Managing Pests in Small Greenhouses

- Greenhouses are great places for plant pests, too! -

Summary: Greenhouses create very favorable growing conditions for both plants and for plant pests like aphids, spider mites and fungus gnats. Pests can increase rapidly and become extremely destructive. Learn ways to combat these insect and mite pests using least-toxic, often even certified-organic, methods.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Greenhouses protect plants from extremes of temperature and humidity, some provide supplemental lighting, and a few even employ specialized irrigation systems. However, these same ideal growing conditions also allow the insects and mites that can damage greenhouse plants to thrive. In addition, greenhouses screen plant pests from regulation by natural enemies that would otherwise keep them in check. Pests and pest damage can therefore grow rapidly along with the greenhouse plants.

Plants growing in greenhouses have a number of additional pest problems that plants growing outdoors generally avoid. Chief among these are certain plant diseases, as well as insect pests like aphids, fungus gnats, thrips, and, perhaps the most damaging of all, spider mites. Fortunately most of these pests can be managed using low toxicity, often certified-organic, methods if problems are caught early.

fungus gnat

an adult fungus gnat - an important but often overlooked greenhouse pest

Steps to better greenhouse pest control

(1) Scout for pests at least weekly starting as soon as plants begin to grow. You don't need to inspect every plant but you should develop a systematic plan to thoroughly inspect a few plants from different areas and record any "suspicious activity". Better yet, take a photo of anything that looks like it might be a problem for later reference.

(2) Restore the natural enemies to the system by releasing specific predators and parasites against your primary pests, this is called augmentative biological control. The exact type of natural enemy you need will depend on your particular mix of plants so be sure to do a little homework and check package instructions carefully. An excellent, in depth resource is this manual from Penn State University.

(3) The next step is to use least-toxic, biorational insecticides to clean up hot spots without disrupting the natural enemy system you deployed in step 2. The best choices in this category are insecticidal soap and/or products that contain neem oil. Both materials are certified organic on many crops but be sure to check your particular crop in this regard before use.  Both will control most soft-bodied pests like aphids, thrips, spider mites and scale crawlers and are virtually non-toxic to you, your plants and any beneficial insects/mites that you release, if used properly.

Suppliers of:

Natural enemies for augmentative biological control - Rincon-Vitova

Biorational insecticides, Traps, Spray Equipment -, our affiliate.

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