Desiccation tolerance among different isolates of the entomopathogenic nematode Steinernema feltiae (Fillipjev).

Poor storage capacity is a major constraint limiting further expansion of the use of entomopathogenic nematodes. In order to prolong shelf life, a quiescent state of the dauer juveniles (DJs) should be induced. This can be attained by means of desiccation of DJs. In this study, 24 natural isolations of Steinernema feltiae were exposed to desiccation stress in non-ionic polyethylene glycol 600. The dehydrating conditions were measured as water activity, a(w)-value. Non-adapted and adapted DJs were tested separately under a series of dehydrating conditions. The mean tolerated a(w)-value (MW50) ranged from 0.85 for the isolate NEP1 to 0.95 for FIN1, ISR5 and ITA2 when not adapted to desiccation stress and from MW50 of 0.822 for CR1 to 0.98 for ISR6 when adapted to the stress conditions. CR1 tolerated the lowest desiccation stress at an a(w)-value for the most tolerant 10% of the population (MW10) at 0.65 when DJs had been adapted to stress. No significant differences were recorded between all isolates in non-adapted DJs populations MW10 was compared. No correlation between tolerance under non-adapted and adapted conditions were found. Most tolerant isolates will now be used for cross-breeding and subsequent genetic selection to enhance desiccation tolerance.

The influence of humidity on the effect of Steinernema feltiae against diapausing codling moth larvae (Cydia pomonella L.) (Lepidoptera: Tortricidae).

Codling moth (CM) is a serious and global pest of pome fruit. It overwinters in cryptic habitats as cocooned diapausing larvae. Field trials with the entomopathogenic nematode Steinernema feltiae (Rhabditida: Steinernematidae) report control of diapausing CM of up to 70%, but results are variable. The objective of this study was to define environmental conditions favouring the performance of the nematodes. Cocooned larvae were more susceptible than non-cocooned larvae. S. feltiae was unable to infect CM at a water activity (aw-values) < or = 0.9. Mortality of cocooned larvae was reported at lower aw-values than of non-cocooned larvae. Exposure time and impact of external relative humidity (RH) was studied. Mortality of cocooned larvae did not further increase after half an hour of exposure, whereas the mortality increased with increasing exposure time in non-cocooned larvae. LC50 and LC90 considerably decreased with increasing RH. The influence of the relative humidity was less pronounced when surpassing 80% than the effect of the volume of applied water. When S. feltiae was formulated in a surfactant-polymer-formulation (SPF), mortality significantly increased when compared to application in water.

Influence of the aeration rate on the yields of the biocontrol nematode Heterorhabditis megidis in monoxenic liquid cultures.

The entomopathogenic nematode-bacterium complex Heterorhabditis megidis Photorhabdus luminescens was cultured in 10-1 internal loop bioreactors with marine impellers at aeration rates of 0.3 vvm and 0.7 vvm. Process parameters like impeller velocity and oxygen saturation were controlled at equal set points. The bacterial density was assessed at 24 h. Nematode dauer juveniles (DJ) were then inoculated and the development to adults after 8 days and final DJ yields after 16 days were recorded. The bacterial population density and the nematode inoculum development was variable and was not influenced by the aeration rate. A significant effect on the yield was recorded at the highest aeration rate. This result was confirmed by a direct comparison in two 5-1 internal loop glass bioreactors at 0.3 vvm and 1.0 vvm, which were inoculated with nematode and bacterium pre-cultures from the same flask culture. Possible reasons for the positive correlation between aeration rate and DJ yield are discussed.