A typical aphid found wherever apple cultivation is widespread is the woolly apple aphid. It is an insect with a very particular life cycle, exclusively occurring on apple trees. To preserve the health of the tree and, consequently, fruit production, it must be kept under control. Infestations can be eliminated using products approved in organic farming; the important thing is to understand how and when to intervene.
In this article, we will discuss agronomic techniques useful for preventing attacks by this apple tree parasite, also known as Eriosoma lanigerum.
Description of the Woolly Aphid
Eriosoma lanigerum is the scientific name of the woolly apple aphid. It is an insect of the Hemiptera order, Aphididae family. As an adult, it does not reach large sizes, stopping at 2 mm in length. It has an elliptical shape and a variegated coloration ranging from brown to violet. However, its appearance is disguised by the white, waxy, and woolly secretions it produces, covering the entire colony. Therefore, recognition is done by observing these whitish, filamentous formations on the tree, hence the common name woolly aphid. The woolly wax is produced after its molt and serves the insect to protect itself from parasites and predators. So, in the early stages, the woolly aphid appears as a nymph, even smaller in size and with a pinkish color.
Damage Caused by the Woolly Aphid on Apple Trees
The woolly apple aphid establishes its colonies on the vital organs of the apple tree and affects all varieties. It can be found on branches, roots, and small crevices of the trunk or secondary branches. If the tree has previously been attacked by other insects, such as the red-banded leafhopper, aphids have an easier time finding spaces to develop their colony. Through trophic nutrition activity, this parasite pierces the plant, extracting vital sap. This causes severe direct damage, resulting in cortical cracks, swellings, and tissue deformations. An apple tree attacked by this aphid shows signs of distress and deterioration, especially if infestations occur over many seasons. Eriosoma lanigerum also causes indirect damage to the tree. The wounds it creates become the entry point for dangerous fungal diseases, such as apple scab.
The woolly aphid completes its entire life cycle on the apple tree, with infestations not affecting other fruit trees. Reproduction of the parasite is asexual and occurs through parthenogenesis by females. It overwinters as a nymph with the arrival of cold weather in December and resumes its activity in the spring months. From April to November, it can complete up to 20 new generations, making it a highly prolific species. During the summer months, winged forms of the woolly aphid develop, allowing them to move to neighboring apple trees and spread the infestation.
Preventing Woolly Apple Aphid
The woolly apple aphid can be prevented by managing the orchard effectively. We recommend:
- Pruning to avoid too many entry points for the parasite;
- Rational irrigation and nitrogen fertilization to prevent excessive vegetative growth, which, in turn, favors the parasite’s development;
- Managing the soil with the cover crop technique to encourage the presence of beneficial insects.
How to Eliminate Woolly Apple Aphid
The woolly apple aphid can be eliminated through biological methods involving the introduction of its natural enemy into the orchard or by using specific biological treatments.
The Natural Enemy
The natural enemy of the woolly apple aphid is the Aphelinus mali, a tiny parasitic wasp capable of parasitizing the aphid and naturally keeping it under control. During the cold season, the natural enemy overwinters inside the woolly aphid, which is found dead in spring. If a colony has been parasitized by the antagonist, it is easy to spot. Insects appear darker and have less waxy covering. If there are colonies of woolly aphids in the orchard parasitized by the parasitic wasp, it is not advisable to take any action. The beneficial insect can effectively control infestations, and intervening with treatments, even biological ones, would hinder its work.
Several organic farming-approved treatments against the woolly apple aphid are available. It is essential to note that these should only be carried out where there is no presence of the natural enemy. The best time to intervene is in the fall/winter. It involves localized sprays with: pyrethrum (100 g per 100 l of water); mineral white oil (250-500 g per 100 l of water). After one week from the intervention, you can perform a wash using soft potash soap to leave the plants free from aphid residues.