The southern pine bark beetle lives in the bark of pine trees. The beetles feed on the tree and live within the bark of the tree, producing offspring. Once the beetles have infested a tree, they will spread to other trees in the same area. Eventually a pine tree infested by the beetle will die. The population of pine bark beetles can be expected to increase in a new habitat where it is introduced. The population would NOT be expected to decrease until what happens?

The beetle population would NOT be expected to decrease until pine population decreases in size.

Explanation:

The southern pine bark beetle is responsible for some of the most important pine forests loss in the last forty years. The insect identifies a pine of a certain advanced age and digs a hole in the bark to establish in places were sugars and starch circulate. There it lays its eggs and feeds. When larvae feed, they introduce a fungus that interrupts water and nutrients circulation between roots and leaves, causing the pine death. When the pine is completely infested or is dead, the insects move to another pine, dispersing and increasing the population size.

As the beetle population size increases, the pine population decreases, after being killed by the insects. When a pine dies, it not only stops producing food for the beetles but it also becomes susceptible to being inhabited by other species that might be predators of these beetles.

The beetles' population starts to decrease when most of the pines in the forest are dying. They hardly have available food and they are more exposed to being eaten by their predators.  

The beetle population would NOT be expected to decrease until the pine population decreases in size.  

It is the cycle of predator-prey, where predators population affect the dinatic of prey population, and viceversa. Populations sizes increase and decrease in these cycles.  Herbivory is a kind of predation, in which the prey is a plant. In this example, the predator is the beetle and the prey in the pine.