6 common greenhouse pests and how to manage them

Outbreaks of pests and diseases in a greenhouse are a serious pain and can get out of hand if not treated properly. Outbreaks of pests and diseases in a greenhouse typically require three main factors: a susceptible host plant, the presence of a pest or disease, and an environment conducive to its proliferation. An effective greenhouse pest control program addresses all three factors simultaneously.

Aphids are small, soft-bodied, sap-sucking insects that feed on the sap from your plant’s leaves. They reproduce quickly, do not need a mate and give birth to live aphids, so it is important to bring them under control immediately. There are many different types of aphids, so you can see them in different colors. The most common aphids seen in greenhouses are at a stage of life where they crawl (don’t fly), so you usually won’t see them on your sticky cards. You will see them on the leaves of plants, especially the underside of the leaves, but not exclusively. You may also see aphid skins on the leaves of the plants. You can also see ants. The ants will “cultivate” the aphids in order to feed on the “honeydew” aphids. So when you see ants, aphids are likely to be present.

Fungal midges
Fungal midges are small, winged, long-legged insects that typically graze on algae and organic matter in your soil. They don’t cause much direct damage to your crops, but they can be a nuisance in the greenhouse and can carry soil-borne diseases that could affect your crops (like pythium). You will see fungal midges on your sticky cards and fly around the base of your plants or other wet areas in the greenhouse. You may also see larvae of white midges in your soil.

White flies are extremely common in greenhouses. They are closely related to aphids and are usually about the same size. However, they’re usually white and winged, so they’ll wander away when you disturb them. They feed on plant sap and, like aphids, can produce a residue of “honeydew” at certain stages of their life cycle. You’ll see them on your sticky cards, on and around plants. They can damage leaves and fruits and slow plant growth.

Mosquito nets can help prevent whiteflies from entering the greenhouse.
Keeping your greenhouse clean of excess debris, plant material and weeds can reduce whitefly hosts. Similar to aphids, on a small scale you can use a strong explosion of water to remove whiteflies from host plants. You can also spray insecticidal soap (such as safer soap) on your plants to kill whiteflies on contact. As with aphids, it can be very effective to spray a safer soap after using the water spray technique. Yellow sticky traps are best used to identify and detect whiteflies, but in a smaller greenhouse, they can also help trap some whitefly populations.

There are many types of mites, but the most common that we see in greenhouses are spider mites. They are very small, can be red, brown, or green, and are usually found under the leaves.
As the population increases, you will see fuzzy straps on and through the leaves of the plant.

There are several species of predatory mites that can be released as a preventative or early action method. Monitor your climate to make sure your greenhouse is not too hot and dry. Spider mites can especially become a problem in hot, dry greenhouse climates or near warmer microclimate in greenhouses (such as right next to a heat source). Over-fertilizing plants can also make plants susceptible to spider mites. Safer soap or other insecticidal soaps can be used on spider mite populations, such as aphids or whiteflies.

Powdery mildew
Powdery mildew will appear as a white, fuzzy fungal spore on the leaves of plants. It can affect all plants, but will usually appear first on broadleaf plants (such as cucurbits) in a diverse planting. PM fungal spores exist in almost all greenhouses, but generally need moist conditions to colonize plant leaves.

You can use circulation fans to increase the air flow through your plant canopy. Prune excess older plant leaves in dense plantings to increase airflow through your plant’s canopy. Reduce humidity in your greenhouse by increasing ventilation (if the season is right). Invest in a dehumidifier or increase your nighttime temperature with a backup heater. Increase the pH of the leaves of your plants to make them a less hospitable environment for the proliferation of particle sores. For example, you can use potassium bicarbonate (small scale baking soda, sulfur burners, or a commercial potassium bicarbonate-based spray like MilStop) as a foliar spray both preventively and in response to particles present.

Thrips are very small, winged insects that are difficult to see without a magnifying glass or magnifying glass. There are many species of thrips, but the most common are the western flower thrips. You can see the damage they do to the leaves of plants in the form of patterned silver spots (which are dead plant cells) which contain small black spots (which are thrips droppings). They mainly scrape and suck chlorophyll from the leaves of the plant, which damages the leaves and reduces the plant’s ability to photosynthesize.

You can also see the deformed plant growth and the deformed flowers.
Yellow or blue sticky cards can help you monitor thrips populations, as you should be able to see trapped adult thrips. Also watch closely for thrips damage to plant leaves. Some growers choose to grow a small flowering crop (like petunias) that naturally attracts thrips. Having these flower attractors allows you to monitor thrips populations in your greenhouses.

A well established population of thrips is very difficult to control.
Prevention through screening is the most effective method. Insect screens (classified for Western Flower Thrips) can be used on all greenhouse outlets. Make sure you install and size your screens correctly so as not to restrict airflow into the greenhouse.
Once installed, clean your screens seasonally and watch for tears or tears so they can be repaired immediately. There are several species of predatory mites that will kill thrips at different stages of their life cycle. Beneficial nematodes can also be used. But these two elements need to be used preventively and repeatedly to have an impact.

Greenhouse pest control is a problem, but it is one that most greenhouse owners have faced at some point. That being said, we hope this blog has provided some useful information in solving your specific pest problems. Remember, no matter the size of your greenhouse and / or the application, pest prevention is always better than pest control for ensuring successful growth in your greenhouse. At Ceres, we design our greenhouses to be biosecure from the start so you can focus on what matters most to you, your plants. We also offer remote or in-person consultation for any pest related issues you may have.

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