Why Are My Lemon Tree Shoots Sticky and Curled?

When the newest growth on your lemon tree arrives distorted and sticky, with small soft-bodied insects clustered tightly along the stems and beneath curling young leaves, you are dealing with an aphid infestation. Several species attack citrus, with the black citrus aphid (Toxoptera aurantii) and the spirea aphid (Aphis spiraecola) being the most frequently encountered. Aphids feed by piercing soft young tissue and extracting phloem sap. As they feed they excrete honeydew, which coats the leaves and stems with a tacky sheen and quickly attracts sooty mould.

Identifying Citrus Aphids

Black citrus aphids are dark brown to black, around 2 mm long, and cluster densely on shoot tips and young leaf stems. Spirea aphids are yellow-green and slightly larger. Both species reproduce parthenogenetically — females produce live young without mating — so a colony can grow from a handful of individuals to hundreds within a week during warm weather. Look for winged adults (alates) during population peaks; these are the forms that disperse to new plants. The leaf curling caused by citrus aphids is more pronounced than with many other host plants because the young citrus leaves are particularly soft and responsive.

Natural Predators and Biological Balance

Outdoors, aphid populations are usually kept in check by ladybirds, lacewing larvae, parasitic wasps (Aphidius spp.), and hoverfly larvae. If your tree has been heavily sprayed with broad-spectrum insecticides in previous seasons, these predator populations may be depleted. Tolerating a small aphid presence on outdoor trees from mid-spring onwards often results in a natural population crash within three to four weeks as predators colonise. Reserve chemical intervention for severe or persistent infestations, or for trees kept entirely indoors where predators cannot arrive naturally.

Physical Removal

For a container tree or a manageable garden specimen, the fastest intervention is physical. Knock the aphids off with a strong jet of water directed at the shoot tips and leaf undersides. This will not harm the tree and dislodges the vast majority of the colony immediately. Repeat every two to three days for two weeks. Alternatively, wear thin gloves and rub the aphid colonies off by hand — unpleasant but highly effective on a small tree. Pinching out the most heavily infested shoot tips removes the highest density colonies and is worth doing before a water wash.

Insecticidal Soap Spray

Pyrethrin-based or fatty-acid insecticidal soap sprays applied directly to the aphid colonies are contact killers with minimal residual toxicity and low risk to beneficial insects once dry. Spray the shoot tips and undersides of young leaves until run-off, ensuring every aphid is wetted. The soap disrupts the cuticle and the insects die within minutes. Repeat every five to seven days for three applications. Do not use soap sprays in bright sunlight or on a dry plant, and rinse the foliage with plain water an hour after application if temperatures are high.

Cleaning the Honeydew Residue

Once the aphid population is controlled, the honeydew residue and any sooty mould coating that has developed will not disappear on its own. Wipe down affected leaves with a soft damp cloth or sponge, or wash the whole canopy with a dilute mild soap solution. This improves photosynthesis immediately and removes the substrate on which sooty mould spores continue to grow. The mould itself is not parasitic — it will not spread into the leaf tissue — but it shades the surface and the discolouration can persist for weeks if the residue is not removed.