Screen the area you intend to use. Carefully follow storage and release instructions when you receive the natural enemies. There are many reasons why biological control may fail. Pesticide residues that can have an impact on beneficials for up to several months. Introduction rates of natural enemies which are too low or too infrequent to begin with.
Pest numbers are too high at the start of the program.
This used to be one of the most difficult aspects of starting biocontrol. Use of many traditional pesticides had to be discontinued well in advance of introducing natural enemies and there weren't as many less persistent pesticides available as there are today. It is important to remember that pest numbers should be as low as possible when starting. The wrong natural enemy may be used. Environmental conditions play an important role in the establishment of many natural enemies.