Looking at the apple cordons planted in our kitchen garden, once the leaves have fallen, we have sometimes discovered whole branches covered with a blanket of cotton as white as snow. Colonies of woolly apple aphids have invaded our fruit trees. This is very worrying, as it is difficult to get rid of this invader from North America, introduced accidentally into Europe at the end of the eighteenth century. It is mainly found on apple trees, occasionally on quince trees and very rarely on pear trees, as well as on several ornamental species.
The insect forms large colonies on young twigs, branches, the trunk and sometimes even the roots. It hibernates in the form of a larval or adult female, often in cracks in the apple tree’s bark. It comes back to activity towards the end of March and continues its development above 7°C. The females do not need to be fertilised and do not lay eggs, but give birth to over a hundred young larvae. Eight to twelve generations follow between spring and summer. Like other aphids, the woolly apple aphid feeds on sap. It extracts it from woody parts and young shoots, but never from the leaves. Heavy attacks lead to cankers, which can be infected by fungi, causing apple canker. This can cause significant deformation and generally weaken the tree.
The first step to take to combat woolly apple aphids is to avoid buying particularly sensitive apple varieties. These include “Reine des Reinettes”, “Reinette du Canada” and “Belle de Boskoop”. Ask at the tree nursery before buying.
For apple trees that are already in place and infested, I recommend the programme we have established in the Villandry gardens:
- In November, as soon as the leaves fall, use a paraffin-based mineral oil also known as white oil. This covers the hibernating forms of parasites (eggs, larvae and adults) and obstructs their respiratory tracts, preventing gaseous exchange and causing death by asphyxiation. Spray the product, diluted in water, all over the tree, focusing particularly on cavities in the bark. It is available from gardening suppliers. Solabiol, for example, markets a product called “Traitement d’hiver” (winter treatment), which is used at a concentration of 25 millilitres per litre of water. I recommend repeating this operation two or three times during the winter, if weather conditions allow.
- In spring, when the buds break through, treat again with white oil or with a rapeseed oil containing pyrethrum, a natural insecticide found under the name of Spruzit from Compo. This product is harmful to bees, so use it in localised areas outside the flowering period.
- Later in the year, when the leaves are established and it is hard to see the colonies of woolly apple aphid, we spray a mixture of black soap, methylated spirit and water. Here is the composition of this cocktail:
For 10 litres of water, add 1.5 litres of methylated spirit and 100 millilitres of black soap. Spray on the interior branches, focusing on the aphid colonies. The diluted methylated spirit will have no effect on the leaves. Once again, there is no miraculous remedy, so keep an eye on the effectiveness of the treatment and repeat if necessary.
To avoid all this labour and maintain healthy trees producing beautiful fruit, we can also enlist the help of beneficial insects such as ladybirds and hoverflies, which should be encouraged in our gardens. Let us stop spraying pesticides and instead plant mixtures of flours at the feet of our trees to protect the microfauna that are so helpful to gardeners.