The effect of artificial light at night on the biomass of caterpillars feeding in urban tree canopies

Alternation of day and night is the oldest cycle on Earth, which is increasingly disturbed by the accelerating rate of urbanization and technological development. Despite the ubiquity of light pollution in cities, many aspects of its influence on urban ecosystems are still poorly understood. Here we studied the effect of artificial light at night (ALAN) on the biomass of arboreal caterpillar populations, which are a major component of the diet of many insectivorous animals. We predicted that increasing ALAN intensity is associated with reduced caterpillar biomass, because ALAN may increase predation risk for both caterpillars and adult lepidopterans (i.e. moths), and can also hinder the moths' reproductive rate. We estimated caterpillar biomass from frass samples (n = 3061) collected from 36 focal trees in two cities in Hungary during four consecutive years. To quantify ALAN we measured light intensity during night at each focal tree (range of illumination: 0.69-3.18 lx). We found that caterpillar biomass of individual trees was repeatable over the four years. This temporal consistency in prey biomass production may be important for birds because it can help predict territory quality, especially in cities where caterpillar abundance is generally low. Our results did not support the negative effect of ALAN on urban caterpillar populations, because ALAN intensity was not related to caterpillar biomass, and this lack of effect was consistent between study sites and tree species. We suggest that the effect of ALAN on urban caterpillar biomass is either weak and thus can be masked by other, local environmental factors, or light pollution may have antagonistic effects acting during different stages of the lepidopteran life cycle. Another explanation could be that even the lower levels of our sites' public lighting are strong enough to cause serious detrimental effects for caterpillars, resulting in their uniformly low biomass.