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Facultatively sexual entomopathogenic nematodes are a promising model for the experimental study of the adaptive values of sex. Our experiments in the laboratory showed that entomopathogenic nematodes display at least two different strategies in regulating the degree of amphimix as a response to nutritional stress. One strategy promotes the production of males, amphimix and the genetic variability of the offspring, improving the chances for a successful new adaptation. Another strategy increases the production of hermaphrodites at the expense of males, increasing the total number of reproductive individuals and thus the total number of offspring produced. Surprisingly, the strategy used depends upon the strain of symbiotic bacteria the nematodes are growing. The relevance of the results, in helping to discriminate between rival theories for the evolutionary maintenance of sex, is discussed.

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The genetic diversity of symbiotic Xenorhabdus and Photorhabdus bacteria associated with entomopathogenic nematodes was examined by a restriction fragment length polymorphism analysis of PCR-amplified 16S rRNA genes (rDNAs). A total of 117 strains were studied, most of which were isolated from the Caribbean basin after an exhaustive soil sampling. The collection consisted of 77 isolates recovered from entomopathogenic nematodes in 14 Caribbean islands and of 40 reference strains belonging to Xenorhabdus and Photorhabdus spp. collected at various localities worldwide. Thirty distinctive 16S rDNA genotypes were identified, and cluster analysis was used to distinguish the genus Xenorhabdus from the genus Photorhabdus. The genus Xenorhabdus appears more diverse than the genus Photorhabdus, and for both genera the bacterial genotype diversity is in congruence with the host-nematode taxonomy. The occurrence of symbiotic bacterial genotypes was related to the ecological distribution of host nematodes.

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When they are mechanically disturbed, all instars of the tropical bont tickAmblyomma variegatum exude droplets of a liquid on the dorsal, lateral, and ventral cuticle. These spread out and quickly evaporate. In this study, the possible role of these secretions was investigated in relation to predators and pathogens. In laboratory bioassays, it was demonstrated that the secretions from engorged larvae, nymphs, and females have an antibiotic activity against the bacteria speciesBacillus thuringiensis andSerratia marcescens, combined with a repellent effect on a potential predator, the fire-antSolenopsis geminata.

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Entomopathogenous nematodes are well known biocontrol agents of insects. They live in the superficial layer of the soil, a location where ticks accomplish their molt and where they oviposit their eggs, making them, theoretically, the preys of infective larvae of nematodes. Seventeen strains of entomopathogenous nematodes: eight strains of Steinernema and nine strains of Heterorhabditis were placed in contact with each of the free living stages of three tick species: Amblyomma variegatum, Boophilus microplus and B. annulatus. The first two species were resistant to all the nematode strains that were tested, whereas B. annulatus was susceptible to all of them. Ovipositing females were more susceptible than females during the preoviposition period. There were no anatomical differences between the two species of Boophilus which can account for such differences in their susceptibility. The use of nematodes to control some species of ticks should be considered.

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In 1992, two cases of abdominal angiostrongylosis were reported in young Guadeloupean children. With a view to determine the natural infestation rate of Guadeloupe rodents (Rattus rattus, R. norvegicus) by Angiostrongylus costaricensis, 656 rats caught in the country have been examined. Both murine species are naturally infested (6.2% in R. rattus and 14.9% in R. norvegicus), which gives a total prevalence of 7.5%. This total prevalence is higher in the tourist area. A complementary investigation proves to be necessary to identify in Guadeloupe the alleged intermediate hosts (slugs, land snails) in which the human illness has its direct origins.

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Some vertebrate species in Guadeloupe are predators of free or parasitic stages of Amblyomma variegatum (Fabricius). Among birds, 1.9% of the 421 identified animals found in the stomachs of grackles (Quiscalus lugubris), 1.6% of the 364 animals found in the stomachs of free-ranging chickens, and 0.3% of the 4642 animals found in the stomach of cattle egrets (Bubulcus ibis) were A. variegatum ticks. The most efficient predator of ticks was the tropical fire ant, Solenopsis geminata, which was observed to only attack engorged stages of ticks. An average of 8% of the 564 individual ticks or batch of ticks released on the ground, mainly in a grass environment, were attacked by this ant species. Mice (Mus musculus) and mongooses (Herpestes auropunctatus) feed on engorged nymphs and female ticks. None of the 15 strains of entomophagous nematodes, genera Steinernema and Heterorhabditis, experimentally put in contact with engorged larvae and nymphs were able to parasitize them. In the laboratory, the insects Megaselia scalaris (Diptera) and Tineola sp. (Lepidoptera) were occasionally found to feed on engorged and unfed ticks, respectively.

