Walking along the edge of Rockcliffe Park I noticed a large purple loosestrife plant riddled with holes in the leaves. Checking more closely I discovered it was covered with dozens of these Galerucella beetles which were introduced to NA from Europe to control this invasive species.

In reply to Carl's comment (which is one that many people have about this introduced biocontrol species): The theory is that these beetles feed only on purple loosestrife and once the loosestrife has vanished, they will die off rather than attack other plants. It is difficult to imagine loosestrife ever vanishing and actually I don't think it will. I think the most we will see is a reduction in its production.