How to Control Aphids in your Greenhouse

There are over 900,000 known insect species on earth, but aphid species are some of the most well known to gardeners. Did you know there are many different species of this notorious pest insect? Although we generally refer to all aphids as a general pest, different species actually affect gardens differently because they have specific winter hosts.

Aphid Biology

Aphids live what’s called a holocyclic life cycle, utilizing different plant hosts in summer and winter. Females reproduce asexually during the summer months, giving birth to live females genetically identical to themselves.

Aphid populations grow so quickly because the majority of females are actually born pregnant! When fall comes, females also begin to give birth to males. At this time of year, male and females mate, producing eggs that will develop over winter on woody hosts specific to the aphid species.

When the weather begins to warm in the spring, aphids emerge from eggs, feed first on their woody winter host plant, and then move to their herbaceous summer hosts. Often your vegetable or flower garden.

Have you ever noticed that some aphids have wings and others don’t? In order to move to a new host, some aphids actually undergo a hormonal change based on their environment that allows them to grow wings. This happens when host plants become too crowded, the food source is poor, or when it’s time to switch from winter to summer hosts. The good news is, aphids are actually poor flyers. They can barely see, and don’t know if they’ve landed on something edible until they actually taste it!

Why are Aphids Considered Pests?

Aphids are considered garden pests because of the context in which gardeners interact with them. Of the hundreds of thousands of known insect species, many don’t affect gardeners. Some are even labeled as beneficial insects because they actually support gardening activities. Others, like aphids, are considered pests because they are destructive to our flowers and vegetables.

Aphids are phloem-feeding insects. They use piercing and sucking mouthparts to tap into the phloem of plants, extracting all the sweet sugars, carbohydrates and other metabolic products produced by plants. Photosynthesis creates these carbohydrates in the leaves and transports them down to the root system. Although this causes little tissue damage to the plant (unless an infestation is very severe), it robs the affected plant of much-needed nutrients and energy.

Aphids can also carry plant diseases and plant viruses, which they transmit during feeding. Feeding also results in honeydew production. Sugary, sticky honeydew on plant surfaces can create an ideal environment for black sooty mold growth. Although aphids themselves can reduce plant vigor and growth, the secondary problems that can result from their presence can also be detrimental to the garden.

Greenhouses are amazing because they create a protected, indoor environment that allows you to grow food and flowers year-round. However, greenhouses also create a protected environment for insects, sheltering them from natural population controls including wind, heavy precipitation, and temperature swings. Because of this, aphids can become very annoying in greenhouse environments if not monitored and managed well.

What Attracts Aphids to Your Garden?

Aphids feeding in your garden are biologically driven, but certain conditions may cause them to end up in your garden, reproduce, and cause damage. Keeping plants in your garden healthy is actually one of the best ways to prevent outbreaks. Here are a few things you can do to avoid stressed plants and over population:

• Maintain nutrient-rich, well-structured soil.
• Develop and follow a watering or irrigation scheme that consistently delivers adequate moisture to plants.
• Maintain proper plant spacing, avoiding overcrowding, and thin when necessary.
• Ensure adequate airflow (critical for aphid control in a greenhouse environment).
• Avoid stressing plants by exposing them to temperature extremes.
• Provide additional nutrients in the form of organic fertilizers only. Avoid too much Nitrogen fertilizer, which stimulates the tender vegetative growth that aphids are most attracted to.
• Select plant varieties that are resistant to aphid infestations and/or the plant diseases they transmit.
• Rotate plant varieties during different growing seasons and use succession planting techniques.
• Maintain proper garden sanitation.
• Ensure your garden is well weeded and that weeds are minimized in the surrounding landscape.

Assessing the Situation

Once you’ve noticed aphids on your plants, you have to decide if pest management is warranted, and what method is appropriate. The decision will be based on both personal tolerance and plant health concerns. Generally speaking, gardeners hit their “aphid tolerance threshold” before plant health becomes a major concern. These insects just bother us, and we immediately want to reduce their numbers.

