Cabbage Aphid

Cabbage aphids are greenish-gray with a white, waxy coating. They commonly occur in dense colonies. Adults can be winged or wingless; the winged adults have a black thorax.

Adult females give birth to nymphs throughout the year in much of California. Cabbage aphids have many generations per year.

Cabbage aphids prefer to feed on the youngest leaves and flowering parts. They are also found deep within the heads of Brussels sprouts and cabbage. The aphid feeds only on plants in the family Brassicaceae (Cruciferae), including weedy mustards in and around fields.

Cabbage aphids do not commonly damage seedlings. Their numbers increase after thinning or transplanting, and large colonies then can stunt or kill small plants. Their most serious economic damage is contamination of the crop at harvest. Because leaves curl around where they feed in dense, waxy colonies, when aphids are abundant it is difficult to reach them with insecticide sprays.

Conserve natural enemies that reduce cabbage aphids, but keep in mind that natural enemies may not provide sufficient control. Use cultural controls and well-timed insecticide application to reduce aphid numbers and avoid harming natural enemies of other pests.

Most fields in coastal cole crop production regions require at least one insecticide application before heading; however, insecticide application before heading is typically not necessary in the low desert. To conserve the parasites and predators of aphids and other pests, delay insecticide application as long as cabbage aphids do not threaten yield quality and quantity. Delay application until just before head formation to maintain natural enemies that can keep infestations of armyworms, cabbage loopers, diamondback moths, and imported cabbageworms below economically damaging numbers, therefore reducing the need for additional insecticide applications.

Many natural enemies can keep aphids at low or moderate numbers. However, if aphid infestations are severe or are located deep within heads, natural enemies may not provide adequate control.

Important natural enemies include the Diaeretiella rapae parasitic wasp, fungal pathogens, lady beetles, and syrphid fly larvae. Maintain hedgerows to sustain and provide refuge for natural enemies.

Use the following cultural methods to control cabbage aphids:

• Destroy crop remnants immediately after harvest.
• Remove alternate hosts, such as mustards and related weeds around field borders.
• Be sure transplants are pest-free before taking them to the field. Infestations on Brussels sprouts can start in seedling beds.
• Remove and destroy infested plants in the field early in the crop cycle.

Use biological and cultural controls in an organically certified crop. Applications of organically acceptable insecticidal soap provide partial control. To increase effectiveness, spray the insecticidal soap during foggy conditions. Soap sprays may be phytotoxic when applied at higher concentrations, especially in Brussels sprouts and cabbage, so use lower rates to avoid crop injury (see the treatment table for more information).

After seedling emergence or right after transplanting, check the field at least twice per week. Sample upwind of field borders and edges next to other crucifers first; this is where aphids tend to first appear.

• If no aphids are found, field samples may not be needed.
• If aphids are found, take field samples in a zigzag pattern.
• Be sure to check all blocks within the field, because aphid populations are often clumped.

Refer to the paragraphs below to make pesticide application decisions for different cole crops. After applying insecticide to control cabbage aphids, check fields frequently. Spray again if aphids reappear.

Broccoli, Cabbage, and Cauliflower

Before heading, check for cabbage aphid in the youngest, highest, and innermost leaves of young plants. Also check for natural enemies.

Until heading, broccoli and cauliflower crops can tolerate up to 100 aphids per plant. Because of the overlapping growth of their leaves, cabbage crops require more careful management and have less tolerance for aphids even during the early vegetative stages. Apply insecticide to cabbage as soon as 1 to 2% of cabbage plants are infested with one or more aphids.

After heading, check the flowering parts of broccoli and cauliflower, and pull back wrapper leaves of cabbage. Once heads begin to form, control cabbage aphids even if only a few are present.

Brussels Sprouts

Use presence-absence and sequential sampling (described below). This sampling method reduces the number of samples required to decide whether an application is necessary. It is based on whether any aphids are present on a leaf, not the number of aphids.

1. Divide the field into four blocks.
2. Randomly select 13 plants from each block that can be sprayed separately.
3. Take 5 samples along the field border and take the rest throughout the field.
4. For each plant, record if aphids are present or not.
5. Use the table below to determine the need for insecticide application or the need to continue sampling.
6. If you take 50 samples and still do not reach a decision, wait until the next sampling date to make a decision.

Brussels sprouts can tolerate 40% infested plants from transplanting until 2 weeks before harvest. This table advises pesticide application at 15% infested plants and is conservative. At topping, apply an insecticide if 1 or 2% of plants are infested with one or more aphids. Insecticides are more effective after topping because coverage is greatly improved.