Controlling

Good plant health, through good nutrition and environmental control, is the first line of defense against both pests and diseases in the greenhouse. But even in the best circumstances, some pests may bother your plants. Aphids, mites, and whiteflies are the most common serious insect problems. Use mechanical controls such as vacuuming, squashing, and washing at the first sign of trouble. Yellow sticky traps are effective against whiteflies. If pest populations continue to grow, soap or pepper sprays or appropriate botanical controls such as neem may be useful.

Biological control with predatory insects such as green lacewings and ladybugs, and parasites such as Encarsia formosa are extremely effective. To meet the needs of these insect allies, set up a small “biological island” in a warm, bright spot with pots of parsley-family members (Umbelliferae), such as chervil and dill, and smallflowered ornamentals, such as scented geraniums, lobelias, and salvias. Kept in bloom for the entire greenhouse season, these plants provide nectar and pollen for the beneficial insects.

Tempting as it is to bring outdoor peppers, eggplants, and herbs into the greenhouse at summer’s end, you’ll be running the risk of importing pests with them. It’s far better to start plants or buy transplants expressly for the greenhouse. If you decide to take the risk of bringing plants in, quarantine the plants inside sacks made of tightly woven translucent material for at least 7 to 10 days. Aphids and damage from such pests as mites and thrips will be easier to see after this time. If problems appear, it’s generally best to throw the plant out.

Fungal diseases are usually the greatest disease problems in a greenhouse. Providing adequate ventilation and spacing between plants and monitoring humidity levels are the best ways to prevent fungal problems. Preventive sprays can also help minimize disease incidence. Fermented nettle tea and dilute compost tea, sprayed on leaves at weekly intervals from the seedling stage onward, inhibit many diseases while also providing trace elements. Sanitation is important, too; isolate or dispose of sick plants, and clean up spilled soil and dropped leaves in the aisles and under benches.  

The warm humid conditions and abundant food in the greenhouse are ideal for pest build up. Problems can be chronic unless recognized and corrected. While insecticides are important tools, successful control of greenhouse vegetables pests relies primarily on cultural factors. Proper cultural practices can minimize the chance for initiation and build up of infestations. Early detection and diagnosis are key to greenhouse pest management, as well as, the proper choice and application of pesticides when pest outbreaks occur.

Cultural Controls

Pests may enter the greenhouse in the summer when the ventilators are open. Others may be brought into the greenhouse on new plant material or in soil. Many are able to survive short periods of time between harvest or plant removal and production of the next crop. Cultural controls are the primary defense against infestation.

Proper cultural practices which will help prevent pest infestations include:

• Maintain a clean, closely mowed area around the greenhouse to reduce pests that develop in rank growth.
• Remove all plants and any plant debris, clean the greenhouse thoroughly after each production cycle.
• Keep doors, screens and ventilators in good repair.
• Use clean or sterile soils or ground media, tools, flats and other equipment.
• At the conclusion of the season remove all plants and any plant debris, clean greenhouse thoroughly possibly fumigate.
• Inspect new plants thoroughly to prevent introduction insect or disease infested material into the greenhouse.
• Watch for leaks or pooled water that can lead to fungus gnat infestations.
• If possible allow the greenhouse to freeze in winter to eliminate tender insects like whiteflies.
• Avoid wearing yellow clothing which is attractive to many insect pests.
• Eliminate infestations by discarding or removing heavily infested material.

Monitoring

Early detection and diagnosis of pest infestations will allow you to make pest control decisions before the problem gets out of hand. It is good practice, therefore, to make weekly inspections of plants in all sections of the greenhouse.

Insect monitoring devices are also available. Yellow stick cards (PT Insect Monitoring & Trapping System, Whitmire) are highly attractive to winged aphids, leafminer adults, whiteflies, leafhoppers, thrips (blue cards can also be used with thrips), various flies and other insects. These can be used to alert you to the presence of a pest and identify hot spots in the greenhouse. One to three cards per 1000 sq. ft. in the greenhouse is recommended and should be changed weekly. If you cannot identify a trapped insect, contact your county extension agent for assistance. Mass trapping products such as sticky tapes are also available for thrips, whitefly, leafminer and fungus gnat detection and management.