Aphids on Hazelnut: What to Do

Several species of aphid attack hazelnut, and a bad infestation can distort young shoots, cover leaves with sticky honeydew, and encourage sooty mould. The good news is that hazelnut aphids rarely threaten the survival of a healthy mature bush, and they have many natural enemies that bring populations under control if given the chance.

Identifying Hazelnut Aphids

The most notable hazelnut aphid is the large hazelnut aphid (Myzocallis coryli), a yellowish-green insect about two millimetres long. It feeds on the underside of leaves and rarely forms dense, easily spotted colonies — instead individuals are scattered across the leaf surface. A related species, the hazelnut-gall aphid, triggers distinctive red or yellow puckered galls on leaves in spring. Black bean aphid occasionally colonises hazelnuts too. All produce honeydew that drips onto surfaces below, leading to a sticky coating and often black sooty mould.

Damage Assessment

Light to moderate aphid infestations on hazelnut rarely reduce the nut crop significantly. The main damage is cosmetic: distorted young leaves, sticky deposits, and sooty mould on foliage and anything below the canopy. Heavy infestations on young trees or in the season when nuts are filling may have a greater impact, weakening the tree's ability to supply carbohydrates to developing kernels. Stress from drought or recent hard pruning can make trees more vulnerable to heavy aphid attack.

Natural Control

Hazelnut aphids are attacked by a range of natural enemies including ladybirds, lacewings, hoverfly larvae, parasitic wasps, and blue tits. These predators are often more effective than any spray if you give them the chance to find the colony. Avoid using broad-spectrum insecticides on or near your hazelnuts, as these kill the predators along with the aphids and typically lead to worse infestations a few weeks later once the natural-enemy population has been destroyed. Many aphid infestations on hazelnut collapse naturally by July without any intervention.

Direct Control When Needed

If the infestation is severe on a young plant or is causing significant distortion to shoots that will carry next year's crop, direct control is justified. A strong jet of water from a hose dislodges aphid colonies effectively. Insecticidal soap or plant-oil sprays can be applied to the undersides of leaves; these kill on contact but leave no persistent residue, so they are much safer for natural enemies than systemic insecticides. Treating in the early morning when predators are less active reduces collateral damage.

Long-Term Prevention

A healthy, well-grown hazelnut is more resistant to severe aphid attack than a stressed one. Avoid excessive nitrogen feeding, which produces the soft sappy growth that aphids prefer. Encourage natural enemies by growing flowering plants around your hazelnuts and avoiding pesticide use in the vicinity. Winter tar oil washes can reduce overwintering aphid eggs on the bark, though their use is now restricted in many countries.