Observing the viability of entomopathogenic nematodes using image processing

The observation of the viability of entomopathogenic nematodes is generally assessed under the microscope; nematodes are considered dead if they do not respond to prodding. However, this strategy is time consuming. To improve the quality and speed of the observation an image processing technique has been developed. Since prodding is incompatible with this technique two chemicals (acetic acid and sodium chloride) were tested for their ability to excite the nematodes during the observation. The addition of either chemical increased the viability as measured using the image processing system. As a result of adding the stimulants the difference between the viability measured by both methods was not significant anymore. Nematode viability as assessed using the two techniques, was found to be the same using either stimulant. However, sodium chloride was selected for all the following observations because of the lower standard error on the results obtained with the image processing system compared with the standard error on the results after the addition of the acetic acid. Based on preliminary experiments, an effect of time after suspending the nematodes in water using the image processing system was suspected and therefore investigated. We did not find a correlation between the viability as observed under the microscope and the time after suspending the nematodes. However, the viability as observed with the image processing technique rose significantly with time; a logarithmic regression curve could be fitted to these latter data.