Aphids do little damage in small numbers. They are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when aphids suck the juices from lettuce leaves. The green peach aphid (Myzus persicae) may also spread lettuce mosaic virus, a disease that dwarfs lettuce plants, rendering them unproductive. The green peach aphid is pale green with three dark lines on the back. Its shiny black eggs spend the winter on the bark of fruit trees. About the time peach trees bloom, the eggs hatch and the young aphids begin feeding, first on the tree; then they migrate to vegetable and flower plants.

Macrosteles fascifrons This insect, also known as the six-spotted leafhopper, feeds on many vegetable and ornamental plants. It generally sucks the sap from the undersides of leaves, which causes stippling. This leafhopper transmits aster yellows, a plant disease that is quite damaging. Leafhoppers at all stages of maturity are active during the growing season. They hatch in the spring from eggs laid on perennial weeds and ornamental plants. Even cold areas where eggs cannot survive the winters are not free from infestation, because leafhoppers migrate in the spring from warmer regions.

Trichoplusia ni Although several worms attack lettuce, the most damaging is the cabbage looper. The looper attacks all varieties of lettuce, as well as members of the cabbage family. Adults lay eggs throughout the growing season. The brownish cabbage looper moth lays pale green eggs on the upper sides of leaves in the evening. The worms eat lettuce leaves and heads, and their greenish brown excrement makes the plants unappetizing. Worms may be present from early spring until late fall. In the South, they may be present year-round. Worms spend the winter as pupae attached to a plant or nearby object.

Several species of cutworms attack plants in the vegetable garden. The most likely pests of lettuce plants in the spring are surface-feeding cutworms. A single surface-feeding cutworm can sever the stems of many young plants in 1 night. Cutworms hide in the soil during the day and feed only at night. Adult cutworms are dark, night-flying moths with bands or stripes on their forewings. In southern parts of the United States, cutworms may also attack fall-planted lettuce.

This plant disease is caused by a fungus (Bremia lactucae) that attacks only lettuce and some of its wild relatives, especially prickly lettuce (Lactuca serriola). It is seen most commonly at harvest time because it usually infects mature plants, but it also infects seedlings and transplants, which may die. Downy mildew is favored by cool (less than 70°F), humid weather. Spores are released on humid mornings with more than 50 percent relative humidity, beginning at sunrise, and are blown by the wind onto plants. Morning dew persisting past 10 a.m. especially favors infection. This fungus mutates readily, and several distinct strains exist.

This plant disease is caused by at least two viruses that attack all varieties of lettuce. The viruses usually enter the garden in infected seed, but they may also live in weeds, including wild lettuce, pokeweed, wild cucumber, and groundsel. Once the infection is present in the garden, aphids-primarily the green peach aphid-spread the viruses from plant to plant. The viruses can also be transmitted mechanically as diseased and healthy plants rub together. Symptoms appear 8 to 14 days after infection. Occasionally a plant will show symptoms but then recover and grow to normal size.