The forests of Denver, Colo., are currently under attack by an invasive insect species from Asia. So what is the scientists’ plan to stop this assault on trees? They’re going with the controversial move of introducing a second invasive species to destroy the first one. If it sounds like that children’s song about the old lady who swallowed the spider to catch the fly, that’s because it basically is. Unleashing a second non-native species might help to eradicate the first species, but it can also unleash a series of other consequences. Previous experiments in this invasive species vs. invasive species tactic have worked out with various degrees of success, as these four examples will show.