Pest and disease outbreaks in a greenhouse are a serious pain and can get out of hand if they aren’t addressed properly. Because this is such a prevalent issue within the greenhouse community, we’ve created this article on how to identify and manage 6 of the most common greenhouse pests. But before we get into pest management, let’s go over some things to consider in order to prevent a pest outbreak in your greenhouse.
How do pest and disease outbreaks start and how do I avoid them?
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.
To address plant hosts, look for plant varieties that are resistant to pest challenges you’ve faced in the past. Experiment with different crops and varieties to see which fare better, and keep good records so you can learn from each growing season and anticipate pest or disease outbreaks before they happen again.
Pest prevention starts with smart greenhouse design
To help prevent the presence of pests, consider biosecurity as early as the design stage of your grow. Consider installing insect screens on all air intakes to prevent pests from entering the greenhouse. Some greenhouses, especially commercial applications, have airlocks or vestibules at the main entry point that allow for clean entry. And in all greenhouse applications, regular scouting is a critical part of pest management. Yellow (or other colored) sticky cards placed strategically around the greenhouse will trap pests and provide a sample of pest populations present in the greenhouse.
Detecting the presence of pests as early as possible will allow you to address the issue in less invasive ways (like introducing beneficial insects, changing cultural/growing practices, removing select plants, or adjusting greenhouse climate) before it becomes a true outbreak (where you may have to use sprays or remove an entire crop).
Pest control with climate control
Finally, greenhouse climate can play a big role in allowing pests or disease to proliferate. A greenhouse that is too hot or cold, too wet or dry, can stress plants and make them vulnerable to pest outbreaks. For example, almost all greenhouses have some powdery mildew spores present, but certain environmental conditions (i.e. high relative humidity and low air flow) will cause PM spores to successfully colonize plant leaves.
So, when you start to see a pest outbreak it’s important to investigate the following: Where are they coming from? What plants are they affecting? What age are those plants (Is the outbreak in the nursery, in the mature plants, etc.)? And is your greenhouse climate in a healthy range or is there one or multiple factors causing plant stress or creating prime conditions for pest presence?
Now that we’ve gone over ways to prevent pest and disease outbreaks in your greenhouse, let’s discuss ways to identify and manage them in the instance of an outbreak.
Common Greenhouse Pests:
- 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 lifestage 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.
- Water – On a very small scale, you can use a strong stream of water to spray aphids off plant leaves. If done repeatedly (everyday or every other day until you no longer see them return) this will interrupt their life cycle and dislodge them from plants.
- Safer Soap – You can also spray an insecticidal soap (like Safer Soap) on your plants to kill aphids on contact. It can be very effective to spray safer soap after using the water spray technique. Safer Soap has minimal impact on beneficial insects.
- Beneficial insects: Ladybugs or lacewings can control aphids very well if they are used preventatively, rather than introduced when a population has already gotten out of control.
- In particularly bad outbreaks, it may make sense to remove entire plants and get rid of them outside your greenhouse to reduce the aphid population.
- Horticultural oils (like neem oil or Azadirachtin) are also an option when other management techniques have not worked.
- ***If using horticultural oils, insecticidal soaps, or other biopesticide sprays, it is very important to follow all label instructions to use them safely and responsibly.
Kucharski & K. Kucharska/Shutterstock
- 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.
- Fungus gnats are attracted to wet areas in your greenhouse, so be sure to keep excess water off the ground or other areas. Remove any excess potting soil off the ground, benches, or tables that may get wet and hold moisture.
- Manage your irrigation well. Do not over water your plants and allow for an appropriate level of dry-down to occur before irrigating pots or nursery seedlings. Using drip irrigation in more permanent plantings (rather than overhead watering) is a very effective way to conserve water resources and eliminate excess water and humidity throughout the greenhouse.
- Eliminate any algae as best you can through irrigation techniques, and by disinfecting pots or other growing equipment between plantings.
- Beneficial nematodes can be used in a soil drench or by dipping cutting and seedlings as a preventative measure.
- White flies 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.
- Prevention! 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.
- Water – Similar to aphids, on a small scale you can use a strong blast of water to knock white flies off plant hosts.
- Safer Soap – 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. Safer Soap has minimal impact on beneficial insects.
- Yellow sticky traps are best used to ID and scout for white flies, but in a smaller greenhouse they can also help to trap some white fly populations.
- Vacuum – shake your plants to put the whiteflies into flight and then vacuum them with a shop vac or similar vacuum cleaner.
- Like aphids, scout for and reduce ant populations as ants will protect and encourage white flies.
- Beneficials – There are multiple species of parasitic wasps that will parasitize whiteflies. These can be released as a preventative or early action method (remove your sticky traps first so you don’t trap the beneficial wasp species). As a rule of thumb, many growers say the right time to introduce beneficials is when you see as many as 5 whiteflies on your sticky cards!
- 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.
- Wiping away webbing from leaves with a soapy sponge.
- Spraying plants down with water from top to bottom to remove webbing and mites.
- Beneficials: 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 white flies.
Photo by AJ Cespedes / Shutterstock.com
- 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.
- 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.
- Generally keep a clean environment in your greenhouse by sanitizing tools, surfaces, and any pots that are re-used.
- 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.
- Utilize a biofungicide (like Cease) to introduce a beneficial and competitive bacteria to your plant leaves.
- ***Again, if using horticultural oils, insecticidal soaps, or other biopesticide/biofungicide sprays, it is very important to follow all label instructions to use them safely and responsibly.
Photo: Matthew Bertone
- Thrips are very small, winged insects that are hard to see without a hand lense or magnifying glass. There are many species of thrips, but the most prevalent are 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 chlorophyl 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 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.
- You can grow a small trap crop (especially something with blue flowers) to attract any present thrips and allow them to spare your main crop. This is effective only if you have small thrips populations.
- If you only see thrips damage on one or a few plants, consider removing those plants entirely.
- Beneficials: 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.
- Horticultural oils (like neem oil or Azadirachtin) are also an option when other management techniques have not worked.
***Again, if using horticultural oils, insecticidal soaps, or other biopesticide sprays, it is very important to follow all label instructions to use them safely and responsibly.
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.
UMass Extension Greenhouse Crops and Floriculture Program
Green Methods Integrated Pest Management
BioWorks Inc. Fact Sheets
Arabico-Organics Fact Sheets and Product Labels
IPM Lecture from Dr. Angie Maderas, UMass Extension Services, Jan. 29th, 2021
University of California Statewide Integrated Pest Management Program
National Pesticide Information Center