Tree-killing insects deserve a category all their own. Mountain pine beetles (Dendroctonus ponderosae), emerald ash borers (Agrilus planipennis), gypsy moths (Lymantria dispar) and more are devastating the world's forests, which is why scientists spend serious time studying their behavior and their biology. In 2012, the emerald ash borer, a beetle not quite the size of a penny, took Connecticut by surprise, drilling holes and laying eggs in the state's ash trees. The beetles killed thousands of trees as they ate through them during each stage of larval development [source: DEEP].
Over the years, the gypsy moth has defoliated millions of arches of hardwoods in the East. In California, the tiny polyphagous shot hole borer (one of the ambrosia beetles) has resisted all eradication efforts. They don't eat the wood of the tree. Instead, they drill tunnels and then deposit fungal spores, which they later harvest to feed their larvae. The fungus spreads, killing the tree. It's tough to kill the shot hole borer with pesticides because the tree shields the insect from the deadly chemical. Scientists even studied whether sex pheromones could lure them out [source: Khan].
Most studies of these and other tree-killing pests center on eradication. Researchers at Northern Arizona University, for example, looked at how sounds could stop armies of bark beetles that have devastated pines in the West. Although Queen, Guns N' Roses and the voice of Rush Limbaugh played backward did little to stop the beetles, recordings of the beetle's own sounds did the trick. Once they heard their own noise, the beetles stopped mating, burrowing and feeding. A few fled. Some killed each other [source: McKinnon].