A  In a word: nothing. It is no more recommended to use prophylactic sprays on orchids than it is to use antibiotics regularly just in case you are exposed to a disease — and for the same reason. All this does is tend to breed more-resistant insects and pathogens. It is far better to keep a close eye on your plants and spray when necessary with appropriate chemicals at the recommended dose and spray intervals. The only exception to this may be in areas, such as yours, with unusually high humid summers, where a prophylactic spraying of fungicide may be indicated. I would recommend that you check with some local growers (the Houston Orchid Society is a fantastic group with many experienced growers) to see what they do. —NedNash.