The most common greenhouse pests are aphids, whiteflies, spider mites, thrips, and fungus gnats. Each one exploits the warm, humid, enclosed environment that makes greenhouses productive in the first place. Knowing what you’re dealing with, how to spot it early, and which controls actually work can mean the difference between a minor nuisance and a devastated crop.
The Five Most Common Greenhouse Pests
Aphids
Aphids feed by piercing plant tissue and sucking out sap. They come in green, black, orange, and other colors depending on the species and host plant. The telltale signs are sticky honeydew deposits on leaves (which attract ants and encourage sooty mold), curled or distorted new growth, and clusters of small soft-bodied insects on stems and leaf undersides. Aphids also spread plant viruses, making them a bigger threat than their size suggests. They reproduce fast, sometimes without mating, so populations can explode within days.
Whiteflies
Whiteflies are tiny, white-winged insects found on a wide range of greenhouse plants. Adults fly up in a cloud when you disturb the foliage, which makes them easy to identify but also easy to spread between growing areas. Their immature stages look completely different from the adults: flat, scale-like, and stuck to the undersides of leaves. Damage shows up as yellowing, stunting, and leaf drop, since both adults and nymphs suck plant sap. Like aphids, they produce honeydew.
Spider Mites
Two-spotted spider mites are the most common species in greenhouses. They’re not insects but arachnids, with adults having eight legs instead of six. They thrive in hot, dry conditions, which is why they often flare up in summer or near heating systems. Damage appears as a fine stippling on leaves, tiny pale dots where mites have punctured individual cells and drained the contents. In severe cases, you’ll see fine webbing stretched across leaves and stems. Spider mites are extremely small and can drift on air currents or hitchhike on clothing, making them easy to spread unknowingly.
Thrips
Western flower thrips are the dominant species in greenhouse production. They feed by rasping open leaf and flower cells and sucking out the fluid, leaving behind silvery or white patches speckled with tiny black dots of waste. They love to hide in tight spaces, especially inside flower buds, which makes them hard to reach with sprays. Thrips are weak fliers but spread readily by hopping between plants or riding on people and equipment. They’re also capable of transmitting serious plant viruses, including tomato spotted wilt virus and impatiens necrotic spot virus.
Fungus Gnats
The adults are small, dark flies that hover around the soil surface. They’re mostly a nuisance. The real damage comes from the larvae, which are translucent with a distinctive black head capsule. Larvae feed on fungi and decaying organic matter in the growing medium, but they also chew on plant roots and can tunnel into crowns and stems. This is especially damaging to seedlings and young transplants. The wounds larvae create become entry points for root rot pathogens like Pythium and Fusarium, compounding the problem.
How to Monitor for Problems Early
Sticky traps are the simplest and most reliable monitoring tool. Yellow sticky traps attract the widest variety of pests, including whiteflies, aphids, and fungus gnats, and are recommended for most situations. Blue sticky traps are sometimes used specifically for thrips since thrips find blue more attractive, but insects are harder to see and count against the blue background.
For general monitoring, place at least one sticky trap per 10,000 square feet of growing area. For whitefly-susceptible crops, increase that density to one trap per 1,000 square feet. Position traps at canopy height, just above the tops of the plants, and check them weekly. Count the insects on each trap and record the numbers so you can track trends over time.
As a rough guideline, management action for whiteflies may be warranted when you’re catching more than about five adults per trap per week on a standard 3-by-5-inch yellow sticky card deployed per 1,000 square feet, particularly early in production. Formal thresholds for ornamental crops are limited because of the sheer number of crop and pest combinations, but consistent weekly counts give you the trend data you need to act before populations spiral.
Biological Controls That Work
Beneficial insects and mites are widely used in greenhouse pest management, and choosing the right one depends on both the target pest and your growing conditions. Temperature and humidity matter more than most growers realize.
For spider mites, the predatory mite Phytoseiulus persimilis is the classic choice, performing best between 70 and 80°F with relative humidity above 60%. If your greenhouse runs warmer and drier, Amblyseius californicus is a better fit since it tolerates temperatures above 80°F and lower humidity. For growers in cooler climates or seasons, Amblyseius andersonii remains active at temperatures as low as 43 to 46°F.
