Sen Selvan

Thermal adaptation of entomopathogenic nematodes: Niche breadth for infection, establishment, and reproduction

1. 1. We determined thermal niche breadths for infection, establishment, and reproduction of twelve species and strains of entomopathogenic nematodes collected from diverse climatic regions 2. 2. Steinernema riobravis infected Galleria mellonella (wax moth) larvae at the widest temperature range (10–39°C), whereas S. feltiae at the narrowest (8–30°C). Thermal niche breadt for establishment within hosts was the widest for S. glaseri, (10–37°C) and the narrowest for S. feltiae (8–30°C) 3. 3. Thermal niche breadth for reproduction was widest for S. glaseri (12–32°C) and the narrowest for S. carpocapsae (20–30°C). Steinernema scapterisci (20–32°C), S. riobravis (20–35°C), and Steinernema sp. (20–32°C) were more adapted to warm temperature reproduction, and S. feltiae to cooler temperatures (10–25°C) 4. 4. Although heterorhabditids are endemic to warmer climates, the upper thermal limits and temperature optima for reproduction of Heterorhabditis bacteriophora and H. megidis were cooler than that of some of the steinernematids from South America and the Caribbean 5. 5. Thermal niche breadths did not differ between conspecific populations isolated from different localities, but were different for different species isolated from the same locality 6. 6. We conclude that entomopathogenic nematode species have well-defined thermal niches which may be unaffected by their locality

Biochemical energy reserves of entomopathogenic nematodes

Quantitative and qualitative analyses of the biochemical energy reserves of 6 species of entomopathogenic nematodes (Rhabditida: Heterorhabditidae and Steinemematidae) were conducted. Lipid, glycogen, and protein contents of 2 species of Heterorhabditis (H. bacteriophora and H. megidis) and 4 species of Steinernema (S. carpocapsae, S.feltiae, S. glaseri, and S. scapterisci) were determined. The quantitative measurement of energy reserves (32-38% lipids and 42-46% proteins) suggested that entomopathogenic nematodes are more similar to free-living nematodes than to parasitic ones in this respect. These results also elucidate relationships between activity levels and life span from an energetic perspective

Male insect-parasitic nematodes: a colonizing sex

An adaptive strategy enhancing reproductive success is described for parasitic nematodes. Male infective juveniles of insect-parasitic nematodes,Steinernema spp. (Rhabditida: Steinernematidae) are dispersers, and take greater risks than females during the host-finding phase of parasitism. They disperse, locate, and establish in distant live hosts before females. Parasitism by male infective juveniles renders the infected hosts suitable for nematode development, and more attractive to female infective juveniles. Such ‘recruitment’ may be a strategy to protect the nematodes against uncertainties of mate finding, thereby enhancing reproductive success.

Changes in foraging behaviour during the infective stage of entomopathogenic nematodes

Studies of foraging strategies are often complicated by competing goals of the forager. In contrast, non-feeding infective juvenile entomopathogenic nematodes forage exclusively for a single host. Two questions were posed: (1) what is the relationship between metabolic rate, energy reserves and foraging strategy and (2) when a foraging strategy fails, will an infective-stage parasite switch strategies? Three species of entomopathogenic nematodes were stored in water and changes in their behaviour, metabolic rate, energy reserves, and infectivity were measured throughout the storage period. Steinernema carpocapsae ambushes insect hosts, whereas S. glaseri and Heterorhabditis bacteriophora cruise forage. Steinernema carpocapsae was least active and had the lowest metabolic rate. Heterorhabditis bacteriophora was more active and had the highest metabolic rate. Steinernema glaseri was most active and had an intermediate metabolic rate. Neither cruising species changed foraging strategy. Steinernema carpocapsae decreased nictation (a behaviour associated with ambushing only) and increased their locomotory rate. Any change in searching strategy occurred without assessment of the profitability or distribution of potential hosts, but the advantage this confers is unknown.

