Greenhouse aphids: identification and control

Identify the colony, not just the symptom

Aphids are small, soft-bodied insects that feed on plant sap. Green, black and other forms can occur under glass. Look first at new leaves, shoot tips, buds and the undersides of leaves. The RHS aphid guide describes curled or distorted growth, sticky honeydew and sometimes dark sooty mould growing on that residue. Whitefly can also leave honeydew, while mites cause a finer stippled pattern, so finding the actual insect matters.

Inspect several plants in the same crop and nearby weeds. A few insects on one shoot are not the same as colonies on every plant. Winged aphids may arrive from outdoors or develop when a colony becomes crowded. Recording where the first colony appeared helps you decide whether a new plant, an overwintered host or an open vent is part of the pattern.

Make a proportionate first response

For a small, accessible colony, the RHS recommends removing insects by hand. Pinch out a badly infested soft tip only where the crop tolerates it; do not remove the growing point from a plant that depends on it. Check again after a few days because eggs or missed individuals can restart a colony. Wipe honeydew from sound, reachable surfaces with water, but do not scrub delicate leaves.

Do not treat every aphid as a crisis. Aphids are food for ladybirds, hoverfly larvae, lacewing larvae and parasitoid wasps. The RHS guide to aphid predators notes that purchased biological controls can also be used under glass. A broad spray can harm those natural enemies and undermine a release. Our biological-controls guide explains when a targeted introduction is worth considering.

Follow the crop and colony through time

New pepper, cucumber and tomato growth deserves regular inspection, especially when plants are crowded and predators cannot easily reach colonies. Turn leaves gently, inspect around flowers and move nearby weeds or spent growth that shelter insects. Bring bought plants into a separate observation area if possible before placing them among established crops. The greenhouse hygiene guide covers how to keep problem material separate.

Healthy plants may tolerate some feeding, but heavy colonies can distort growing points and contaminate fruit with honeydew. Aphids can also transmit plant viruses. If a plant has unusual persistent mosaic or severe distorted growth, identify the plant problem rather than assuming killing aphids will cure already infected tissue.

Be careful with products marketed as control

Do not choose a pesticide solely because it is sold for garden pests. UK authorisation is product- and use-specific. The HSE garden-pesticide guidance says to check the authorised-product database and read the exact label, including crop and harvest restrictions. A product described as “natural” is not automatically selective or permitted for a food crop. Start with identification, removal and inspection; use a current authorised product only if truly needed and if its precise label covers the situation.

If the insects are white and fly when disturbed, use the whitefly guide instead. If fine webbing and pale stippling dominate, check the red spider mite guide. Distinguishing these problems saves unnecessary treatment and helps preserve useful wildlife under glass.