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There are several species, the most well known in Spain being red scale, melon aphis and cotton aphis, of which there are different families. They work by extracting the sap from the plant using their sucking mouthparts. They expel part of the sap in a kind of treacle, except the red scale species which does not secrete any sweet liquid. 

If the leaves of the plant turn yellow, discolour or become deformed, if they become sticky due to the treacle or a black fungal growth develops (on top of the treacle), you will have to keep an eye on the plant until you discover the woodlouse. It may also indicate the presence of other insects, such as plant lice or whitefly, which cause similar damage. It is worth studying the plant carefully in order to identify the pest type. The woodlouse can be distinguished by its shell, which has a different colour depending on the species. 

In order to fight this pest you can use insecticides or natural treatments. The latter include beneficial “pests” such as butterflies and vedalia ladybirds, common woodlouse predators. 

However, in order to prevent it, the most effective method is to use chemical products designed for larvae. You spray the plants twice, firstly at the end of spring, and then again three weeks later. In this way it is possible to get rid of the insect, but if it is not successful, the best thing to do is to treat it with specific insecticides which kill the woodlice and chemicals which work on the black fungal growth. Both actions are complemented by removing the affected parts of the plant and by moving the plant pots and window boxes to cooler places, where the insect is not as comfortable as in a sunny, warm garden. 

Fruit trees are the favourite targets of specific species such as the San José louse and the California louse, two common pests in Spain, which attack the trunk, branches, leaves and fruit, and can even kill the tree if they are not killed when they are larvae. Where fruit trees are concerned, vigilance and prevention are essential, treating them in the winter so as to prevent subsequent attacks in the summer. Since there are many different varieties of this species of pest, it is recommended that you consult a garden centre expert about the best solutions and insecticides for the plants and trees you wish to protect. If the plant is already damaged, you can show the garden centre professional a leaf so that they can accurately identify the type of pest. 

Whitefly
Like the plant louse and the woodlouse, it sucks the sap from the plant leaves using its beak and secretes a sugary liquid or treacle on which the black fungal growth appears. As such it damages the plant twice, it removes its vitality by stealing part of the sap required for its growth and brings it an unpleasant guest that darkens the leaves and impedes the photosynthesis process.  

The most well-known species of whitefly are those which attack in greenhouses, targeting cabbage and orange trees, and those which attack ornamental plants and various fruits and vegetables. It can be detected by the yellowing and discolouration of the leaves, which end up falling off. You can also detect the stickiness caused by the treacle.

Natural treatments for getting rid of whitefly are quite effective. Applying them with infusions of products used in organic farming, washing the plant with soapy water, and hanging sticky yellow sheets in greenhouses so that the insect sticks to them, having been attracted by the colour, are perfect solutions which usually get rid of the problem.  

You can also rely on the predators of this species, such as Cales noacki and Encarsia Formosa wasps, two parasites which have been used for decades to control whitefly on all sorts of plants, and which are particularly effective for the species which strikes in greenhouses. 

Insecticides are another alternative. It is best to apply them as soon as you detect the presence of the insect, spraying them according to the manufacturer’s instructions. It is advisable to remove weeds from the garden, as they are the best hiding place for this pest.