Fungus gnats are very common but have never caused a problem. Ants love to invade a mealybug culture if they can find a way into a cage and they usually do. I have been very pleased using a pesticide called hydramethylnon for ant control. The best formulation for me has been Combat Ant Killing Granules. It is slow to solve ant invasions but will provide a very long period of control.
Parasite vigor can decline in artificial conditions used to raise parasites.
Eventually an artificial environment will select for those best suited for such an environment. When these parasites are released into a greenhouse or the outdoors the environment may be very different. Different temperatures and lack of an easy source of water could lead to a short life. The parasite may also not be effective at finding a host because it was reared in a cage where it was easy to find a host. To solve this problem I occasionally restart a new culture from greenhouse collected parasites.