SUMMARYStudies 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.
Abstract. 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.Key words. Nematoda; insect parasites; reproductive strategies; host-finding; mate recruitment; reproductive success; risk factors.Males of many animal species are dispersers, and take greater risks than females to enhance their reproductive success ~. A male's genetic contribution to the next generation often depends on the number of females fertilized, whereas females are restricted by the number of eggs produced. It follows that males take the more active role in mate-acquisition behavior, and will consequently encounter more risks 7. We provide the first evidence of such an adaptive phenomenon in the parasitic Nematoda. The soil-inhabiting infective juveniles of insect-parasitic nematodes in the families Steinernematidae and Heterorhabditidae (Nematoda: Rhabditida) locate and parasitize suitable insect hosts in which they develop to adults, mate, and reproduce ~2. This group of nematodes has a mutualistic association with the bacteria of the genus Xenorhabdus 13. After gaining access to host's hemocoel, the bacteria harbored by the infective juveniles are released, killing the insect within 24-48 h and providing optimum conditions for nematode growth and reproduction 12 14. As development and mating of these insect-parasitic nematodes can occur only within hosts, selection pressure should operate to evolve means of successful infections wherein mating and reproduction is maximized ~5 16. These insect parasites have evolved two distinct reproductive strategies. Infective juveniles of all the known species of Heterorhabditis develop into hermaphrodites (thus not requiring mates) in the first generation, the progeny of which consist of male and female (monoecious) individuals 14. The members of Steinernema spp. are dioecious, and require at least one infective juvenile of each sex to invade a host to allow mating and reproduction. We describe here how Steinernema spp. may have evolved a means of enhancing their likelihood of successful reproduction. Materials and methods"Third-stage infective juveniles of the following insectparasitic nematode species were reared 17 in last-instar Galleria mellonella L. larvae at 25 ~ Steinernema anomali (Kozodoi), S. carpocapsae (Weiser), S. Jeltiae (Filipjev), S. glaseri (Steiner), and S. scapterisci (Nguyen and Smart). As it is difficult to distinguish morphologically the sex of juvenile nematodes 16,18, we used a sand c...
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.
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