Light pollution influences web placement in grass spiders (Agelenopsis spp.) with implications for insect prey populations

Approximately one-third of insects attracted to artificial light at night (ALAN) die from exhaustion or predation by sunrise. Insectivorous vertebrates hunt near artificial light to catch more prey, but few studies have tested this in invertebrate predators. We tested how ALAN and prey affect web placement in grass spiders (Agelenopsis). We kept spiders in enclosures with overhead lights on a 12:12h light: dark cycle. A second light source outside one corner either stayed on (light treatment) or off (control) all night, and we compared web placement between treatments. We then added prey in a separate unlit corner and repeated the experiment. Without prey, web placement was random for control (N = 11, k = 3, p = 1), but not light treatment (N = 10, k = 9, p < 0.001) spiders; light treatment spiders made webs significantly closer to the light (χ²_1,36 = 7.259, p = 0.007). When prey was present, control spiders web placement was no longer random (G_2,11 = 6.996, p = 0.030), and light treatment spiders built webs significantly closer to the light (χ²_1,48 = 23.312, p < 0.001) and farther from prey (χ²_1,48 = 10.491, p = 0.001) than control spiders. Agelenopsis prioritized ALAN, which, outside, likely draws them to lit, prey-dense areas. Recent reports, however, show evolved reductions in flight-to-light behavior in urban insects. Therefore, ALAN could become an ecological trap for spiders if fewer insects approach light, but spiders prioritize light over prey.