiate cultures are described. The capacity of a range of media to promote growth and development of the nematodes has been evaluated and current approaches to in vitro outlined.     

Laboratory Culture and Life History of Heleidomermis magnapapula in Its Host, Culicoides variipennis (Diptera: Ceratopogonidae)

The mermithid parasite Heleidomermis magnapapula was maintained in larvae of the midge Culicoides variipennis for 20 months in enamel pans containing nutrient-rich water and polyester pads as a substrate. Inseminated female mermithids were introduced to the pad surface when the host was in the late second or early third-instar. Host larvae were harvested from the pans 9 days after exposure and held in tap water for nematode emergence. Preparasite yield was positively correlated with female nematode size and averaged 1,267 preparasites/female. Male and female nematodes emerged an average of 12.2 and 13.4 days after host exposure, respectively. Supplemental host food (Panagrellus) during the final days of parasitism did not alter time of emergence. Parasites emerging singly were 64% females, whereas superparasitized hosts yielded males (up to nine/host). Nematode carryover into the adult midge normally occurred at a level of 0.5-2.5%. Parasite load (nematodes/ parasitized individual) in midge adults was lower than that of larvae from the same cohort, and adult midges were more likely to harbor female parasites. Exposure of fourth-instar host larvae resulted in higher levels of adult parasitism (up to 17%).     

Biological Control Potential of Neoaplectanid Nematodes

The neoaplectanids are among the most studied of all entomogenous nematodes. Because these nematodes kill their insect hosts, they are regarded as having excellent potential as biological control agents. While the host specificity of most entontogenous nematodes tends to limit their potential usefulness, the broad host range and high virulence of neoaplectanids make them attractive candidates for industrial development. Also, recent development of economical mass rearing procedures appears to make production on a commercial basis feasible. Infective stages may be stored for years trader various laboratory conditions. Although entomogenous nematodes, as parasites, are exempt from govermnent registration requirements, the mutualistic association of neoaplectanid nematodes with a bacterium will likely necessitate a detailed safety evaluation. Studies conducted to date indicate a lack of pathogenicity to mammals. Field trial success has been limited by the intolerance of infective stages to mffavorable environmental conditions, particularly low moisture. Applications against pests on exposed plant foliage have been especially disappointing. More encouraging anti consistent results have been obtained in more favorable environments, including soil and aquatic habitats, but the most promising treatment sites ntay be cryptic habitats where infective stages are shehered from environmental extremes. Cryptic habitats also exploit the ability of neoaplectanids to actively seek out hosts in recessed places where conventional insecticide applications are intpractical.     

Biological Control of Mosquitoes with Mermithids

Mermithid nematodes parasitizing mosquitoes have substantial potential for vector control. Studies on the physiological ecology of Romanomermis culicivorax have defined some of the general requirements of mermithid nematodes and produced general guidelines for the experimental release of mermithids in biological control. Experimental field studies have established the biological control potential of R. culicivorax, but further development and ulilization of this parasite will require a substantial commitlnent of scientific man-years and ftmds.     

Occurrence of five distinct clades of mermithid nematodes (Nematoda: Mermithidae) infecting black fly larvae (Diptera: Simuliidae) in tropical streams in Malaysia

Mermithids are the most common parasites of black flies and are associated with host feminization and sterilization in infected hosts. However, information on the species / lineage of black fly mermithids in Southeast Asia, including Malaysia requires further elucidation. In this study, mermithids were obtained from black fly larvae collected from 138 freshwater stream sites across East and West Malaysia. A molecular approach based on nuclear-encoded 18S ribosomal RNA (18S rRNA) gene was used to identify the species identity / lineage of 77 nematodes successfully extracted and sequenced from the specimens collected. Maximum likelihood and neighbor-joining phylogenetic analyses demonstrated five distinct mermithid lineages. Four species delimitation analyses: automated simultaneous analysis phylogenetics (ASAP), maximum likelihood Poisson tree processes with Bayesian inferences (bPTP_ML), generalized mixed yule coalescent (GMYC) and single rate Poisson tree processes (PTP) were applied to delimit the species boundaries of mermithid lineages in this data set along with genetic distance analysis. Data analysis supports five distinct lineages or operational taxonomic units for mermithids in the present study, with two requiring further investigation as they may represent intraspecific variation or closely related taxa. One mermithid lineage was similar to that previously observed in Simulium nigrogilvum from Thailand. Co-infection with two mermithids of different lineages was observed in one larva of Simulium trangense. This study represents an important first step towards exploring other aspects of host – parasite interactions in black fly mermithids.     

