When insecticides are used to suppress insects, they often are used in a curative manner. Used in this way, nothing is done until insect numbers become high, at which time a pesticide is applied.
Insecticides are capable of quickly suppressing high pest numbers, in many cases.
Fungicides, for control of plant diseases, are not used in the same way. They are used in a preventative manner because the fungal pathogen, once inside the plant, cannot be killed. Fungicides act by preventing infection, and hence must be applied before disease develops to high levels. Biocontrol agents differ from insecticides in that they must be used in a preventative manner, rather like the way we use fungicides. For example, whiteflies on poinsettias are easier to control if wasp releases are begun very early in the crop, at a time when whitefly numbers are low and do not yet present a damaging problem.