Day/night variations of feeding and immune activities in larvae of the European grapevine moth, Lobesia botrana

Daily varying intensities of exposure to infectious enemies should select for the evolution of a daily structure of host immunity with a marked peak and trough (i.e. a daily rhythm). Such nychthemeral variations have been documented for insect inducible immunity (responsiveness to microbial challenge), while the existence of similar daily patterns in basal immunity remains unexplored. Basal immunity is defined as the background, readily available protection aimed at preventing infection at any time. Daily rhythmic elevation of basal immunity should bear substantial costs and be performed only when facing highly predictable changes in threat of infection. This could be the case for risk of food-borne infection, presumably fluctuating with daily patterns of host feeding activity. This study investigated the existence of day/night variations in feeding activity and basal immunity, using larvae of the moth Lobesia botrana (Lepidoptera: Tortricidae) raised under a realistic daily cycle of light and temperature conditions. At night, larvae (i) spent a greater amount of time feeding, (ii) displayed a higher total phenoloxidase activity and (iii) underwent subtle changes in the balance of some haemocyte types newly described for this species (increase in the relative abundances of prohaemocytes and plasmatocytes, decrease in the one of oenocytoids). These data provide the first evidence of nychthemeral variations of basal immunity in an insect model, with peaks in some immune effectors co-occurring with maximal feeding activity (during nighttime). The ecological implications and the contribution of this work to improve the general understanding of the temporal structure of immunity are discussed.