Is Survival After Pathogen Exposure Explained by Host's Immune Strength? A Test with Two Species of White Grubs (Coleoptera: Scarabaeidae) Exposed to Fungal Infection

It is usually assumed that a host's survival after pathogen exposure should correlate with the host's immune strength. In the laboratory and using two species of white grubs, Phyllophaga polyphylla (Bates) exhibited a higher survival than Anomala cincta (Say), when inoculated with Metarhizium anisopliae and Beauvieria bassiana (two fungus pathogens of the above white grub species). We tested whether such survival difference correlates with differential expression of immune ability. Thus, immune response (phenoloxidase [PO], and nitric oxide production [NOP]) and survival after experimental fungal infection were compared among and within (challenged versus control groups) white grub species. As expected, results showed that P. polyphylla had higher PO and survival values compared with A. cincta. However, only A. cincta produced NOP. Thus, our study provides support for the idea that survival correlates with host's basal immune strength that never-theless only applies to PO, but not to NOP. The interspecific difference in PO and NOP may be because of the distinct pathogenic pressures that each grub species faces according to their different feeding regimes. Paradoxically, no differences in survival and immune response were found within each white grub species. This suggests that the difference observed between species is better explained by their basal immune response but not to challenge. We discuss possible scenarios to explain why white grubs are attacked by fungus in the field but not in our laboratory conditions.