Insect Immune Evasion by Dauer and Nondauer Entomopathogenic Nematodes

The immune response of animals, including insects, is overcome by some parasites. For example, dauer larvae (DL) of the obligate entomopathogenic nematodes (EPNs) Heterorhabditis and Steinernema can invade insects, evade their defenses, and cause death. Although DL were long assumed to be the only infective stage of nematodes, recent reports suggest that L2-L3 larvae of facultative EPNs are also capable of killing insects. There are no studies, to our knowledge, about the role of nonimmunological barriers (the exoskeleton and its openings) in avoiding infection by DL and L2-L3 larvae, or whether these larval stages evade the host immune system in the same way. The objective of this study was to examine these questions by infecting Galleria mellonella with the facultative parasitic nematode Rhabditis regina. DL or L2-L3 larvae were either deposited on or near the moths or injected into their hemocoel. Once nematodes reached the hemocoel, the following host immune response parameters were quantified: prophenoloxidase, phenoloxidase, lytic activity, and the number of granular hemocytes. DL showed a greater ability to penetrate the exoskeleton than L2-L3 larvae. Once inside, however, both went unnoticed by the immune system and killed the insect. A higher number of granular hemocytes was activated by L2-L3 larvae than DL. We show for the first time that L2-L3 larvae can penetrate and evade the insect immune system. Further research is needed to compare facultative and specialized EPNs to determine which is more likely, with both DL and L2-L3 larvae, to evade insect defense barriers and produce death. The results will contribute to understanding the evolution of virulence in entomopathogenic nematodes.