They suck plant sap from the leaf. The affected leaves curl and crinkle or form cups, completely lined with the aphids. In severe infestations, the plants wilt and die. The plants, if not killed, are dwarfed, grow slowly and form small light heads. Badly infested plants become covered with a mass of the small soggy aphids, and the dying leaves and plants rapidly decay.
Aphids are more troublesome during cool, dry weather.
Because these pests are difficult to control, treatments should be applied early. On a smaller scale, as in a vegetable garden, spray foliage with soapy water, then rinse with clear water or use insecticidal soaps. Planting in aluminum foil-covered beds and filling yellow pans with water to trap the aphids are both helpful as control measures. On a larger scale, two or three insecticide treatments at five-day intervals may be needed to clean up plants. When 2 percent of the plants are infested with aphids, an insecticide application should be made with high spray volume and adequate pressure to thoroughly wet foliage.