Leaf beetles damage willow trees by stripping away their leaves, but a combination of parasites and predators generally keeps populations of these beetles in check  Researchers have found that severe air pollution results in reduced predator populations
The parasites, by contrast, are not adversely affected by pollution; nevertheless, the researchers' discovery probably does explain why leaf beetles cause particularly severe damage to willows in areas with severe air pollution, since
where air pollution is not especially severe, predators have much more impact on leaf-beetle populations than parasites do