If you take a hose inside your greenhouse, and gently spray down the plants, maybe once a week, that can wash off a lot of aphids. It will also humidify the air for you, which is probably a good thing where you are?
A mild soapy water solution, about 1/2tsp. of dish soap to a quart of water in a spray bottle is also a good preventive measure. If you spray that on the whole plant (be sure to get the undersides of the leaves as well as the tops) on a regular basis, it will kill most sucking insects like aphids, whiteflies and spider mites.
