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

Specificity of Association between Paenibacillus spp. and the Entomopathogenic Nematodes, Heterorhabditis spp.

Endospore-forming bacteria, Paenibacillus spp., have recently been isolated in association with insect pathogenic nematodes Heterorhabditis spp. Sporangia adhere to nematode infective juveniles (J3) and are carried with them into insects. Paenibacillus proliferates in the killed insect along with Heterorhabditis and its obligate bacterial symbiont, Photorhabdus, despite the antibiotic production of the latter. Nematode infective juveniles leave the insect cadaver with Paenibacillus sporangia attached. The specificity of the relationship between Paenibacillus and Heterorhabditis was investigated. Sporangia of nematode-associated Paenibacillus adhered to infective juveniles (but not other stages) of all Heterorhabditis species tested, and to infective juveniles of vertebrate parasitic Strongylida species, but not to a variety of other soil nematodes tested. Paenibacillus species that were not isolated from nematodes, but were phylogenetically close to the nematode-associated strains, did not adhere to Heterorhabditis, and they were also sensitive to Photorhabdus antibiotics in vitro, whereas the nematode-associated strains were not. Unusual longevity of the sporangium and resistance to Photorhabdus antibiotics may represent specific adaptations of the nematode-associated Paenibacillus strains to allow them to coexist with and be transported by Heterorhabditis. Adaptation to specific Heterorhabditis- Photorhabdus strains is evident among the three nematode- associated Paenibacillus strains (each from a different nematode strain). Paenibacillus NEM1a and NEM3 each developed best in cadavers with the nematode from which it was isolated and not at all with the nematode associate of the other strain. Differences between nematode-associated Paenibacillus strains in cross-compatibility with the various Heterorhabditis strains in cadavers could not be explained by differential sensitivity to antibiotics produced by the nematodes' Photorhabdus symbionts in vitro

Specificity of Association between Paenibacillus spp. and the Entomopathogenic Nematodes, Heterorhabditis spp.

Endospore-forming bacteria, Paenibacillus spp., have recently been isolated in association with insect pathogenic nematodes Heterorhabditis spp. Sporangia adhere to nematode infective juveniles (J3) and are carried with them into insects. Paenibacillus proliferates in the killed insect along with Heterorhabditis and its obligate bacterial symbiont, Photorhabdus, despite the antibiotic production of the latter. Nematode infective juveniles leave the insect cadaver with Paenibacillus sporangia attached. The specificity of the relationship between Paenibacillus and Heterorhabditis was investigated. Sporangia of nematode-associated Paenibacillus adhered to infective juveniles (but not other stages) of all Heterorhabditis species tested, and to infective juveniles of vertebrate parasitic Strongylida species, but not to a variety of other soil nematodes tested. Paenibacillus species that were not isolated from nematodes, but were phylogenetically close to the nematode-associated strains, did not adhere to Heterorhabditis, and they were also sensitive to Photorhabdus antibiotics in vitro, whereas the nematode-associated strains were not. Unusual longevity of the sporangium and resistance to Photorhabdus antibiotics may represent specific adaptations of the nematode-associated Paenibacillus strains to allow them to coexist with and be transported by Heterorhabditis. Adaptation to specific Heterorhabditis- Photorhabdus strains is evident among the three nematode- associated Paenibacillus strains (each from a different nematode strain). Paenibacillus NEM1a and NEM3 each developed best in cadavers with the nematode from which it was isolated and not at all with the nematode associate of the other strain. Differences between nematode-associated Paenibacillus strains in cross-compatibility with the various Heterorhabditis strains in cadavers could not be explained by differential sensitivity to antibiotics produced by the nematodes' Photorhabdus symbionts in vitro

Controlling the large pine weevil, Hylobius abietis, using natural enemies

The large pine weevil is the most serious pest of conifer reforestation in Ireland. The adult weevil causes damage by feeding on the bark of young transplants. With up to 100,000 adult weevils emerging per hectare on recently felled conifer sites, it is not uncommon for 100% of young plants to be killed. Pine weevils are susceptible to attack from a number of natural enemies such as fungi, parasites and predators. Of these, insect killing nematodes (microscopic worms) are the most promising biological control agent: capable of reducing weevil populations by up to 70%. The practicalities of using nematodes, their environmental safety and their potential for use as part of an integrated pest management strategy are discussed

