Pest and disease outbreaks in a greenhouse usually require three main factors: a susceptible host plant, the presence of a pest or disease, and the right environment for it to proliferate. An effective greenhouse pest management program addresses all three factors simultaneously.
Aphids are small, soft-bodied, sap-sucking insects that will feed on the sap in your plant leaves. They reproduce rapidly, do not need a mate, and give birth to live aphids, so it is important to get them under control right away. There are lots of different types of aphids, so you may see them in different colors.
The aphids most commonly seen in greenhouses are at a life stage where they crawl (don’t fly), so you typically will not see them on your sticky cards. You will see them on plant leaves, especially the underside of leaves, but not exclusively. You may see aphid skins on plant leaves as well. You may see ants as well. Ants will “farm” aphids in order to feed on the “honeydew” aphids produce. So when you see ants, aphids are likely to be present.
Fungus gnats are small, winged, long-legged insects that typically graze on algae and organic matter in your soil media. They do not cause a lot of 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 fungus gnats on your sticky cards and flying around the base of your plants or other wet areas in the greenhouse. You may also see white fungus gnat larvae in your soil media.
Whiteflies are extremely common in greenhouses. They are closely related to aphids and are typically about the same size. However, they are typically white and winged so they will swarm away when you disturb them. They feed on plant sap and can, like aphids, produce a “honeydew” residue at certain stages of their lifecycle. You will see them on your sticky cards, and on and around plants. They can cause leaf and fruit damage, and stunted plant growth.
Insect screens can help keep whiteflies out of the greenhouse.
Keeping your greenhouse clean from excess debris, plant material, and weeds can reduce hosts for whiteflies. Similar to aphids, on a small scale you can use a strong blast of water to knock whiteflies off plant hosts. You can also spray an insecticidal soap (like Safer Soap) on your plants to kill whiteflies on contact. As with aphids, it can be very effective to spray safer soap after using the water spray technique. Yellow sticky traps are best used to ID and scout for whiteflies, but in a smaller greenhouse, they can also help to trap some whitefly populations.
There are many types of mites, but the most common we see in greenhouses are spider mites. They are very small, can be red, brown, or green and are typically on the underside of leaves.
As the population grows, you will see fuzzy webbing on and throughout the plant leaves.
There are multiple 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 close to warmer micro-climates in greenhouses (like right next to a heat source). Over-fertilizing plants can make plants susceptible to spider mites as well. Safer Soap or other insecticidal soaps can be used on spider mite populations, similar to aphids or whiteflies.
Powdering mildew will show up as a fuzzy, white fungal spore on plant leaves. It can affect any plants, but will typically show up first on broad leaf plants (like cucurbits) in a diverse planting. PM fungal spores will exist in almost any greenhouse but typically need humid conditions to colonize plant leaves.
You can utilize circulation fans to increase airflow in your plant canopy. Prune out excess, older plant leaves in dense plantings to increase airflow into your plant canopy. Reduce humidity in your greenhouse by increasing ventilation (if seasonally appropriate). Invest in a dehumidifier, or increase your nighttime temperature with supplemental heating.
Raise the pH of your plant leaves to make them a less hospitable environment for PM sores to proliferate. For example, you can utilize potassium bicarbonate (baking soda on a small-scale, sulfur burners, or a commercial potassium bicarbonate-based spray like MilStop) as a foliar spray both preventatively and in response to PM present.
Thrips are very small, winged insects that are hard to see without a hand lens or magnifying glass. There are many species of thrips, but the most prevalent is the western flower thrips. You can see the damage they cause to plant leaves as patterned silvery patches (which are dead plant cells) that contain small black specks (which is thrips frass). They primarily scrape and suck the chlorophyll out of plant leaves, which damages leaves and reduces the plant’s ability to photosynthesize.
You may also see deformed plant growth and flower deformation.
Yellow or blue sticky cards can help you monitor for thrips populations, as you should be able to see adult thrips trapped. Also, monitor closely for thrips’ damage on plant leaves. Some growers choose to grow a small flowering crop (like petunias) that naturally attract thrips. Having these flower attractors allows you to monitor and pest thrips populations in your greenhouses.
A well-established thrips population is very hard to control.
Prevention through screening is the most effective method. Insect screens (rated to western flower thrips) can be used on all greenhouse intakes. Be sure to install and size your insect screens correctly so that you do not reduce airflow in the greenhouse.
Once installed, clean your screens seasonally and monitor for any rips or tears so they can be fixed immediately. There are multiple species of predatory mites that will kill thrips at various stages in their life cycle. Beneficial nematodes can also be used. But these both have to be used preventatively and repeatedly to have an impact.
Greenhouse pest control is a hassle, but it’s a hassle that most greenhouse owners have at some point had to deal with. That being said, we hope this blog has provided some useful information for solving your specific pest issues. Remember, no matter what your greenhouse size and/or application, pest prevention is always better than pest management to ensure successful growth in your greenhouse. At Ceres, we design our greenhouses to be biosecure from the get-go so that 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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Ceres Greenhouse Solutions