Caterpillar Infestation on Helvellyn
If you look at the photograph of the peak of Helvellyn below, you'll notice that it's awfully brown. On our ascent of the fell, we noticed hundreds of small caterpillars along the path. It turns out that Helvellyn is in the midst of a periodic large-scale infestation of antler moth caterpillars. The moths lay their eggs on the fells, and every eight to ten years there is a large-scale emergence of caterpillars, which strip the grass and leave the fells brown and bare.
BBC: "Caterpillars devour Lakes grass"