Thrips are managed with predatory mites like Neoseiulus cucumeris, which works well between 70 and 75°F and around 65% relative humidity, making it a good match for cooler greenhouses. The alternative, Amblyseius swirskii, needs temperatures above 64°F and performs better in warmer conditions. The predatory bug Orius is highly effective against thrips but needs day lengths over 12 hours and temperatures above 60°F. At northern latitudes, Orius females enter a dormant phase from October through March under short days, making them impractical for winter use.
For whiteflies, the parasitic wasp Encarsia formosa is the standard biocontrol agent, working best above 70°F. Against aphids, parasitic wasps in the Aphidius genus are effective, with different species suited to different temperatures. The predatory midge Aphidoletes works best between 68 and 80°F at 70 to 80% humidity and is primarily active at night.
Fungus gnat larvae are effectively controlled with parasitic nematodes (Steinernema feltiae) applied as a soil drench. The growing medium needs to be moist and between 50 and 86°F. Apply early in the morning or late in the evening, and water crops both before and after application to help nematodes move through the soil.
Organic and Low-Toxicity Sprays
When biological controls need backup, several biorational products can fill the gap without wiping out your beneficials entirely.
Insecticidal soap (potassium salts of fatty acids) works on contact against aphids, whiteflies, thrips, and mites, and is also active against powdery mildew. It kills only what it touches, so thorough coverage is essential. Be aware that it can burn some plants, especially in hot weather. Test a small area first and avoid spraying when temperatures are above 85°F or when plants are stressed. Soaps can also harm some beneficial insects, so timing matters.
Neem oil extract controls mites and has some activity against fungal diseases like powdery mildew and botrytis. It works best as a preventive measure when pest populations are still low, and repeat applications are necessary. Azadirachtin, the active compound in neem, acts as an insect growth regulator, repellent, and feeding deterrent. It disrupts the development of immature pest stages and is labeled for aphids, thrips, whiteflies, leafminers, and several other pest groups. Like neem oil, it works best before outbreaks establish.
Beauveria bassiana is a naturally occurring fungus that infects and kills aphids, whiteflies, and thrips. It penetrates the insect’s outer shell, grows inside the pest, and eventually releases new spores. Apply in the evening since the fungus is sensitive to UV light. As with other biorationals, it performs best when used preventively based on your monitoring data, before pest numbers climb.
Spinosad works as both a contact and stomach poison against caterpillars, leafminers, and thrips. It’s somewhat toxic to some beneficial insects, so rotate it with other products rather than relying on it exclusively.
Rotating Controls to Prevent Resistance
Greenhouse pests develop resistance fast because of their short generation times and the enclosed, heavily treated environment. The key principle is simple: don’t treat consecutive generations of a pest with the same type of product.
Pesticides are classified into groups based on how they kill pests. The treatment interval approach ties rotations to the pest’s generation time. You can apply the same type of product more than once within a single generation, but when the next generation comes along, you need to switch to a product with a different mechanism. For example, if you used a product from one chemical group for the first generation of whiteflies, the next generation should be treated with a completely different group, or left untreated.
Insecticidal soaps and horticultural oils are valuable additions to any rotation because pests don’t develop resistance to them. Their physical mode of action, smothering or disrupting cell membranes on contact, bypasses the biological pathways that pests evolve resistance to. You can insert these products between chemical applications to reduce selection pressure while still keeping populations in check.
Environmental Conditions That Reduce Pest Pressure
The greenhouse environment itself is your first line of defense. Spider mites thrive in hot, dry air, so maintaining adequate humidity during warm months helps suppress them. Conversely, fungus gnats and root diseases flourish in overly wet conditions, so avoiding waterlogged growing media is critical. The balance point varies by crop, but monitoring vapor pressure deficit (the gap between how much moisture the air can hold and how much it actually holds) gives a more useful picture than relative humidity alone.
Air circulation is often underestimated. Inside a dense plant canopy, relative humidity can be 20 to 40% higher than in the air above, creating a microclimate that favors both pests and diseases. Continuous air movement from horizontal fans, fan-jet systems, or even running furnace blowers without heat breaks up this stagnant layer, keeps leaf surfaces drier, and makes conditions less hospitable for pests. It also helps distribute any biocontrol agents or sprays more evenly through the crop.
Sanitation rounds out the picture. Removing plant debris, spent flowers, and weeds eliminates hiding spots and breeding habitat. Screening vents and openings keeps flying pests from entering, though the mesh size needs to be fine enough for the target pest. Thrips, for instance, can slip through surprisingly small gaps. Inspecting new plant material before it enters the greenhouse prevents the most common route of pest introduction in the first place.