The worldwide expansion of artificial light at night (ALAN) and its alarming rate of increase makes it an imperative issue to study effects of this anthropogenic disturbance on the natural world. It has been shown that ALAN has negative impacts including disturbing the circadian rhythms of herbivory in nocturnal insects. However, there is a lack of studies on how this disturbance on circadian rhythms affects diurnal insects. I studied the effects of ALAN on a diurnal insects behavior and performance. My study system consisted of monarch caterpillars and common milkweed. These organisms are ideal to study effects of light pollution on diurnal insects species because milkweed often grows in disturbed that are exposed to ALAN. I conducted an experiment to examine the effects of ALAN on feeding frequency and habitat selection, specifically how high individuals perched on their host plants. I conducted a second experiment to examine effects of ALAN on larval performance, in which I measured effects on days to pupation (of 2nd instar larvae) and pupal weight.
Results/Conclusions
I found that the feeding frequency of caterpillars at night was 91% higher under ALAN than under ambient light. Additionally, there was a marginally significant effect of ALAN on feeding frequency during daytime; caterpillars under ALAN fed 58% more often than caterpillars under ambient light. In contrast, I found no effects of ALAN on habitat selection. In the performance experiment, there was no significant effect of ALAN on the developmental rate of the caterpillars. However, there was a marginally significant effect of ALAN on pupal weight such that the pupae of caterpillars exposed to ALAN weighed 8% more than the pupae of caterpillars exposed only to ambient light. The increased feeding at nighttime under ALAN could possibly be caused by disturbance to circadian rhythms of herbivory. A potential explanation for why this feeding carried over into daytime is that ALAN may have stimulated the production of hormones that promote feeding and remain in the body for a lengthy period of time. The increase of pupal weight may have been caused by the increased feeding frequency we observed in the behavioral experiments. This study is likely the first to show that ALAN increased the nighttime feeding of a diurnal insect. It is imperative to study any effects of human disturbances, like ALAN, on diurnal insects especially those like the monarch butterfly that are undergoing population declines.