I've been battling mealies and sometimes other pests since I started growing orchids. I have come to the conclusion that such pests are impossible to fully eradicate. Even if you were to kill every pest in your collection new pests will undoubtedly arrive shortly. Pests are introduced by a variety of means, some of which are beyond our ability to prevent.
Newly acquired plants frequently carry the seeds of an infestation in the form of larvae or immature adults. A plant can seem completely pest-free on the surface, only to harbor a nascent colony of nasties just below the surface of the media. The isolation of new plants may be beneficial, but I suspect that in this respect growers may talk a good game, but most of us fail to take effective action. It is difficult or even impossible for most growers to provide such isolation while providing adequate growing conditions. Few of us have the facilities for that.
I assume that all incoming plants have some level of infestation. This may be taken to imply that all commercial growers suffer from some level of infestation. This is probably a gross generalization, but who can say at any point in time just which facilities are currently pest-free? Better to err on the side of caution. Accordingly, I spray all new plants upon arrival. After flowering I always repot every new plant, dousing the entire plant (especially the roots, the incubation area for pests) in an Orthene WP spray.
In maintaining a collection it behooves the grower to become sensitive to even the subtle signs of the presence of pests. As a slipper grower I have become particularly adept at detecting the manifestations of a potential mealybug infestation. Mealies can be associated with the color white, that is, they are white, their nests are white and their immature young appear as white. If I see anything white on a plant, from an individual to a fuzz to even just a speck, I spray the affected plant immediately.
The key in dealing with pests is regular inspection of each plant. I'm not sure if this can be accomplished in a really large collection, but it would be beneficial. Greenhouse growers tend to treat their orchids collectively while home growers treat their plants individually. The greenhouse grower treats benches, while the home grower treats individual plants. Though the greenhouse approach saves time and energy, the second is far more effective in spotting problems on individual plants.
I maintain a spray bottle of pre-mixed Orthene at all times. Since insecticides degrade in the presence of light, I cover the spray bottle with black electrical tape. I use Orthene at about double the recommended strength with a spreader-sticker added. I have found that the recommended strength just isn't effective, and a larger dose is a sure killer.
I have heard that a plant should be well watered prior to spraying for pests. I take the opposite approach, that the medium is more likely to maintain residual or systemic effects if it is allowed to absorb the spray. It seems far more likely to do so if it is sprayed in a dry condition. I have seen no detrimental effects from doing so.
I don't think pests can be eradicated, but they can be controlled. Start by upping the insecticide dosage, and protect yourself at all times.
Regards,
Rick