P. S. Grewal

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

1. n1. We determined thermal niche breadths for infection, establishment, and reproduction of twelve species and strains of entomopathogenic nematodes collected from diverse climatic regions n n2. n2. 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) n n3. n3. 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) n n4. n4. 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 n n5. n5. Thermal niche breadths did not differ between conspecific populations isolated from different localities, but were different for different species isolated from the same locality n n6. n6. We conclude that entomopathogenic nematode species have well-defined thermal niches which may be unaffected by their locality

Host finding behaviour as a predictor of foraging strategy in entomopathogenic nematodes

Foraging strategies of eight species of entomopathogenic nematodes were predicted from their response to host volatile cues and dispersal behaviour on 2-dimensional substrates. Positive directional response to chemical cues and similar distances travelled on smooth (agar) or nictation substrates (agar overlaid with sand grains) by Heterorhabditis bacterio-phora, Heterorhabditis megidis, Steinernema anomali , and Steinernema glaseri suggest their cruising approach to finding hosts. The absence of directional response and less distance travelled on nictation substrate, than on smooth agar by Steinernema carpocapsae and Steinernema scapterisci suggest their ambushing mode of foraging. Steinernema feltiae and Steinernema sp. responded directionally to host volatiles, but travelled less distance on the nictation substrate than on smooth agar; the two species also did not nictate. The cruiser species located hosts more effectively in the sand columns, whereas the ambushers were more effective at finding hosts on filter paper. Steinernema feltiae and Steinernema sp. performed equally on filter paper and in the sand column. We conclude that H. bacteriophora, H. megidis, S. anomali and S. glaseri cruise to find hosts, whereas S. carpocapsae and S. scapterisci ambush hosts. Steinernema feltiae and Steinernema sp. are intermediary in the search continuum sharing some characteristics of both ambush and cruise foragers.

Response of infective stage parasites (Nematoda : Steinernematidae) to volatile cues from infected hosts

Volatile infochemicals play a significant role in the interactions between trophic levels. Volatile infochemicals may allow species within the third trophic level to avoid patches where conspecifics or heterospecifics are present. We show odor-mediated resource assessment by entomopathogenic nematodes in the family Steinernematidae. We hypothesized that the infective juvenile nematodes may reduce inter- and intraspecific competition by responding differently to unparasitized hosts vs. hosts parasitized by conspecific or heterospecific nematodes. All Steinernema spp. except S. carpocapsae were attracted to hosts that were not parasitized. Steinernema carpocapsae infective juveniles were repelled from hosts infected for 4 hr with all heterospecific infections except S. anomaliwhereas S. glaseri were repelled only from S. riobravis-infected hosts. Steinernema feltiae did not differentiate any heterospecific or heterogeneric infections. Steinernema glaseri were attracted to four of five heterospecific infections and S. anomali and S. riobravis were attracted to two of five heterospecific infections. Both S. anomali and S. glaseri were more attracted to hosts infected with the out-group Heterorhabditis bacteriophora than those infected by conspecific nematodes. Infective juvenile S. carpocapsaeS. anomaliand S. glaseri were more attracted to insects colonized by conspecific nematodes than to uninfested insects. Infective juvenile S. carpocapsae were repelled from the 24-hr-old conspecific infections, whereas S. glaseri were less attracted to 24- than to 4-hr-old conspecific infections. Experiments with insects injected with bacteria from the nematodes suggested the latter as the source of active volatiles. We suggest that odor-mediated host recognition by infective juveniles may reduce inter- and intraspecific competition among Steinernematidae.

Host recognition by entomopathogenic nematodes : behavioral response to contact with host feces

Host recognition by entomopathogenic nematodes may occur through contact with insects excretory products, cuticle, or gut contents. We analyzed the behavioral responses of four species of entomopathogenic nematodes during contact with feces of natural or experimental hosts. Host recognition by nematodes was manifested in alterations in the frequency and/or duration of one or more search parameters including forward crawling, headwaving, body-waving, stopping, backward crawling, head-rubbing, and headthrusting.Heterorhabditis bacteriophora andSteinernema glaseri showed behavioral responses to contact with feces of their natural hosts,Spodoptera exigua (Lepidoptera) andPopillia japonica (Coleoptera), and to the experimental hosts,Acheata domesticus (Orthoptera) andBlatella germanica (Blatteria).Steinernema carpocapsae responded only toB. germanica feces, whereas5. scapterisci did not significantly respond to any of the insect species. During contact with cockroach feces, all nematodes, exceptS. scapterisci, showed avoidance behavior. We suggest that ammonia present in cockroach feces is inhibitory to nematodes. Specific host recognition by entomopathogenic nematodes may be an important mechanism to maintain host affinities.

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.

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.