Answer: The new ATTRA publication Cole Crops and Other Brassicas: Organic Production contains a section on Organic Integrated Pest Management that may be helpful to you, since it addresses aphids. Here are some additional considerations:
Encourage natural enemies by diversifying the habitat and their food sources, and refraining from use of broad-spectrum pesticides. The primary parasite of cabbage aphids in the Northeast is a very small, black wasp, Diaeretiella rapae, which lays its eggs inside the aphid. The parasite larva feeds inside the aphid, turning it a bronze color and killing it. It may take 2-3 weeks from the time the parasite lays its eggs inside the aphid until an adult parasite emerges from the dead aphid. Generally, there is a lag period between the outbreak of aphids and control by the parasite, so some other control should be used, but one that does not harm the parasite. A diversified cropping system with several potential aphid hosts can allow D. rapae to maintain itself in an area during periods of low levels of aphid abundance on one crop. Many other insects, such as ladybird beetles and Aphidoletes aphidimyza, can also be effective biological control agents against aphids. (See Biological Control: A Guide to Natural Enemies in North America, from Cornell University.)
Materials Approved for Organic Production
1. Soap: Scout brassica plantings once or twice a week, especially in the fall, and apply insecticidal soap sprays if aphids are found. Do not wait until aphids reach high numbers and dense colonies; apply when numbers are low. Repeat applications two or three times and ensure coverage of the parts of the plant where aphids live, including undersides of leaves and in the buds, shoots or heads of Brussels sprouts, broccoli, cabbage, etc. In recent studies, soaps have been ineffective against green peach aphid. Other studies we examined indicated 5 good, 1 fair, and 2 poor results against other aphid species.
2. Rotenone is recommended in the older literature (currently, no rotenone products are on the OMRI approved list).
3. Neem products can provide some control. Based on a limited number of studies, neem products gave good control of turnip aphid (2 studies); fair (4) to poor (3) control of green peach aphid; and mostly good control of other aphids (2 good, 2 fair, 1 poor). There are several different types of neem products.
4. Summer oils (2 fair and 3 poor results) will provide some control.
5. Kaolin clay will reduce aphid populations but will leave a white residue that may affect marketability.
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