I always repot and inspect plants carefully before moving them into the collection. The species of mealybugs that are dominant in your greenhouse may change with the seasons depending on moisture and temperature. Monitor these changes and evaluate the effectiveness of your control measures at different times. Only adult male mealybugs fly, but the young nymphs will walk to find suitable food. Ants will sometimes bring them back into the greenhouse once you have eliminated them.
Outside vegetation can harbor mealybugs.
I have found them living happily in ivy growing on the side of the house in midwinter. I have been killing and removing the ivy, both to eliminate an ideal mealybug nursery and to stop damage to the house. Mealybugs have developed some resistance to common greenhouse chemicals, so the use of biological controls is especially important. I used to hand pick and kill mealy bugs with rubbing alcohol, but it often caused scaring on the growing tips and could be fatal to seedlings. Biological controls have so far been the safest and most effective approach in dealing with mealybugs.