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How do I control aphids in my high tunnels?

Answer

Greenhouses and high tunnels are great tools, but they also provide a nice protected environment for certain insect pests. Normally, aphids are kept in check by predators, parasites and by wind and rain which can knock them off plants. In the protected environment of a greenhouse or high tunnel, they can become a problem because there are few or no natural controls. In the right conditions, aphids and other pests, such as whiteflies and spider mites, can explode in numbers very quickly.

Aphids are sucking insects that weaken plants by sucking up sugars and other fluids from crops. They are not easy to see, since they are the same color as the plant stem and generally like to feed on stems, buds, and underneath leaves.

Step one for control is to monitor your crops on a regular basis. Infested plants are often stunted and can be a lighter green or yellow. Look closely at the stems under the leaves for aphids. A magnifying glass or loop can help. A good approach that saves time is to flag “sentinel” plants. Mark plants in a grid pattern and only monitor those specific plants on at least a weekly basis. If you find significant numbers of aphids on these plants, you know it is time for control options. Yellow sticky cards are another good monitoring device. Adult female aphids have wings and are strongly attracted to the color yellow, which mimics the color of sickly plants. The cards are coated in sticky glue, which traps them. Check cards at least weekly for signs of adult aphids. Sticky cards need to be replaced frequently to work well. Fortunately they are cheap and available through any greenhouse supply company.

If you have any infested plants in a greenhouse or high tunnel, you should begin control options since their numbers can explode quickly.

Here are your options as an organic farmer:

Biological control

There are a number of predators and parasites available for purchase and release. Ladybug larvae are the most familiar, but there are parasitic wasps, lacewings and others available now as well. Biological controls work best to keep aphid levels down, but may not give good control if you already have a problem. They simply cannot reproduce as quickly as aphids (which can actually reproduce without males - females give birth to clone daughters). Beneficial insects work best as preventative controls. A number of companies sell beneficial insects, including Arbico Organics and Buglogical .

Non-Chemical Spray

Soapy water will kill aphids. The soap strips away their waxy cuticle and they die of dehydration. In order for this to work, they must be directly sprayed with the soapy water. Use a sprayer and mix one tablespoon of liquid soap per gallon of water. (Dr. Bronner’s is pure soap. Be careful not to use soaps with perfumes, dyes or other synthetic additives.) There are also many ready-to-use brands that are OMRI listed including the common Safer Insecticidal Soap.

Allowed Chemical Sprays

Remember that all insecticides approved for organic use are “restricted use” products. You can use them only when your other control options have failed, and you must notify your certifier if you intend to use a new product and the reason you must use it. Pyrethrum/pyrethrin-based sprays will work on aphids, but have a very short residual effect and must come in contact with the aphids. The product Pyganic works well, since it is pyrethrin mixed with oil, which coats and kills aphids and many other insects. Your certifier should be able to provide a list of approved pyrethrum/pyrethrin sprays or check the OMRI website for a list.

Aphids also love plants that are over-fertilized with nitrogen. If they are a constant problem despite other control efforts, you might be adding too much nitrogen to your potting mix or through fertilizer applications. A tissue test to determine nitrogen levels may be in order if you are having ongoing issues with aphids and other sucking pests.

As always, crop rotation and good sanitation practices can help control aphids in the long run.

Posted: Jun 2017
Answer By: Joe Pedretti