Biological control does not have to be with an introduced species. It can also be accomplished by either artificially inflating the number of existing predators.
E.g. Spruce bud worm has a natural predator in the form of a tiny wasp. But budworm can spread through a stand faster than the wasp can. By moving popluatins of the wasp to the forefront of the budworm advance, you get most of the 2nd generation budworms. The trees have a bad needle year but aren't killed.
It can also be done with habitat modification. If you create conditions that favour the existing predator, then there are more predators in place. E.g. create places for ladybug overwintering to reduce aphid populations. (I have no idea if this would work.)
Anohter example -- although ecological controll may be better than biological -- I had a significant mosquito problem. Lots of small puddles in spring. I created a permanent pond. This has resulted in much smaller mosquito population.
- The pond is a local year round water supply for insectivorous birds.
- The pond is by far the largest water body around, so tends to get most of the mosquito eggs.
- The pond now has a permanent population of boreal and chorus frogs. Tadpoles of these make it tough to be a mosquito larvae.
Is this species introduction? Only if you consider immigrant species from neighborin populations to be introduction.