Factor Affecting Sex Ratios of a Mermithid Parasite of Mosquitoes

The ratio of male to female Reesimermis nielseni Tsai and Grundmann, a nematode parasite of mosquito larvae, increased as the number of parasites per host increased. Hosts with a single nematode produced 9% males compared with essentially 100% males in hosts with more than 7 parasites; hosts with 3 nematodes produced about equal numbers of males and females. Males of R. nielseni generally emerged before females because of the earlier death of multiple-infected mosquitoes. The species of the host mosquito influenced the sex ratio, but the size of a specific host at the time of invasion did not. Host diet also had a noticeable influence on the sex ratio of the nematode: singly infected hosts from a starved population produced 92% males compared with 13% in the normally fed group. The importance of these factors in the mass rearing of R. nielseni is discussed.     

Influence of Host Nutrients on the Parasitic Development of Mermis nigrescens (Mermithidae)

Schistocerca gregaria nymphs and adults of both sexes were infected with eggs of Mermis nigrescens. Mermithid larvae grew more slowly in nymphal hosts, and emerging larvae were smaller than those from adult hosts. The longer the larvae remained in the host, the greater their size. Those developing in adult female hosts were longest. Single mermithid larvae that were transferred to a second host continued to grow and were significantly longer at emergence than larvae that developed solely in one host. In adult hosts that were infected with 40-300 M. nigrescens eggs, the percentage of mermithids that became males was strongly dependent on host weight at infective doses of 90 eggs or more. Results are discussed in relation to nutrient stress on the larvae and its importance in developing in vitro culture techniques.     

Detection and Description of Soils with Specific Nematode Suppressiveness

Soils with specific suppressiveness to plant-parasitic nematodes are of interest to define the mechanisms that regulate population density. Suppressive soils prevent nematodes from establishing and from causing disease, and they diminish disease severity after initial nematode damage in continuous culturing of a host. A range of non-specific and specific soil treatments, followed by infestation with a target nematode, have been employed to identify nematode-suppressive soils. Biocidal treatments, soil transfer tests, and baiting approaches together with observations of the plant-parasitic nematode in the root zone of susceptible host plants have improved the understanding of nematode-suppressive soils. Techniques to demonstrate specific soil suppressiveness against plant-parasitic nematodes are compared in this review. The overlap of studies on soil suppressiveness with recent advances in soil health and quality is briefly discussed. The emphasis is on methods (or criteria) used to detect and identify soils that maintain specific soil suppressiveness to plant-parasitic nematodes. While biocidal treatments can detect general and specific soil suppressiveness, soil transfer studies, by definition, apply only to specific soil suppressiveness. Finally, potential strategies to exploit suppressive soils are presented.     

Evolution of Parasitism in Insect-transmitted Plant Nematodes

Nematode-insect associations have evolved many times in the phylum Nematoda, but these lineages involve plant parasitism only in the Secernentean orders Aphelenchida and Tylenchida. In the Aphelenchida (Aphelenchoidoidea), Bursaphelenchus xylophilus (Pine wood nematode), B. cocophilus (Red ring or Coconut palm nematode) (Parasitaphelenchidae), and the many potential host-specific species of Schistonchus (fig nematodes) (Aphelenchoididae) nematode-insect interactions probably evolved independently from dauer-forming, mycophagous ancestors that were phoretically transmitted to breeding sites of their insect hosts in plants. Mycophagy probably gave rise to facultative or obligate plant-parasitism because of opportunities due to insect host switches or peculiarities in host behavior. In the Tylenchida, there is one significant radiation of insect-associated plant parasites involving Fergusobia nematodes (Fergusobiinae: Neotylenchidae) and Fergusonina (Fergusoninidae) flies as mutualists that gall myrtaceous plant buds or leaves. These dicyclic nematodes have different phases that are parasitic in either the insect or the plant hosts. The evolutionary origin of this association is unclear.     

Tetradonema solenopsis n. sp. (Nematoda: Tetradonematidae) Parasitic on the Red Imported Fire Ant Solenopsis invicta Buren from Brazil

Explorations in Brazil to find parasites of the red imported fire ant, Solenopsis invicta Buren, have uncovered a new species of the Tetradonematidae, Tetradonema solenopsis n. sp. The nematode parasite was fatal to about 25% of the colony. The female nematodes are large and sausage shaped and the males are small and difficult to find, which is typical of the genus. The ant is a new host record for tetradonematids.     

