Trees get diseases just like people. They don't catch colds, but they get fungus, bacteria, cankers and rot. Bugs attack them too. Parasitic plants - plants that don't make their own food - sponge off trees. Some trees die of old age. Diseases, insects, and even the parasitic plants are part of the forest ecosystem. "Forest pathologists" are people who study how bugs and diseases affect trees.

Insect populations are controlled by natural forces like predators and weather. Most of the time the damage they do isn't noticeable. They look for trees weakened by fire, drought or old age. If the conditions are right, wham, they overtake a weak tree. Neighboring trees can get hit too. If there's a lot of weak trees, and a lot of insects, guess what happens to the forest?

Bark beetles are small but really destructive. Some are smaller than the tip of a pencil! Bark beetles bore through the bark. They lay eggs just under the bark. When the eggs hatch, the larvae eat the thin cambium layer (the only layer that makes new cells for the tree.) The larvae are tiny and white with no legs and BIG jaws. (Better to eat wood with!) Thousands of beetles gang up to conquer a tree.

How about those parasitic plants that sponge off trees? One parasite is dwarf mistletoe. It's a small leafless plant. It deforms branches and weakens conifer trees. Mistletoe makes the trees grow a cluster of branches where it's infected. They're called "witches brooms." Dwarf mistletoe attacks a tree in a really sneaky way. First, a sticky seed explodes from water pressure built up in the fruit. It lands on some needles. When the seed gets wet, it slides onto a branch and begins to germinate. The roots grow into the branch and start spreading. The roots grow and spread inside the tree, sucking its energy. Sort of like an alien taking over a body.