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.
Relative humidity and temperature in particular impact natural enemies a daily basis. Day length must also be considered since a number of natural enemies diapause under short day conditions. This can limit their use in winter or on crops that require short days to flower. If the program has been properly planned and backup resources are maintained, then many of these problems are unlikely to occur. Q. I have heard some presentations indicating that using biological controls are more expensive than traditional chemicals.