Answer: Grenhouses are so much fun -- until there's a problem!
Tossing badly infested plants is one of the best ways to control an infestation, as is emptying the greenhouse and starting over. In addition, a bleach and water solution and careful scrubbing should help and is definitely a good idea in any case, but this is primarily aimed at general disinfection and algae, fungus and molds rather than insects. (Surfaces should be wet for at least ten minutes, do not apply to plants!) You may also need to pay special attention to the floor if it is made of gravel rather than a hard surface. In my experience it has also helped to leave an unheated empty greenhouse wide open in midwinter to freeze any remaining adult pests.
Unfortunately, the pests may re-enter the greenhouse once it is back in use. They may hitch-hike in with new plants or infiltrate on their own. Aphids, for instance, overwinter outdoors in the egg stage and are certainly present outdoors every year -- you might consider putting screens on your greenhouse if they have been a truly severe problem in the past. Weed control in the area near the greenhouse can also help.
Once the greenhouse is reopened, check all new plants and quarantine them before bringing them in. Ongoing operations may also need some modification if the pests reappear. For example, plants which are oversupplied with nitrogen and are growing very soft lush new growth tend to be attacked by aphids and mealybugs both, so if this is the case, you may need to cut back on your fertilization schedule.
The key to success is careful, even daily, monitoring. This is critical for catching pests before their numbers escalate so far out of control. If you catch them early, indentify them, and understand their life cycle then you can use least toxic control methods and avoid losing plants. Good luck with your greenhouse!
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