Problems and Strategies Associated with Long-term Use of Nematode Resistant Cultivars

Plant-parasitic nematodes are obligate parasites, and planting cultivars that are highly resistant to these organisms places extensive selection pressure on the target species and affects nontarget nematodes as well. Problems encountered with long-term planting of cultivars resistant to nematodes include shifts in nematode races or species and the occurrence of multiple species of nematodes within the same field. These problems can be alleviated to some extent when crop management is used to lessen the selection pressure for change on the nematode populations. Race shifts within populations and possibly shifts between nematode species can be delayed by rotating susceptible cultivars and nonhost crops with resistant cultivars. Nematicides in conjunction with resistant cultivars may be used to limit damage by multiple species of nematodes. Some cultivars have resistance to multiple species of nematodes, but greatly increased research effort is needed in this area. More intensive plant breeding effort will be required to make nematode resistant cultivars competitive in quality and yield with more productive, susceptible cultivars.     

Entomopathogenic Nematodes Are Not an Alternative to Fenamiphos for Management of Plant-Parasitic Nematodes on Golf Courses in Florida

With the cancellation of fenamiphos in the near future, alternative nematode management tactics for plant-parasitic nematodes (PPN) on golf courses need to be identified. The use of entomopathogenic nematodes (EPN) has been suggested as one possible alternative. This paper presents the results of 10 experiments evaluating the efficacy of EPN at managing PPN on turfgrasses and improving turf performance. These experiments were conducted at various locations throughout Florida over the course of a decade. In different experiments, different EPN species were tested against different species of PPN. Separate experiments evaluated multiple rates and applications of EPN, compared different EPN species, and compared single EPN species against multiple species of PPN. In a few trials, EPN were associated with reductions in certain plant-parasite species, but in other trials were associated with increases. In most trials, EPN had no effect on plant parasites. Because EPN were so inconsistent in their results, we conclude that EPN are not acceptable alternatives to fenamiphos by most turf managers in Florida at this time.     

Plasmodium relictum infection and MHC diversity in the house sparrow (Passer domesticus)

Antagonistic coevolution between hosts and parasites has been proposed as a mechanism maintaining genetic diversity in both host and parasite populations. In particular, the high level of genetic diversity usually observed at the major histocompatibility complex (MHC) is generally thought to be maintained by parasite-driven selection. Among the possible ways through which parasites can maintain MHC diversity, diversifying selection has received relatively less attention. This hypothesis is based on the idea that parasites exert spatially variable selection pressures because of heterogeneity in parasite genetic structure, abundance or virulence. Variable selection pressures should select for different host allelic lineages resulting in population-specific associations between MHC alleles and risk of infection. In this study, we took advantage of a large survey of avian malaria in 13 populations of the house sparrow (Passer domesticus) to test this hypothesis. We found that (i) several MHC alleles were either associated with increased or decreased risk to be infected with Plasmodium relictum, (ii) the effects were population specific, and (iii) some alleles had antagonistic effects across populations. Overall, these results support the hypothesis that diversifying selection in space can maintain MHC variation and suggest a pattern of local adaptation where MHC alleles are selected at the local host population level.     

Nematode parasites reduce carotenoid-based signalling in male red grouse

Carotenoids determine the yellow-red colours of many ornaments, which often function as signals of quality. Carotenoid-based signalling may reliably advertise health and should be particularly sensitive to parasite infections. Nematodes are among the commonest parasites of vertebrates, with well-documented negative effects on their hosts. However, to date, little is known about the effects that these parasites may have on carotenoid-based signalling. Tetraonid birds (grouse) exhibit supra-orbital combs, which are bright integumentary ornaments pigmented by carotenoids. We tested the effect of the nematode parasite Trichostrongylus tenuis on signalling in free-living male red grouse Lagopus lagopus scoticus. We show that experimentally reduced nematode infection increases plasma carotenoid concentration and comb redness, demonstrating for the first time that nematodes can influence carotenoid-based signals.     

