Aphids do little damage in small numbers. They are extremely prolific, however, and populations can rapidly build up to tremendous numbers during the growing season. Damage occurs when the aphid sucks the juices from bean leaves. The aphid is unable to digest fully all the sugar in the plant sap, and it excretes the excess in a fluid called honeydew, which often drops onto the leaves below. Ants feed on the sticky substance and are often present where there is an aphid infestation.

Two widespread bacterial blights that affect all kinds of beans are common blight (caused by Xanthomonas phaseoli) and halo blight (caused by Pseudomonas phaseolicola). Common blight is more severe in warm, moist weather; halo blight is favored by cool temperatures. The bacteria are usually introduced into a garden on infected seed and can live on infected plant debris in the soil for as long as 2 years. They are spread by rain, splashing water, and contaminated tools. The bacteria multiply rapidly in humid weather. If water-conducting tissue is invaded, bacteria and dead cells eventually clog the veins, causing leaf discoloration. Often bacteria ooze from infected spots in a yellow or cream-colored mass.

Bean common mosaic and bean yellow mosaic are widespread viral diseases that are difficult to distinguish from one another. Common mosaic virus affects only French and snap beans; yellow mosaic virus affects lima beans, peas, summer squash, clover, gladiolus, and other perennial flowers. Both diseases are spread by aphids, which transmit the virus as they feed. In warmer parts of the country with large aphid populations, the disease spreads rapidly. Common bean mosaic is also spread in infected seed. If the infection occurs early in the season when the plants are young, the plants may not bear pods. Infection later in the season does not affect pod production as severely.

Epilachna varivestis This beetle is found throughout the United States. It prefers lima beans but also feeds on pole and bush beans and cowpeas. Feeding damage by both adults and larvae can reduce pod production. Adult beetles spend the winter in plant debris in the garden and emerge in late spring and early summer. They lay yellow eggs on the undersides of leaves. Larvae that hatch from these eggs, in early to midsummer, are green at first and gradually turn yellow. One to four generations occur per year. Frequently, all stages of beetles appear at the same time throughout the season. Hot, dry summers and cold winters reduce the beetle population.

Hylemya platura The seedcorn maggot feeds on many vegetables, including beets, beans, peas, onions, cucumbers, and cabbage. It damages seeds and germinating seedlings by tunneling into them and feeding on the tissue. If the seed is only partially destroyed, the plants that sprout are weak and sickly and soon die. These pests can be responsible for snakeheads. The seedcorn maggot is favored by cold, wet soil with a high manure content. It spends the winter as a larva or a pupa in soil, plant debris, or manure and becomes active in the early spring. The adults-flies slightly smaller than houseflies-emerge from May to July and lay eggs in the rich soil or on seeds or seedlings. The overwintering maggots cause the most damage. Those of later generations are of less importance. Three to five generations occur per year.

Tetranychus urticae These mites, related to spiders, are major pests of many garden and greenhouse plants. They cause damage by sucking sap from the undersides of leaves. As a result of their feeding, the plant''s green leaf pigment disappears, producing the stippled appearance. Spider mite webbing traps cast-off skins and debris, making the plant messy. Mites are active throughout the growing season but are favored by hot, dry weather (70°F and up). By midsummer, they have built up to tremendous numbers.