There are many, many non-native trouble-making species in this world. They come in all shapes and sizes, but they share in common several features. They are introduced (accidentally or deliberately) by humans, spread like wildfire, cause economic and/or environmental damage, and require money to manage. A species that fills the above criteria is considered to be a non-native invasive species.
Among the brute force methods (i.e. burning, cutting, hunting, trapping, removing vegetation, applying pesticides), we have also increasingly been introducing additional non-native species as a method of controlling the invader. These are predators or parasites (or parasitoids) of the invader species, and they are called “biocontrol agents”.
All in all, we have been underwhelmingly successful in the management of invaders. I’ve discussed the horror of the cane toad in Australia, and there are many other failed examples.
The lady bird beetle (or lady bug, Coccinellidae) was used as early as the late 1800′s as a biocontrol agent of scale insects in citrus crops, and later as a biocontrol agent for mealy-bug infestations.
The ladybird lost some of its popularity recently, when the introduced Japanese ladybird beetle started infesting homes in late summer in the northern hemisphere.
My lab studies invasive thistles, for which several biocontrol agents have been introduced. There is the rosette-boring weevil Trichosirocalus horridus (also, what a great name, right?):
The thistle tortoise beetle:
And the other weevil Rhinocyllus conicus, which lays its eggs on the flower head. Its larvae bore into the capitulum (flower head) and destroy seeds:
These biocontrol agents vary in effectiveness, but none has successfully suppressed the spread of the thistles. Introducing these organisms also brings about the risk that they will start attacking native thistles (which they have).
So biocontrol is a tricky subject. Although it has had some success in the past, it is extremely difficult to predict the impact of introducing a species, even one which you believe will help control a pest.