Why are the leaves on my buddleia curled and yellow?

The leaves on buddleias across the land have been showing curled, yellow symptoms which have left gardeners puzzled. The butterfly bush or buddleia, Buddleja davidii, is a beloved feature in UK gardens, famed for its fragrant, nectar-rich blooms that attract clouds of butterflies and bees through the summer months.

However, this usually hardy and resilient shrub can sometimes fall victim to aphid infestations – and now, most notably the new kid on the block, is the melon-cotton aphid Aphis gossypii, a pest that’s increasingly found on buddleia as well as many other garden plants.

The melon-cotton aphid is a small, soft-bodied insect, typically green, yellow, or occasionally black. It’s best known for attacking, perhaps unsurprisingly, cucurbits (like melons and cucumbers) and cotton plants, particularly in areas with warmer climates than those in the UK.

However, this highly adaptable species can colonise a wide range of ornamental plants including, unfortunately, buddleia. I have also seen the same aphid on cardoons and globe artichokes, as well as a number of other garden plants. These aphids reproduce rapidly in warm weather, forming dense colonies on young shoots, the undersides of leaves, and even flower buds. Not only can they lay eggs – particularly useful in the autumn to allow the insects to overwinter – but they can also give birth to live young during the spring and summer.

The babies quickly grow into mature adults that start to produce their own offspring in just a few weeks. It’s not surprising, therefore, that their number can soon get out of hand!

Aphids feed by piercing plant tissues and sucking out sap, and injecting toxins into the cells which cause growth to be affected and which can have several negative effects on buddleia.

Heavy feeding by aphids drains the plant’s energy, resulting in weak, stunted shoots. The toxins which they inject into the tissues cause the new growth to be distorted or curled, and the overall vigour of the bush is reduced.

As aphids extract sap, the leaves in particular, may curl as they contain less moisture which helps them to support their weight. The removal of the sugary sap also deprives the leaf of energy it requires to stay alive and support cell division. The leaf may turn yellow – either entirely or in part – and drop prematurely, diminishing the buddliea’s lush appearance and reducing its ability to make its own food from sunlight by photosynthesis.

With less energy in the plant in the form of sugar, the plant may be unable to make new growth and produce fewer flower buds. These may also be distorted, particularly if the aphids are feeding directly on them. Buddleia stressed by aphid infestation may produce fewer and smaller flower spikes, limiting its value as a pollinator magnet and garden highlight.

A secondary problem is the production of honeydew and infection with sooty mould fungus. Aphids excrete a sticky substance called honeydew, which derives from the sugar which they suck from the plants on which they are feeding. This honeydew drops down onto the foliage of the infected plant, as well as that of any other plants beneath it. This sticky coating them attracts the microscopic spores of sooty mould fungi, which develops into unsightly black patches on the foliage. This, in turn, blocks light from the leaf surface and further reduces photosynthesis and plant health.

The sugary honeydew also attracts ants directly to the aphids, which are ‘farmed’ by the ants. They may protect aphid colonies from natural predators, making infestations harder to control.

While chemical sprays are available to control aphids, many gardeners prefer to avoid them to protect pollinators, maintain organic principles, or simply because non-chemical options can be highly effective. There are some useful ways to manage melon-cotton aphids on Buddleia without resorting to pesticides, although bear in mind that you may not get rid of them altogether. Aphids are, after all, part of the natural food chain to provide for birds, insects and other wildlife.

Ladybirds, lacewings, hoverfly larvae, and tiny parasitic wasps are all natural enemies or predators of aphids. Attract these helpful insects by planting a diversity of nectar-rich flowers nearby and avoiding broad-spectrum insecticides that could harm them. A healthy garden ecosystem is your best long-term defence. Many small garden birds, such as tits and sparrows, also feed on aphids. Installing bird feeders and nesting boxes attracts these natural pest controllers to your garden.

Meanwhile for light infestations, simply squashing aphids with your fingers or brushing them off with a soft paintbrush can be effective. For larger colonies, a strong jet of water from a hose will dislodge many aphids – just take care not to damage the young shoots and flowerbuds of your plants. If aphids are concentrated on the tips of shoots or on specific branches, prune off and dispose of the affected parts. This not only removes the pests but also encourages healthy new growth.

Some gardeners find that interplanting Buddleia with strongly scented herbs like garlic, chives, or mint can help deter aphids. While not a guaranteed solution, companion planting can contribute to a more resilient and diverse garden environment. As a last resort, a mild solution of pure soap and water (never detergent) can be sprayed onto affected areas to suffocate aphids. Always test on a small section first, as even gentle sprays can sometimes cause leaf scorch. Use sparingly and only when necessary, as soap sprays can also affect beneficial insects.