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Greenhouse Insect Pests
- Greenhouses are a great place for pests, too! -
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Summary: In addition to creating favorable growing conditions, greenhouses separate plant pests from their natural enemies. This is why pests increase so rapidly and are so destructive in greenhouses. Learn ways to combat these pests in home, or hobby, greenhouses.
Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)
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 Hobby greenhouses have become popular additions to many home gardens. These enclosures provide a means to start plants earlier in the year thus extending the growing season. Greenhouses also are great places to overwinter plants and during the growing season they provide increased humidty or shade.
Metal structures with polycarbonate glazing are the easiest to construct but redwood framing and glass glazing are the classic materials. Redwood, or cedar, and glass greenhouses require considerably more maintenance.
Plants growing in greenhouses have a number of pest problems, however, that plants growing outdoors generally avoid. Chief among these are aphids, spider mites, whiteflies, scale insects, thrips and sometimes fungus gnats. See our control suggestions below.
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| fungus gnat - an important greenhouse pest |
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| Insect and mite pests are often more severe in greenhouses than in gardens because these enclosures inadvertantly separate pests from the natural enemies that would normally keep them under biological control. Greenhouse also create favorable environmental conditions for pests as well as plants.
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Greenhouse pest control
To counter the loss of natural enemies we can artificially release certain predators such as lacewings, lady beetles and syrphid flies into greenhouses. This procedure is called augmentative biological control (see below).
Second, there are biorational insecticides available for use in home greenhouses. Your main insecticide should be insecticidal soap. Insecticidal soap will control most soft-bodied pests and is virtually non-toxic to you, your plants and any beneficials that you release, if used properly.
Last, the key to greenhouse pest control is regular (daily, if possible) monitoring and prompt treatment (see below) when pests are detected. Aphids and spider mites, in particular, can increase to damaging levels very quickly if not detected early.
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How to control ...
Aphids - Lacewings [more information]** and lady beetles can be released into greenhouses. Insecticidal soap is an effective spray and will not interfere with predators.
Spider Mites & Thrips - Spray leaves with insecticidal soap. Be sure to treat the undersides of leaves as this is where spider mites often feed.
Whiteflies - Release the parasitic wasp called Encarsia [more information]**.
Scale Insects - Use insecticidal soap to treat the crawler stage.
Fungus Gnats - Treat potting soil with the bioinsecticide Gnatrol. Yellow sticky card traps can be used to monitor for fungus gnats and thrips. Plan to treat with Gnatrol at least three times, with treatments about 1 week apart.
** use the search box in the upper left corner at Gardens Alive! to find more information about these products.
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