Desiccation survival of the entomopathogenic nematode Steinernema feltiae: induction of anhydrobiosis

The present study describes different desiccation tolerance traits of three strains of Steinernema feltiae (IS-6, IS-15, and N8). A slow dehydration regime (pre-conditioning at 97% relative humidity \[RH] for 3 days at 23 degrees C) induced a quiescent anhydrobiosis state in all strains, which enabled them to survive at lower humidities (75% and 85% RH). The IS-6 strain isolated from the Negev desert region of Israel exhibited the best desiccation tolerance. The second best tolerance was observed in the IS-15 strain isolated from Galilee, in the northern part of Israel. The poorest tolerance was exhibited by the N8 strain, which was obtained from Germany. The higher desiccation tolerance of the IS-6 and IS-15 strains was associated with a dispersal response of the aggregated infective juveniles (IJs) at the slow dehydration regime. This allowed the coiled IJs to enter into anhydrobiosis individually, whereas the IJs of the N8 strain remained clumped together. In the present study, the IS-6 strain was chosen to determine the optimal conditions for induction into, and recovery from, anhydrobiosis. A high correlation (r = 0.875, P < 0.05) was found between the survival of individual IJs at 85% RH and the initial numbers of IJs (ranging from 70 to 7700) in the pre-conditioned clump. The same recovery rates of pre-conditioned IJs exposed to 85% RH over a period of 12 days were obtained with either direct immersion in distilled water or immersion in distilled water after 24 h exposure to 100% RH. No significant differences in virulence and ability to penetrate Tenebrio molitor larvae were observed between non-desiccated IJs and rehydrated IJs that had been pre-conditioned and desiccated for 5 days at 85% RH. Survie en etat de deshydratation du nematode entomopathogene Steinernema feltiae: induction de l'anhydrobiose - La presente etude decrit differents aspects de la tolerance a la dessiccation chez trois souches de Steinernema feltiae (IS-6, IS-15 et N8). Une deshydratation lente- preconditionnement a une humidite relative (RH) de 97% pendant 3 jours a 23 degrees C - induit un stade de quiescence anhydrobiotique chez toutes les souches, ce qui les rend capables de survivre a des humidites faibles (RH 75% et 85%). La souche IS-6 isolee dans le desert du Negev (Israel) fait montre de la meilleure tolerance a la dessiccation. Vient ensuite la souche IS-15 isolee en Galilee (partie nord d'Israel). La plus faible tolerance est celle de la souche N8 provenant d'Allemagne. La tolerance plus elevee a la dessiccation des souches IS-6 et IS-15 est associee a une reaction de separation des juveniles infestants (IJ) dans les agregats lors de la deshydratation lente. Cette separation est suivie d'une entree en anhydrobiose des separes, enroules sur eux-memes, tandis que les IJ de la souche N8 restent agglomeres. La souche IS-6 a ete choisie pour determiner les conditions optimales induisant l'anhydrobiose et la sortie de ce stade physiologique. Une forte correlation (r = 0,875, P < 0,05) a ete observee entre la survie des IJ separes a une RH de 85% et la presence d'agregats (comptant 70 a 7700 IJ) au moment du preconditionnement initial. Les memes taux de reviviscence sont obtenus, soit par immersion directe dans l'eau distillee, soit par exposition a une RH de 100% avant immersion dans l'eau. Aucune difference significative dans la virulence et la capacite a penetrer les larves de Tenebrio molitor n'a ete observee entre les IJ dessechees, apres le preconditionnement pendant 5 jours a une RH de 85% et les IJ n'ayant pas subi ce traitement.