Life cycle of entomopathogenic nematodes

The life cycle of EPNs consists of three phases: a free phase, an entomopathogenic phase and a necrotrophic phase (see BIBINE team). IJs in the L3 stage are found in the soil in search of an insect to parasitize. There are two hunting strategies. Some nematodes are very active in searching for the insect larva (cruiser) while others are on the lookout for passing insects (ambusher)(Crow, 2003; Campbell et al, 2003). IJs detect their potential prey using molecules and thermal, tactile or vibratory signals (O'Halloran et al, 2006). Infesting larvae enter their prey through natural orifices (mouth, anus, spiracles) or by penetrating the cuticle (case of the genus Heterorhabditis) and find their way into the insect. Entomopathogenic bacteria, living in symbiosis with the EPNs, are then released and will start to multiply in contact with the insect's blood (haemolymph) and cause septicemia. These bacteria secrete antimicrobial molecules that prevent the gut microbial flora from colonizing the cadaver. In addition, the secreted enzymes help to decompose the remains by converting them into nutritive resources that can be used by the nematodes for their development and multiplication. Under optimal conditions, the insect dies within 24 to 48 hours.

The insect larva colonized by the genus Heterorhabditis takes on a brick red color (larva on the left). This color is linked to pigments produced by the symbiotic bacterium Photorhabdus. Infestation by the genus Steinernema does not give the larva any color (right larva).

The food shortage triggers the emergence of thousands of larvae in the IJ stage.

These escape from the dead insect and are again free in the soil in search of a new insect (Murfin et al, 2012).

Bibliography

Crow, W.T. 2003. Using nematodes to control insects : overview and frequently asked questions. University of Florida. https://journals.flvc.org/edis/article/download/108640/103820.

Campbell, J.F., Lewis, E.E., Stock, S.P., Nadler, S., Kaya, H.K. 2003. Evolution of host search strategies in entomopathogenic nematodes. J nematol, 35, 142–145. https://journals.flvc.org/jon/article/view/69480/67140.

O’Halloran, D.M., Fitzpatrick, D.A., Burnell, A.M. 2006. The chemosensory system of Caenorhabditis elegans and other nematodes. In Chemical Ecology: From Gene to Ecosystem. Volume 16, pp 71-88. Dicke, M. & Takken, W. (eds), Wageningen, The Netherlands. https://edepot.wur.nl/136894.

Murfin, K., Dillman, A., Foster, J., Bulgheresi, S., Slatko, B., Sternberg, P., Goodrich-Blair, H. 2012. Nematode-bacterium symbioses - cooperation and conflict revealed in the "omics" age. Biol Bull, 223, 85-102. DOI : 10.1086/BBLv223n1p85.