How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Cabbage aphids are green gray with a white, waxy coating. They commonly occur in dense colonies, often covered with waxy droplets. They prefer to feed on the youngest leaves and flowering parts and are often found deep within the heads of cabbages or Brussels sprouts. The aphid has a simple life cycle with adult females giving birth to live offspring throughout the year in most parts of California. Both winged and wingless adults occur; the winged adults have a black thorax and lack the waxy coating. The aphid does not infest noncruciferous crops but can survive on related weed species when cole crops are not in the field.

DAMAGE

Cabbage aphids do not normally affect seedlings but build up after thinning or transplanting. Large colonies can stunt or kill small plants, but the most serious problem is contamination of the harvested crop. Dense populations cause leaves to curl around them, making them harder to reach with pesticide applications.

MANAGEMENT

Cultural practices and biological control agents can reduce aphid infestations and delay or prevent the need for pesticide use. Try to delay using insecticides for as long as possible while maintaining yields and quality. Most fields require at least one application against aphids at preheading; however, if you can delay applications until just before head formation, you will save the expense of additional applications and may also be able to maintain the natural enemies that will keep caterpillar pests, including loopers, imported cabbageworms, armyworms, and diamondback moths, below economically damaging levels.

Biological Control
Cabbage aphids have many natural enemies and these can sometimes control low populations; however, short crop life, use of pesticides for other pests, the tendency for the aphids to be deep within the head, and various other factors make it difficult for natural enemies to keep rapidly rising aphid populations from reaching economic levels. Important natural enemies include lady beetles, syrphid fly larvae, fungal diseases, and the parasitic wasp, Diaeretiella rapae. Protect habitat for natural enemies so that they can survive and increase their population levels.

Cultural Control
Destroy crop remnants immediately after harvest and remove or control alternate hosts, including mustards and related weeds, around field borders. Infestations on Brussels sprouts can start in seedling beds, so be sure transplants are clean before taking them to the field. Roguing (removal and destruction) of infested plants from the field can be effective early in the crop cycle.

Organically Acceptable Methods
Biological and cultural control are organically acceptable, as well as sprays of insecticidal soap, which can give partial control. Soap sprays, however, may be phytotoxic under some conditions, especially in Brussels sprouts and cabbage. For most effective control, apply during foggy conditions.

Monitoring and Treatment Decisions
Check each field at least twice a week. A sequential sampling program is available for Brussels sprouts. Sample upwind field borders and edges next to other crucifers first; this is where aphids tend to appear first. If no aphids are found, you may not need to take field samples. Take field samples in a zigzag pattern. Remember to check all quadrants of the field because aphid populations are often clumped.

Cabbage, broccoli, and cauliflower. Check for cabbage aphid in the youngest, highest, and innermost leaves of young plants. After heading, check the flowering parts of broccoli and cauliflower and pull back wrapper leaves of cabbage. Also check for natural enemies. Broccoli and cauliflower crops can tolerate up to 100 aphids per plant up to heading. Once heads begin to form, cabbage aphids must be controlled even if only a few are present. 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; treat as soon as 1 to 2% of plants are infested with one or more aphids. After treating, recheck fields frequently and treat if populations reappear.

Brussels sprouts. A presence-absence sequential sampling program is available for making treatment decisions in Brussels sprouts. In this program you do not need to count actual numbers of aphids on a leaf but need only to determine if aphids are present. The program also reduces the number of samples required when aphid populations are low. Start by sampling 13 randomly selected plants for each block that can be sprayed separately; take 5 samples along the field border and the rest scattered throughout the field. For each plant, simply record if the aphid is present or not. Use the table below to determine need for treatment or continued sampling. If you take 50 samples and still don't reach a decision, wait until the next sampling date to make a decision.

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

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgments for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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