It’s July, and the hot weather is setting in. Our tomatoes have been in the ground for about a month, and little growth is happening with our plants. What gives? On closer inspection of our plants, green peach aphids are noticeable on the stems and under the leaves of the plants.
Aphids on tomato plants can lead to early stunted growth and leaf curling of young plants. Green peach aphids, Myzus persicae, are the dominant species in our area and can attack a variety of plants and weeds, as well as transmit viruses to vegetable plants. Organic control of aphids is especially important in early summer when tomato plants are developing new stems and leaves. Aphids feed primarily on new growth areas of plants and tend to avoid well-developed, larger leaves and stems.
Prevention of aphids on tomato plants
- Avoid over fertilizing – Since aphids feed on new growth, increased nitrogen in soil can encourage additional green growth of plant and increase in aphid numbers. A well balanced fertilizer with increased phosphorous will encourage flowering and fruiting rather than green growth and will prevent aphids feeding on susceptible, fragile, new leaves. Generally, look for fertilizers with about twice as much phosphorous as nitrogen, ex. 5-10-5.
- Co-plant with aphid unfriendly plants – Mint, catnip, chives, onion, garlic, dill, and fennel all contain volatile aromatic oils that repel aphids. Co-planting tomatoes with these herbs can discourage aphids on tomatoes as well as attract beneficial insects. Choose dwarfed varieties of these herbs so they do not out-compete the tomatoes.
- Top-watering – Top-watering tomato plants at least once a week with a higher intensity spray of water can knock aphids off plants and decrease their numbers which can assist in control of these pests. Avoid top-watering during periods of intense sunlight (ie. mid-afternoon) to prevent leaf burns from the water. Also avoid top-watering after sundown to prevent fungus. The best time for watering plants is early to mid morning to allow leaves to dry before the sun is at its highest.
- Reduce nearby red ant populations – Certain species of red ants are attracted to the honeydew produced by aphids and can form symbiotic relationships with aphids. Some ants go as far as farming aphids by carrying them to new plants, as well as protecting them from predators. Decreasing or eliminating nearby ant colonies can reduce aphid numbers on plants and prevent their spread.
- Introduce predatory insects – Lacewings, lady beetles, and some small wasp species feed on small insects including aphids. These insects can be released on plants to hunt and destroy aphids. When using beneficial insects, it is important to avoid chemicals or organics that may injure these helpful insects. Also, to encourage beneficial insects to remain on your plants and not fly off, insect exclusion netting can be first placed over the tomato cage and insects released under the netting to keep them on the plants.
Organic chemical control
- Neem oil – A variety of neem oil preparations are available to control insects and fungus on vegetable plants. Neem oil is generally not toxic to animals. Neem oil should be avoided on heavily flowering plants as it will discourage pollinators from visiting flowers. The underside of leaves and stems must be sprayed to treat areas where aphids like to hang out. Ensure you use a product labeled for this use and use according to manufacturer directions.
- Insecticidal soaps – Insecticidal soap interferes with an insect’s ability to breathe, as well as its ability to adhere itself to the plant which can kill or encourage an insect to leave the plant. Insecticidal soaps only work when in direct contact with the insect and will not harm pollinators to the plant once the plant is dry. These must be sprayed frequently to decrease insect numbers and are available at most garden stores. Follow directions on label for use.
- Dish soap – Highly diluted, natural dish soap can also be used to spray leaves and stems and decrease insect numbers. Choose a natural dish soap without added chemicals or artificial scents. Dr. Bronner’s unscented or peppermint makes a great plant wash. Shake mixture to encourage foaming and spray it using a clean spray bottle. Avoid spraying soap onto plants in very bright sunlight or on very hot days as leaf burn might occur.