Large-scale inoculative releases of the entomopathogenic nematode Steinernema glaseri: Assessment 50 years later

Abstract Billions of nematodes were released from 1939 to 1942 throughout the state of New Jersey (563 sites) in an effort to colonize the entomopathogenic species Steinernema glaseri for biological control of the Japanese beetle, Popillia japonica. Because of the onset of World War 11 and the postwar development of chlorinated hydrocarbon insecticides, little effort was expended to evaluate the outcome of these introductions. We evaluated this colonization program by collecting soil samples in 1991 from 304 geographically and ecologically diverse sites across New Jersey. The soil samples were assayed for entomopathogenic nematodes using the Galleria bait method. Overall, 66 (21.7%) soil samples were positive for entomopathogenic nematodes: 24 steinernematids and 42 heterorhabditids. The most common species isolated was Heterorhabditis bacteriophora (38 isolates), followed by S. glaseri (14), S. carpocapsae (4), S. feltiae (4), Heterorhabditis spp. (4), and Steinernema spp. (2). S. glaseri was recovered only from the southernmost third of the state. We conclude that the colonization effort initiated more than 50 years ago was unsuccessful. The reasons remain uncertain, but intolerance of S. glaseri to temperate climates is one likely explanation. That is, southern New Jersey appears to represent the northernmost range of this neotropical species. Moreover, early workers were unaware of the nematodes mutualistically associated bacterium, Xenorhabdus poinarii, which plays important roles in killing insect hosts and in nematode reproductive potential. We show that the bacterium is inhibited by antimicrobial compounds used by these workers during mass rearing, so it is probable that only the nematode portion of the nematode-bacterium complex was released.

Water Content and Fatty Acid Composition of Infective Juvenile Entomopathogenic Nematodes during Storage

Limited shelf life of infective juveniles is a major obstacle in realizing the full biological control potential of entomopathogenic nematodes. We studied some of the physiological parameters affecting storage stability. Heterorhabditis bacteriophora, Steinernema carpocapsae, and Steinernema glaseri were cultured in the wax moth larva, Galleria mellonella, and the freshly emerged infective juveniles were stored in water at 25 C. Shelflife, estimated by a minimum of 20% survival, varied among species: infective juvenile S. glaseri survived for 36 wk, whereas S. carpocapsae and H. bacteriophora survived only 16 and 7 wk, respectively

Heat shock enhances thermotolerance of infective juvenile insect-parasitic nematodesHeterorhabditis bacteriophora (Rhabditida: Heterorhabditidae)

Insect-parasitic nematodes possess many of the attributes of ideal biological control agents, but intolerance to extreme temperatures can restrict their use. We examined whether heat-shock treatments could improve nematode survival and infectivity at temperatures that normally inhibit their activity (35 and 40°C). Nematodes exposed to a sub-lethal temperature (35°C) for 3 h with a latency period of 1–2 h at 25°C killed insects at 35 and 40°C. Correlative evidence was obtained between increased thermotolerance and the synthesis of 70-kDa heat-shock proteins (hsps). These results provide the first evidence of hsp synthesis in the development of thermotolerance and biological activity in the non-feeding, developmentally arrested, infective juvenile nematodes.

Polymorphism in heat shock protein gene (hsp70) in entomopathogenic nematodes (rhabditida)

Abstract 1. 1. All organisms tested respond to a sudden increase of temperature by synthesizing heat-shock proteins, which helps organisms to survive high temperature. A correlation with increased thermotolerance and production of major 70 kDa protein has been observed in many organisms. 2. 2. Many studies have been designed on a large number of animal species to assess their adaptation to different thermal environments. Genetic analysis of the hsp 70 gene in entomopathogenic nematodes inhabiting different environments may provide insight into the physiological roles of hsps in these nematodes and could be useful for ecological studies. 3. 3. To assess variation among species of entomopathogenic nematodes for thermotolerance, we initiated a search for the molecular organization of heat-inducible hsp 70 genes in these nematodes. Five Heterorhabditis species/isolates with different temperature optima for survival and one warm-adapted species of Steinernema were tested 4. 4. PCR and RFLP analyses of hsp 70 in Heterorhabditis species and S. scapterisci demonstrated a putative homology with the Caenorhabditis elegans hsp 70 A gene, thus indicating evolutionary conserved nature among different nematode species. 5. 5. RFLPs with the hsp 70 A gene probe revealed different banding patterns for Heterorhabditis species and isolates. 6. 6. This is the first report on the identification of any hsp 70 gene in entomopathogenic nematodes. 7. 7. Our observation establishes a unique correlation between geographical distribution and polymorphisms for hsp 70 A gene in these nematodes.