Enhanced levels of plant cell cycle inhibitors hamper root-knot nematode-induced feeding site development

Root-knot nematodes (RKN) are highly specialized, obligatory plant parasites. These animals reprogram root cells to form large, multinucleate, and metabolically active feeding cells (giant cells) that provide a continuous nutrient supply during 3–6 weeks of the nematode’s life. The establishment and maintenance of physiologically fully functional giant cells are necessary for the survival of these nematodes. As such, giant cells may be useful targets for applying strategies to reduce damage caused by these nematodes, aiming the reduction of their reproduction. We have recently reported the involvement of cell cycle inhibitors of Arabidopsis, named Kip-Related Proteins (KRPs), on nematode feeding site ontogeny. Our results have demonstrated that this family of cell cycle inhibitors can be envisaged to efficiently disrupt giant cell development, based on previous reports which showed that alterations in KRP concentration levels can induce cell cycle transitions. Herein, we demonstrated that by overexpressing KRP genes, giant cells development is severely compromised as well as nematode reproduction. Thus, control of root-knot nematodes by modulating cell cycle-directed pathways through the enhancement of KRP protein levels may serve as an attractive strategy to limit damage caused by these plant parasites.     

A Semiquantitative Method for Enumerating and Observing Parasites and Predators of Soil Nematodes

A laboratory method was developed to count and observe antagonists of soil nematodes and simulate their relationships in the soil. A 10- to 25-cc soil sample is suspended in water and washed through a series of small standard sieves. Residues are washed into a small beaker and collected on a 24-mm filter paper disk in a filter holder under vacuum. The disk is placed on corn meal agar in a petri dish. Microfauna and flora present in the sample colonize the organic matter on the disk and move onto and into the agar where they can be observed easily. Distinct successions of organisms usually occur and within 6-18 days or more, parasites and predators of nematodes are often abundant, especially nematode-trapping fungi. Counting predation events and parasitized nematodes in replicate dishes after specific incubation periods allows quantitative comparisons between soil samples. The method has distinct advantages over others for enumerating organisms which attack nematodes.     

Review of Pasteuria penetrans: Biology,Ecology, and Biological Control Potential

Pasteuria penetrans is a mycelial, endospore-forming, bacterial parasite that has shown great potential as a biological control agent of root-knot nematodes. Considerable progress has been made during the last 10 years in understanding its biology and importance as an agent capable of effectively suppressing root-knot nematodes in field soil. The objective of this review is to summarize the current knowledge of the biology, ecology, and biological control potential of P. penetrans and other Pasteuria members. Pasteuria spp. are distributed worldwide and have been reported from 323 nematode species belonging to 116 genera of free-living, predatory, plant-parasitic, and entomopathogenic nematodes. Artificial cultivation of P. penetrans has met with limited success; large-scale production of endospores depends on in vivo cultivation. Temperature affects endospore attachment, germination, pathogenesis, and completion of the life cycle in the nematode pseudocoelom. The biological control potential of Pasteuria spp. have been demonstrated on 20 crops; host nematodes include Belonolaimus longicaudatus, Heterodera spp., Meloidogyne spp., and Xiphinema diversicaudatum. Pasteuria penetrans plays an important role in some suppressive soils. The efficacy of the bacterium as a biological control agent has been examined. Approximately 100,000 endospores/g of soil provided immediate control of the peanut root-knot nematode, whereas 1,000 and 5,000 endospores/g of soil each amplified in the host nematode and became suppressive after 3 years.     

Perspectives on the Behavior of Entomopathogenic Nematodes from Dispersal to Reproduction: Traits Contributing to Nematode Fitness and Biocontrol Efficacy

The entomopathogenic nematodes (EPN) Heterorhabditis and Steinernema are widely used for the biological control of insect pests and are gaining importance as model organisms for studying parasitism and symbiosis. In this paper recent advances in the understanding of EPN behavior are reviewed. The “foraging strategy” paradigm (distinction between species with ambush and cruise strategies) as applied to EPN is being challenged and alternative paradigms proposed. Infection decisions are based on condition of the potential host, and it is becoming clear that already-infected and even long-dead hosts may be invaded, as well as healthy live hosts. The state of the infective juvenile (IJ) also influences infection, and evidence for a phased increase in infectivity of EPN species is mounting. The possibility of social behavior - adaptive interactions between IJs outside the host - is discussed. EPNs’ symbiotic bacteria (Photorhabdus and Xenorhabdus) are important for killing the host and rendering it suitable for nematode reproduction, but may reduce survival of IJs, resulting in a trade-off between survival and reproduction. The symbiont also contributes to defence of the cadaver by affecting food-choice decisions of insect and avian scavengers. I review EPN reproductive behavior (including sperm competition, copulation and evidence for attractive and organizational effects of pheromones), and consider the role of endotokia matricida as parental behavior exploited by the symbiont for transmission.