Spontaneous and induced activity of Heterorhabditis megidis infective juveniles during storage

Infective juveniles (IJ) of three Heterorhabditis megidis isolates, HF85, EU17 and UK211, were stored in water at 20C for up to 10 weeks. At 2-week intervals, activity, infectivity, energy reserves and survival were measured. There was no difference between the three isolates in infectivity, which increased significantly over the first 2 weeks and declined gradually thereafter. IJ became inactive during storage. Out of storage, the highest activity was recorded in week 0: nearly all IJ were active within the first minute of observation and remained active for the 20-min observation period.With increasing storage time, an increasing proportion of IJ were inactive in the first minute, reaching 83-96% by week 6. The time taken by 50% of the IJ to become active (AT50) initially increased with nematode age, reaching a maximum of 3-7 min in week 4 or 6 (depending on isolate) but subsequently declined to 2-4 min in week 10. By the time the IJ were becoming more readily activated in weeks 6-8, 75% of the lipid reserves had been depleted, and IJ had started to die. This greater propensity to become active with age may represent a switch to risk-taking behaviour in the face of impending starvation

Effects of Paenibacillus nematophilus on the entomopathogenic nematode Heterorhabditis megidis

The insect parasitic nematodes Heterorhabditis spp. are mutualistically associated with entomopathogenic bacteria, Photorhabdus spp. A novel association has been detected between H. megidis isolate EU17 and the endospore-forming bacterium Paenibacillus nematophilus. P. nematophilus sporangia adhere to infective juveniles (IJs) of H. megidis and develop in insect hosts along with the nematodes and their symbiont. We tested the eVects of P. nematophilus on H. megidis. The yield and quality (size, energy reserves, and storage survival) of IJs were not aVected by co-culture in insects with P. nematophilus. Dispersal of IJs in sand and on agar was inhibited by adhering P. nematophilus sporangia: fewer than 2% of IJs with P. nematophilus sporangia reached the bottom of a sand column, compared to 30% of the control treatment. Sporangia signiWcantly reduced infectivity of H. megidis for wax moth larvae in sand, but not in a close contact (Wlter paper) assay. The results suggest that P. nematophilus may reduce the transmission potential of H. megidis through impeding the motility of IJs

The infectivity and behaviour of exsheathed and ensheathed Heterorhabditis megidis infective juveniles

The consequencesof sheath loss on infectivityand behaviourof infective juveniles (IJ) were investigatedin Heterorhabditis megidis. Ensheathed IJ were more infective, killing 32% of wax moth larvae, compared to 18% killed by exsheathed IJ. The percentage of time engaged in seven behavioural activitieswas recordedfor individuallystored IJ but no differenceswere found between exsheathed and ensheathed IJ. Immobility was the most common behavioural category exhibited by both exsheathed and ensheathed IJ, occupying one third of the observation time. Storage conditions affected the rate of exsheathment; 40% of IJ stored for 28 days in water in bulk (50 in 8 ml) exsheathed compared to only 23% of those stored individually (1 in 2 ml)

Morphological characterisation of three isolates of Heterorhabditis Poinar, 1976 from the "Irish group" (Nematoda: Rhabditida: Heterorhabditidae)and additional evidence supporting their recognition as a distinct species, H. downesi n. sp

The morphological variation of three representative isolates of the "Irish group" of Heterorhabditis was examined. First generation hermaphrodites were characterised by having a blunt and mucronate tail. Females (second generation) and third-stage infective juveniles were also distinguished by the morphology of the tail and the presence of a refractile projection in the tail tip. Males were characterised by the position of the excretory pore and by the value of ratio SW. These morphological features do not fit the description of currently recognised Heterorhabditis species, and provide additional evidence in support for the consideration of the Irish group as a new species. A description of this species, as H. downesi n. sp., is provided