Aphids: Identification and Control in Scottish Gardens

“Planting marigolds among vegetables and roses repels aphids”

French marigolds release limonene, a compound shown to deter whitefly and thrips on tomatoes, and marigold roots can suppress soil nematodes. Neither of those documented effects is about aphids. A direct expert answer on the specific question of marigolds and aphids states there is no documented study showing marigolds keep aphid problems away, and that it is questionable whether the foliage protects against aphids at all.

Our read The confusion likely comes from generalising one real marigold benefit (whitefly and nematode suppression) to pests it was never tested against.

What to do: Grow marigolds for their proven benefits, deterring whitefly and thrips and feeding pollinators, but plan a separate approach for aphids rather than treating marigolds as a shield.

“Seeing ladybirds in the garden means aphids are already under control”

Ladybirds and their larvae are genuine aphid predators and can wipe out established colonies by early to mid-summer. But aphid numbers build up earliest in spring, before predator populations are active in sufficient numbers to keep pace, so the presence of a few ladybirds does not mean an early infestation is already being handled.

Our read The gap matters most on soft new growth in spring, exactly when gardeners are most likely to spot the first aphid colonies and assume predators will deal with it unaided.

What to do: Treat ladybirds as a helpful, free-running control that strengthens through the season, not an immediate fix, and check spring growth regularly rather than assuming predators have it covered from the start.

“Spraying aphids with washing-up liquid diluted in water is a safe way to kill them”

Aphids are genuinely among the pests most susceptible to soap sprays, since the fatty acids in soap disrupt their cell membranes and dehydrate them. But that effectiveness data comes from commercial insecticidal soap, a refined product, not household washing-up liquid. Dish and laundry detergents contain additives that a US university extension factsheet describes as far too harsh for plants, with a substantially increased risk of leaf damage compared with proper insecticidal soap.

Our read The soap kills aphids either way; the safety part of the claim is the false element, since dish detergent's extra additives are what put the plant at risk.

What to do: Use a proper horticultural soap product rather than kitchen washing-up liquid, and if using dish soap anyway, test it on one or two leaves first and wait several days to check for scorch before spraying the whole plant.

“Aphids only attack weak or stressed plants, so a healthy plant is naturally safe from them”

Aphids target soft, new plant growth rather than weakness as such, and a US extension pest note links high nitrogen fertiliser directly to bigger aphid populations because it produces exactly the lush, soft growth aphids feed on best. A peer-reviewed study on wheat found the opposite of the stress theory for at least one common stressor: aphid reproduction and population growth actually declined as water stress increased, because drought triggered stronger plant defences rather than making the plant an easier target.

Our read If anything, vigorously growing, well-fed plants can be more attractive to aphids than a stressed one, the reverse of the popular claim.

What to do: Do not rely on plant vigour as aphid protection; instead avoid over-fertilising with nitrogen, especially in fast bursts, since that is what reliably drives aphid numbers up.

“Aphid honeydew directly causes sooty mould to grow on the plant itself”

Sooty mould does not feed on or infect plant tissue. It grows on the sugar-rich honeydew that aphids excrete onto leaf surfaces, using it purely as a food source. The mould still harms the plant, but indirectly, by forming a dark coating that blocks light and reduces photosynthesis rather than by attacking the plant directly.

What to do: Wipe sooty mould off with a damp cloth if it is heavy, but focus control effort on the aphids producing the honeydew rather than treating the mould itself as a plant disease.