The best way to avoid pest problems in a greenhouse is to keep them out to begin with. The list that follows gives many ways to help keep a greenhouse pest free. The more of these that can be integrate into greenhouse gardening practices, the better chance a gardener will have of winning the war against undesirable garden pests.
Start Plants from Seed
If plants are bought at a nursery or a garden center, one can not be assured that the plants are perfectly clean. If one has been getting plants from a reputable producer and has not had problems in the past, it would be a good idea to stick with that grower, even if the prices are higher. Treated seeds are safer for starting your greenhouse plants. Untreated seeds are more likely to carry a seed-borne bacterial or fungal disease.
Repot Plants Outside the Greenhouse
Repotting plants should be done outside of the greenhouse, and any used pots should be cleaned and disinfected with a 10% bleach solution before use. Commercially available soilless mix should be used as the media for seed starting and potting greenhouse plants. This will allow one to avoid introducing insect and microbial pests that often live in soil.
Protect the Work Area
Protect the ground on the floor of the greenhouse with a barrier to keep soil born pests from digging their way in from the outside. Work in the greenhouse first before working in the outside garden. Outside plants should not be kept near the greenhouse door. These plants can be a safe harbor for bugs waiting for a chance to get in the greenhouse.
Hands should always be washed before going into the greenhouse. This is particularly important after working with plants, or produce in the kitchen.
If one has been in close contact with plants, grass or dirt/mud, a change of clothing may be in order before entering the greenhouse. If one has been walking through grass or mud, it is a good idea to remove footwear, before entering your greenhouse. If one will be walking in the woods or a wooded area under trees, or even just walking on dirt paths, try to do it after working in the greenhouse.
Consider possible contamination by visitors to the greenhouse. Visitors should not enter the greenhouse after being in another greenhouse, a garden or an agricultural field.
Clean the Tools
Insects, mites or diseases can be taken into the greenhouse on garden tools that have been used outside. Tools should be thoroughly washed and disinfected with a 10% bleach solution before bringing them in the greenhouse and in between working on separate plants.
Items that have been exposed to plants or produce are a source of contaminants. Used plant shipping boxes and produce shipping boxes may be very useful, but they should never be taken into the greenhouse.
No Pets Allowed
Dogs and cats, that live or spend time outdoors, should never be allowed in the greenhouse.
A Few More Things
Screen air intakes to the greenhouse with a very fine mesh. The screen area should be at least five times the area of the greenhouse air intake, as to not restrict airflow.
Consider if a double door is possible. This is particularly helpful in keeping moths and butterflies out. Moths and butterflies are not generally a problem themselves, but when they lay their eggs on your plants, they will soon hatch caterpillars and start to eat their hosts. On your daily bases remove any of them that are present.
Sticky fly traps can help in early detection of some flying and crawling pests.
Inspect plants as often as possible for visual predators or damage caused by harmful pests, fungus, bacteria or disease.