Insects that damage plants feed and reproduce very well in a greenhouse environment. If not controlled they can multiply rapidly and significantly damage a plant that may become no longer sell able. Typically, the bad bugs are sprayed with pesticides, however a few will always manage to survive, either by avoiding the spray (by hiding in the cryptic structures of the plant, protected by special wax on their skin, or by a dense canopy) or by developing resistance to chemicals.
Since we have been using a biological control, and the good bugs can search tirelessly for pest even in their hiding places. Predators eat the bad bugs and parasitoids lay eggs and reproduce inside the pest. They are hard at work and because they feed and reproduce on the pest, they can increase in numbers when their populations reach high levels, they can completely eliminate the pests. Bad bugs do not develop resistance to good bugs, so they are always at risk of being killed by their biocontrol agents.
Beneficials are sometimes so small, that a magnifying glass is needed to find them, however you may notice on leaves some granular materials like vermiculite or bran, that is the carrier the beneficial insects are shipped in.