The black bean aphid, Aphis fabae, is a common insect pest in gardens and agriculture. This small, soft-bodied insect belongs to the order Hemiptera, which includes true bugs that feed on plant sap. Often referred to as “blackfly,” these aphids can rapidly establish large colonies on a wide range of host plants, leading to noticeable plant damage.
Identification and Life Cycle
Black bean aphids are small, pear-shaped insects, 1.5 to 3 millimeters long. Their bodies are dull black or dark green, though some may appear purplish-black. Wingless females are common and form dense colonies. Winged forms, more slender with shiny black heads and thoraxes, develop when colonies become overcrowded or plant conditions decline. These winged aphids are responsible for dispersing to new host plants.
Black bean aphids reproduce rapidly, often through parthenogenesis, where females give birth to live young without mating. A single female can produce up to 30 offspring, leading to quick population explosions. In temperate regions, black bean aphids overwinter as eggs laid on primary woody host plants, such as spindle (Euonymus europaeus) or viburnum species. These eggs hatch in spring, and several generations of wingless females are produced before winged forms emerge to migrate to summer host plants.
Host Plants and Damage
Black bean aphids use various plants throughout their life cycle, switching between primary and secondary hosts. Their primary overwintering hosts are woody shrubs like common spindle (Euonymus europaeus) and various Viburnum and Philadelphus species. In spring, winged aphids migrate to many secondary summer hosts, where they cause the most noticeable problems. These include cultivated plants such as broad beans, French beans, runner beans, sugar beets, spinach, celery, potatoes, and nasturtiums.
Aphids feed by inserting their piercing-sucking mouthparts into plant tissues to draw out nutrient-rich sap. This feeding can lead to symptoms such as yellowing and curling leaves, stunted growth, and distorted shoots and pods, especially on young, tender growth. As they feed, black bean aphids excrete a sticky, sugary waste product called “honeydew.” This honeydew coats plant surfaces and provides a substrate for black sooty mold, reducing photosynthesis and diminishing crop value.
The Role of Ants
Ants form a mutualistic relationship with black bean aphids. Ants are attracted to the sweet honeydew excreted by aphids, which serves as a valuable food source for their colonies. In return for this sugary reward, ants actively “farm” and protect the aphid populations.
Ants gently stroke aphids with their antennae to stimulate honeydew production, much like milking livestock. They also guard aphid colonies from natural predators, such as ladybugs and lacewings, sometimes even removing predator eggs or larvae from the vicinity. This protective behavior allows aphid populations to thrive and expand. A noticeable presence of ants on your plants can indicate an aphid infestation.
Managing an Infestation
Controlling black bean aphid infestations in a home garden involves several approaches. For small infestations, a strong jet of water from a garden hose can dislodge aphids from plant stems and the undersides of leaves. This method should be repeated regularly until the population is reduced.
Encouraging natural predators is another sustainable strategy. Ladybugs, lacewings, hoverfly larvae, and parasitic wasps are natural enemies of aphids and can significantly reduce their numbers. Planting diverse flowering plants that attract these beneficial insects helps establish a healthy garden ecosystem.
For more direct treatment, horticultural oils and insecticidal soaps are effective solutions. These products work by suffocating aphids or disrupting their outer layer, leading to dehydration. When applying these sprays, ensure thorough coverage, especially on the undersides of leaves where aphids congregate, and apply during cooler parts of the day to avoid plant stress.