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The amount of water lost out of the fresh faeces during the first hours after they are laid is dependent on the hour, the site and the nature (isolated or aggregated dropping) of the faecal deposit. We studied the effect of desiccation by deposing contaminated faeces in different natural conditions. It appears that the number of infective larvae for one hundred deposited eggs is higher when the rate of desiccation is slower, mainly for O. circumcincta. The results for T. colubriformis and H. contortus are more variable, the number of larvae being lower. N. spathiger is less sensitive to the desiccation. In laboratory experiments, the optimum humidity of faeces, as far as the development rate is concerned, extended from 60 to 65%. With higher and lower levels, the developing rate rapidly decreases. N. spathiger is hardly more resistant to low levels of water content. Embryonated eggs and infective larvae are the most resistant stages, mainly for O. circumcincta. The importance of desiccation of fresh faeces during the first hours after deposition and the viability of Trichostrongyle eggs is discussed.

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A flock of 50 Lacaune ewes grazed three paddocks (approximately 17ha each) in rotation. Thirteen areas were defined in each of the two selected paddocks on the account of vegetation and spatial utilization by the ewes. These areas represented one tenth of the total area of the paddock. For each area the following measures were estimated: species of grasses, bushes, grass disponibility, amount of faeces deposited, infective larval population of strongyles, duration of the utilization by the ewes for resting or grazing. The measures were achieved during two rotational cycles from May to July 1981. Among the several methods utilized for the assessment of the various parameters influencing the probable infection of animals, one was unsatisfying (estimation of grass intake) and another was of particular good value (estimation of the amount of faeces deposited by an index method). The areas at risk were characterized by a vegetation with Bromus erectus associated either with Festuca duriuscula or Brachypodium pinnatum. The infectivity of each area (number of larvae X duration of grazing) was correlated to the number of strongyle eggs deposited, the amount of faeces accumulated and to an index of pasture quality (pastoral value X biomass of available grass). This work demonstrates that in heterogeneous pasture, even if the level of parasitism (Ostertagia sp. and Nematodirus sp.) is low, area at risk could be determined.

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Experiments were done in natural and laboratory conditions with ovine Trichostrongyle species by depositing infective larvae on the top or inside a tube filled with soil. After different periods, extractions were done from soil samples taken after every 5 cm. Patterns of larval positions in the soil layers were similar for the 4 species in the experiments, though numbers of recovered larvae were not similar. In outdoor conditions at the end of winter, larvae could migrate from the surface to 40 cm depth or more in two weeks even if soil temperature was as low as 6 degrees C. These downward migrations seemed mainly due to water percolation through soil. Active migrations through soil were favoured according to the amount of soil water content but larvae had the ability to actively migrate upwards even in relatively dry conditions.

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An overall study of gastro-intestinal parasitism was initiated in a flock of sheep in the Western central region of France (Limousin) comprising studies on the animals and on the pasture. Results of the study on animal parasitism and the possibilities of using pepsinogen blood level measurements were published previously (Hubert, Kerboeuf and Gruner, 1979). This report is concerned with the distribution of parasites on the pasture and a discussion on the epidemiology of these strongyles. The pasture was contamined throughout most the year, first by the ewes and then by the lambs from the second month onwards of their presence on the land. Maximum contamination occurred in Summer, but a precocious wave of Ostertagia and Haemonchus larvae on the pasture was responsible for the high incidence of O. circumcincta infections. Unfavourable summer weather depressed the development of eggs and two secondary waves of larvae at the end of August and the end of September brought about peaks of H. contortus and Trichostrongylus in lambs at the end of the summer. Far less Nematodirus were found than normal. In comparison with the weight gain of lambs treated regularly, untreated animals suffered a drop in their rate of growth from the beginning of July to the beginning of September corresponding to the ingestion of a great number of infective larvae. These preliminary results for the year 1977 show the necessity of continuing these observations for several years to enable the effect of preceding year's contamination to be properly assessed.

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