Greenhouses create warm, humid, sheltered environments that are ideal for growing plants, and equally ideal for the insects and mites that feed on them. The most common greenhouse pests include whiteflies, aphids, thrips, fungus gnats, spider mites, and scale insects. Globally, plant pests and diseases destroy up to 40 percent of crop production each year, costing the global economy over $220 billion. In a greenhouse, where populations can explode without natural predators or weather to keep them in check, catching problems early makes all the difference.
The Most Common Greenhouse Pests
A handful of pest species cause the vast majority of greenhouse damage. Each one feeds differently and leaves distinct signs on your plants.
Aphids are small, soft-bodied insects that cluster on new growth, stems, and the undersides of leaves. They suck plant sap and excrete a sticky residue called honeydew, which attracts sooty mold. Heavy infestations cause leaves and shoots to curl and become distorted.
Whiteflies look like tiny white moths and scatter into the air when you disturb an infested plant. Like aphids, they feed on sap and produce honeydew. They reproduce rapidly in greenhouse conditions and can weaken plants to the point of stunted growth and leaf yellowing.
Thrips are slender, barely visible insects that scrape plant cells and feed on the contents. They leave silvery feeding scars on leaves and petals, and heavy feeding causes distortion of flowers and new growth. Thrips also transmit plant viruses, making even small populations a serious concern. Brown, necrotic spotting near white feeding scars is a telltale sign.
Spider mites thrive in hot, dry greenhouse conditions. They feed on the undersides of leaves, producing a fine stippling pattern that eventually turns leaves bronze or yellow. Fine webbing between leaves and stems signals a well-established infestation.
Fungus gnats are small, dark flies that hover around the soil surface. The adults are mostly a nuisance, but the larvae feed on roots and organic matter in the growing medium, which can damage seedlings and young plants. You can check for larvae by pressing a fresh potato disk (about an inch across and half an inch thick) into the soil surface. After two days, flip it over: fungus gnat larvae will be visible on the underside.
Scale insects attach themselves to stems and leaves and feed on sap beneath a protective waxy covering. Armored scales can cause dieback of entire plant parts and are easy to overlook because they don’t move once they’ve settled.
Caterpillars and leafminers are less common indoors but can arrive on incoming plant material. Caterpillars chew visible holes in leaves and stems, while leafminer larvae tunnel between the upper and lower surfaces of leaves, creating winding, translucent trails.
How Pests Get Into Greenhouses
The most common entry point is new plant material. Pests or their eggs hitchhike on purchased plants, cuttings, and even soil. This is why inspecting and quarantining incoming stock before moving it into production areas is one of the most effective prevention steps you can take. Isolate new plants in a separate area for at least a week and look carefully at leaf undersides, stems, and the soil surface.
Pests also fly or crawl in through doors, vents, and gaps in the structure. Installing fine mesh screening over vents and intake openings blocks thrips, aphids, whiteflies, and even larger pests like tarnished plant bugs. Some growers have reported reducing pesticide use by 90 percent after installing proper screening. The trade-off is reduced airflow: fine mesh restricts ventilation, which can lead to overheating. Increasing the screened area by building a frame or enclosure over the vent opening compensates for the restriction. Positive air pressure systems, like swamp coolers installed around the greenhouse perimeter, create outward airflow at openings that physically resists pest entry.
Monitoring Before It Becomes a Problem
Regular scouting is the backbone of greenhouse pest management. The goal is to catch pests at low numbers, before populations grow large enough to cause visible damage. For small greenhouses under 4,000 square feet, you can scout the entire space as one unit. Larger operations should be divided into sections of 2,000 to 3,000 square feet. Plan to spend 5 to 10 minutes per 1,000 square feet, inspecting at least 20 randomly chosen plants each time. Propagation areas, where young plants are most vulnerable, should be scouted every 3 to 4 days.
Yellow sticky traps are a standard monitoring tool. Use bright yellow cards, at least 3 by 5 inches, coated with sticky material. Place them in a grid pattern throughout the greenhouse, positioning the cards 4 to 6 inches above the crop canopy. For general monitoring, one card per 10,000 square feet is a minimum. For whitefly-susceptible crops, increase to one trap per 1,000 square feet. Always place additional traps near doors and vents, since these are the entry points where you’ll first detect pests migrating in from outside. Change the cards weekly and keep a record of what you find.
