Why Are There Tiny White Flies Under My Bean Leaves?

A cloud of tiny, white, moth-like insects rising from bean foliage when the plants are disturbed — and found resting on the undersides of leaves when undisturbed — are whitefly, most likely the glasshouse whitefly (Trialeurodes vaporariorum) or, outdoors in summer, the cabbage whitefly (Aleyrodes proletella). Whitefly feed on plant sap by piercing the underside of leaves, producing sticky honeydew that falls onto lower leaves and pods and may grow black sooty mould. On outdoor beans, they are rarely as damaging as they are under glass, but large populations in a warm summer can weaken plants and contaminate produce.

Identifying whitefly

Adult whitefly are 1.5–2 mm, white, and triangular-winged — they look like tiny moths. They rest on the undersides of leaves and fly up in a cloud when disturbed. Eggs are tiny, pale, and laid in rings on the leaf underside; the flat, scale-like nymphs (immature stages) are also found on the underside. All stages excrete honeydew, a sticky, sugary liquid that drops onto lower leaves and pods. A black coating (sooty mould) on the honeydew is often the most noticeable symptom — the leaves themselves may otherwise look normal.

When to take action

On healthy, established outdoor bean plants, small to moderate whitefly populations cause little actual damage. The plant can usually grow faster than the pest can cause harm. Action is more justified when populations are very heavy (hundreds of adults per plant) — causing noticeable yellowing, honeydew drip contaminating pods, and sooty mould making produce unappetising. Wash affected pods thoroughly before use — the sooty mould rinses off easily. The main practical concern is heavily infested plants reducing neighbour-plot quality and building up a reservoir for re-infestation next season.

Control options

Yellow sticky traps hung above the plants catch adult whitefly. Encourage natural predators — parasitic wasps (Encarsia formosa) are available for greenhouse use; outdoors, various parasitoids naturally reduce populations through the season. A jet of water on the leaf undersides disrupts the colony and dislodges nymphs. If control is needed, an organic insecticide based on fatty acids (soft soap solution) applied directly to leaf undersides contacts and kills soft-bodied stages.