A functioning immune system is crucial for protection against disease and illness,yet increasing evidence suggests that species living in urban areas could be suffering from immune suppression,due to the presence of artificial light at night (ALAN).This study examined the effects of ecologically relevant levels of ALAN on three key measures of immune function (haemocyte concentration,lyric activity,and phenoloxidase activity) using a model invertebrate species,the Australian black field cricket,Teleogryllus commodus.We reared crickets under an ecologically relevant daily light-cycle consisting of 12 hr bright daylight (2600 1x) followed by either 12 h darkness (0 1x) or dim environmentally relevant ALAN (1,10,100 1x),and then assessed immune function at multiple time points throughout adult life using haemolymph samples.We found that the presence of ALAN had a clear negative effect on haemocytes,while the effects on lyric activity and phenoloxidase activity were more complex or largely unaffected by ALAN.Furthermore,the effects of lifelong exposure to ALAN of 1 1x were comparable to those of 10 and 100 1x.Our data suggest that the effects of ALAN could be large and widespread,and such reductions in the core immune response of individuals will likely have greater consequences for fitness and survival under more malign conditions,such as those of the natural environment.