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Controlling Aphids in Fall Vegetable Gardens

Oct 25, 2013
By Charlotte Glen,
When planted in late summer, cool season crops like cabbage, collards, broccoli, kale, and turnips usually thrive with minimal care. One pest that can cause problems for these crops that has shown up in our area this fall is the aphid. Though individually tiny, aphids frequently occur in huge numbers and can cause serious problems for vegetable crops. Aphids are easy to manage with either organic or synthetic insecticides, but before you treat take look to see if nature is taking care of the problem for you.
kale leaf
Leaves infested with aphids may be distorted or misshapen.
All About Aphids
Aphids are tiny (1/8”), oval to pear-shaped, soft bodied insects, sometimes referred to as plant lice. They come in a variety of colors, including green, yellow, orange, brown, and black. Like all insects, aphids have six legs. One thing that sets them apart from other insects is a pair of cornicles on their rear end which look like two exhaust pipes. Another unique feature of aphids is that they rarely lay eggs, and instead give birth to live young, which are clones of their mother. This ability results in rapid, explosive increases in aphid numbers in a very short time.
All aphids are sap feeders, meaning they feed on plant sap with their needle like mouthparts, rather than eating leaf tissue. The two most common aphids that cause problems in vegetable gardens at this time of the year are the cabbage aphid and the turnip aphid. These two aphids are extremely similar in appearance. Both are green in color and feed on plants in the crucifer family, such as cabbage, collards, kale, turnips, mustard, broccoli, cauliflower, brussel sprouts and radish.
parasitized aphids
Parasitized aphids appear puffed up and bronze in color.
Detecting Aphid Activity
Cabbage and turnip aphids are most prevalent in cool dry weather. Because they feed on plant sap from the underside of leaves, they are often not noticed until their numbers become severe. Aphid feeding can cause plants to produce crinkled, cupped or deformed leaves. Feeding by large populations will stunt plants and can kill small plants. Aphids also excrete honeydew, a sticky, sweet, clear substance that can coat plant leaves and attract ants and wasps. Gardeners should inspect the backside of plant leaves, particularly tender new leaves, for aphids each week.
Beneficial Insects
The good news is that aphids have many natural enemies, including ladybugs, parasitic wasps, and hoverfly larvae. Inspect plants closely before treating to determine if beneficial insects are already taking care of the problem for you. Look for hoverfly larvae, as pictured above, ladybugs, and aphid mummies (picture at top of article), which have been parasitized by tiny wasps. Aphid mummies are bronze or tan in color and appear ‘puffed up’ when compared to living aphids.
Controlling Aphids
Because aphids reproduce extremely rapidly, they can damage young vegetable plants before beneficial insects are able to control the outbreak. In addition, cabbage and turnip aphid are not killed by cold weather in our area and survive through the winter, most commonly on collards; if you have an aphid outbreak now it is likely to get worse over the next few weeks unless beneficials are already present.
If you don’t find any beneficial insects on your plants you will likely need to treat. If you only find a few aphids, squish them or break off infested leaves and remove them from the garden. When control is needed, both organic and synthetic insecticide sprays are available. When using either, make sure to cover plants completely, especially the backside of leaves, since aphids often shelter in pockets and crevices underneath leaves and in buds. Repeated applications are usually necessary to control this pest.
Organic insecticides that are effective for aphid control in vegetable gardens are insecticidal soap, pyrethrin, and neem oil. For these products, vegetables can be harvested from treated crops the same day of application.
Synthetic insecticides for aphid control on vegetables include those containing the active ingredients permethrin or bifenthrin. Both of these products are available under several name brands, so check product labels to find these in local garden centers. Permethrin has a one day post harvest interval, meaning you have to wait at least one day after treating to harvest. For bifenthrin, the post harvest interval is seven days.
Aphids can be washed off of harvested leaves with running water or by submerging harvested crops in soapy water and then rinsing with clean water. Vegetables that aphids have fed upon are safe to eat and if you don’t get every single aphid off don’t worry — they will just add a little protein to your meal!