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Greenhouse Pest Control

Monitoring for pests is essential. Yellow sticky cards distributed throughout the greenhouse will provide a good early warning for many pests. Blue sticky cards are sometimes recommended for thrips but we found thrips harder to count than on yellow cards.  Card placement is important.  Thrips and whiteflies tend to be caught near the plant canopy. Fungus gnats are best caught near the soil surface. Sticky cards do not work well for monitoring aphids, worms, and spider mites.  Adult aphids usually have wings and migrate only when looking for new food sources.  It only takes a few moths to lay hundreds of eggs that later turn into tomato worms.  Spider mites do not have wings, and although they may disperse on wind currents they require good magnification to identify on cards.   Therefore, be alert for any signs of pests on the leaves, such as frass (droppings) from worms, white cast skins from molting aphid immatures that resemble whiteflies, sik and stipled leaves from mites, and curled leaves from pinworms. It is ususally important to introduce natural enemies as soon as pests appear. In some cases their appearance can be anticipated as the outside weather warms in spring.

Monitoring Small sticky cards (3" by 5") should be distributed throughout the greenhouse, both near the soil and near the plant canopy. This allows identification of problem areas, and initial infestations. The yellow color is attractive to many insect pests, possibly because if indicates a plant under stress. The blue color is said to be more attractive to thrips. However, we have not found any difference and it is much easier to count thrips on a yellow background.

Biological Controls

There are many suppliers of natural enemies but only a few producers. European greenhouses were the first to start using biologicals and thus have had a headstart in research and production. Many natural enemies provided by North American suppliers originated in European rearing facilities and were shipped overnight once an ordered. Although these usually arrive in good shape you should be aware that not all European reared insect species seen on some web sites are available for use in North America. Many US based suppliers are more than willing to assist growers with problems and often provide excellent educational web sites, some of which we list. It is not the purpose of our site to reproduce this material but rather to provide information on what we found to work best in our area. Researchers are always searching for new and better natural enemies. A very promising recent addition is a rove beetle that is a generalist predator within the soil or potting media.

A small hand lens comes in handy. This one is 20X but lower power will also work.

Chemical Controls

Natural pesticides are available for greenhouse use. Natural pyrethrum causes rapid knockdown and mortality although many pests have developed some degree of resistance. Many products of natural pyrethrum also contain a synthetic compound to enhance efficacy. Neem is extracted from the fruit and seed of an Asian tree and has multiple modes of action. It is an antifeedant (tastes bad), blocks the molting hormones, causes mortality, and reduces fecundity (birth control). It is more compatible with natural enemies than synthetic pesticides but must be applied regularly and takes a few weeks to show results. Bacillus thuringiensis products are effective against fungus gnats (Gnatrol®) and early instars of fruitworms (Dipel® and others) . These products are derived from various strains of a bacteria that contains a protein crystal that is toxic in insect guts. In the highly alkaline gut of an insect it dissociates and becomes active, disrupting the gut lining and allowing pathogenic bacteria to enter. These products are natural products that can be used in organic production systems. The different strains provide good specificity and thus they are compatible with use of natural enemies. Various oils and soaps can be effective against softbodied insects, including aphids and whiteflys. Oils suffocate eggs and disrupt membranes. Soaps kill by removing the outer waxy cuticle and allowing the insect to dessicate. Although many types of oils and soaps work, only those derived from natural plant sources are considered organic. Natural enemies that are not soft-bodied are compatible to a degree with use of soaps and oils.

Synthetic Pesticides. Most insect pests found in vegetable greenhouses are also found in ornamental greenhouses where pesticides are heavily used. This heavy exposure to pesticides has been associated with resistance to many types of pesticides. Because of the higher standards required to label a pesticide for food use, the number of pesticides available for vegetable greenhouses are few. The older pesticides such as Malathion are not very effective against whiteflies or thrips. Newer pesticides with different modes of action and more specificity have become available for field grown vegetables. Rarely, there are allowances for vegetable greenhouses. A few insect growth regulators have been registered for special use. Talus® (Buprofezin) inhibits development of insects chiten exoskeleton and has been registered for control of whiteflies on greenhouse tomatoes. Distance® (pyriproxyfen), a juvenile hormone mimic, prevents molting and has been granted special local needs registrations (whiteflies, fungus gnats) for indoor tomatoes in Colorado, Texas and Virginia. The signal word on both products is "caution" and re-entry times are 12 hours.

Reliance on pesticides to control pests in vegetable greenhouses is risky. Greenhouse use of pesticides is more restricted than field use, partially because of the longer times pesticides often take to degrade under greenhouse conditions. By the time new pesticides become labelled for use on greenhouse vegetables it is likely resistance will already have started to develop from heavy use in ornatmental greenhouses. Natural enemies make a more attractive long-term solution.