• Minor infestation: Gently remove with your hand.
• Medium infestation: Use integrated pest management techniques described below.
• Remove heavily infested plants entirely from our Growing Dome greenhouse.

Really, one of the toughest concepts for many gardeners to come to terms with is that APHIDS ARE PART OF THE GARDEN ECOSYSTEM, and aiming to completely eradicate them is futile. Instead, it’s more productive to consider ways to manage and control aphid populations so they cause the least amount of harm and annoyance.

Aphid Control Techniques

Once you’ve determined the severity of the infestation, there are a number of integrated pest management techniques you can use to control them. These techniques include cultural, mechanical, chemical, and biological controls.

Cultural controls include many methods discussed above in “What Attracts Aphids to Your Garden”, including proper irrigation, crop rotation, nutrient management, plant variety selection, and proper plant spacing. Often, the most effective management regimes combine a few techniques that fall into each of these categories.

If you’ve decided to manage the aphid population in your garden, there are many management method choices to consider:

• Squishing the aphids (mechanical) – works best if you have a good amount of time, are feeling vindictive, and when aphid populations aren’t extremely dense.
• Spraying aphids off your plants with a strong stream of water (mechanical) – can kill some aphids, but mostly just interrupts their feeding on your plants.
• Shaking aphids off of your plants (mechanical) – interrupts aphid feeding, but doesn’t actually harm them for the most part.
• Removal of heavily infested leaves or whole plants (mechanical, cultural) – physically remove aphids from your garden, decreasing populations and removing plants that have lost vigor and are highly attractive to aphids.
• Planting “decoy” plants to attract aphids away from highly desired plants (cultural) – Planting species like marigolds and calendula may lure aphids away from your prized lettuces and other vegetables.
• Introduction of beneficial insects (biological) – physically introducing natural enemies to your garden. Ladybugs are the most commonly introduced beneficial insect. Keep in mind it’s important to create a habitat for your new beneficial insects so they stick around!
• Encouraging beneficial insects (biological, cultural) – creating an ideal habitat for aphid predators that already exists in your area so they will reproduce in your garden and help to manage pest insects. One good way to do this is to cultivate plants that attract beneficial insects including lacewings, ladybugs and spiders.
• Organic sprays (chemical) – although “chemical” sounds a little scary, organic sprays actually fall into this category, but are a much better alternative to synthetic chemical sprays. Spray ingredients often include soaps, garlic, neem oil and pepper among others.

Growing Spaces Organic Spray Recipe

Interested in the recipe for the organic spray we used in this video? Here’s how Dana made it:

1. Boil 1-gallon water in a deep sauce pot.
2. Add 3 tablespoons red pepper flakes to the boiling water, cover, and allow to boil for 15-20 minutes.
3. Turn off the burner and let the red pepper flakes sit in the water for 24-36 hours. (This allows the capsaicin in the red pepper to soak into the water. Capsaicin is the oil that makes chilis hot, and inflicts nervous system damage on soft-bodied insects, killing them.)
4. Strain the red pepper flakes from the mixture using a mesh pasta strainer or cheesecloth.
5. Put the red pepper mixture in a storage container or sprayer.
6. Add a few drops of Dr. Bronner’s Peppermint Soap (the fatty acids in some soaps injure soft-bodied insects).

You can apply this pepper/soap spray to the undersides of leaves affected to curb and manage populations. Be very careful to avoid your own eyes when spraying, and keep in mind that using an organic spray such as this one can also injure beneficial insects. If you’ve seen beneficial insects around, you may want to avoid using a spray and consider different cultural and mechanical management methods.

For more information, visit our latest blog post on How to get Rid of Aphids Naturally.

Do you have your own organic pest control method? Still have questions about pests in the garden? Ask a question or let us know what works in your own outdoor garden or greenhouse in the comment section below!