Sticky traps are useful for tracking trends and detecting new arrivals, but they aren’t reliable for deciding whether to treat. Always combine trap counts with direct visual inspection of plants for feeding damage and live pests.
Prevention Through Sanitation and Culture
Between crop cycles, clear the greenhouse of all plant debris, weeds, old growing media, flats, and tools. Weeds and leftover “pet plants” under benches harbor pest populations that will quickly colonize your next crop. Fix drainage problems, since standing water encourages fungus gnats and creates conditions for disease.
Use footbaths with disinfectant between greenhouse compartments, particularly between propagation and stock plant areas. Remove dead and dying plants immediately, placing them in sealed garbage containers rather than leaving them under benches. Dedicate specific wheelbarrows or carts for hauling diseased material so contaminated equipment isn’t used for other tasks. Employees handling infested plants should wear disposable gloves or wash hands thoroughly with soap before touching healthy stock.
Bench surfaces and walkways should be cleaned and disinfected before each new crop. Common greenhouse disinfectants include diluted bleach solutions and commercial products containing chlorine dioxide or quaternary ammonium compounds, all mixed according to label directions.
Biological Control: Beneficial Insects
Releasing natural enemies inside the greenhouse is one of the most effective long-term strategies for pest suppression, especially for operations that want to reduce chemical inputs. Several commercially available species target the most common greenhouse pests.
For aphids, parasitic wasps in the genus Aphidius lay their eggs inside live aphids. The parasitized aphids swell into gray-brown “mummies,” each one eventually releasing a new wasp. Lacewing larvae (both green and brown species) are generalist predators that consume large numbers of aphids, mites, and other small insects. Ladybeetle adults and larvae, particularly the convergent ladybeetle, feed on aphids, mites, insect eggs, and small larvae. Syrphid fly larvae, the immature stage of hoverflies, are also voracious aphid predators.
Minute pirate bugs are especially valuable against thrips. Both adults and nymphs prey on thrips, aphids, spider mites, and insect eggs. For caterpillar pests, the spined soldier bug is an important predator of many larval species, and parasitic wasps like Cotesia target specific caterpillars such as tomato hornworm.
Biological control works best as a preventive measure. Releasing beneficials early, before pest populations spike, gives them time to establish and keep numbers in check. Once an infestation is severe, beneficial insects alone often can’t catch up.
Low-Toxicity Treatment Options
When pest numbers exceed what prevention and biological control can handle, several biorational products offer effective control with less impact on beneficial insects and the greenhouse environment.
Insecticidal soap (potassium salts of fatty acids) works well against soft-bodied pests like aphids, whiteflies, and spider mites, though it’s generally ineffective against eggs. The pest must be directly contacted while the spray is still wet. Once the soap dries, it has no residual activity, which means thorough coverage and repeat applications are necessary. One important detail: insecticidal soap doesn’t mix well in hard water, which reduces its effectiveness. If you’re unsure of your water hardness, a simple jar test (mixing soap with water and checking for curd or scum) will tell you.
Horticultural oil works by smothering pests on contact. Like soap, it must cover the target organism while still wet and has zero insecticidal effect once dry (as quickly as 15 to 20 minutes after application). Avoid applying oils to tender new foliage, which can be damaged by the coating. Don’t spray if freezing temperatures are expected within 24 to 48 hours.
Bacillus thuringiensis (Bt) is a naturally occurring bacterium that’s lethal to caterpillars when they eat treated foliage. It’s most effective against young caterpillars and should be applied as soon as larvae are detected. Bt breaks down in storage and should be used promptly after mixing. Adding a sticker-spreader to the tank helps the product stay on leaf surfaces longer.
Insect growth regulators disrupt the molting process in immature insects, preventing them from reaching adulthood. These are particularly useful against whiteflies and fungus gnats, where targeting the juvenile stages can collapse a population over time.
Putting It All Together
Effective greenhouse pest management combines all of these tools in layers. Start with structural exclusion (screening, positive pressure) and strict sanitation between crops. Inspect and quarantine every plant before it enters production. Scout regularly using both sticky traps and hands-on plant inspection, keeping written records so you can track trends week to week. Introduce biological control agents early and preventively. Reserve biorational sprays and targeted treatments for situations where monitoring shows pest numbers rising beyond what your beneficials can manage. Each layer reduces the pressure on the next, and growers who commit to this integrated approach consistently use far less pesticide while maintaining healthier crops.