Various species of cochineal attack citrus. They look like tiny black, brown or white bubbles or crusts stuck to small branches and leaves, sometimes in tight groups. They attach themselves to the plant to feed on the sap and cover themselves with a protective wax. When they swarm, they cause bark injuries, leaf deformations and deficiencies. They also secrete a honeydew on which a harmful fungus, fumagine, develops, forming sticky black patches and reducing photosynthesis.
White shell scale is the most common type of scale on lemon trees, forming cottony white clusters on the bark of branches.

To remove them, use soapy water, beer, alcohol or a mixture of those. Apply to infested areas with a sprayer, brush or cloth.

To avoid their appearance, the recommended essential oils are peppermint and geranium. In very low doses as always for essential oils: 5 to 10 drops of each, 20 drops of any food oil (eg sunflower) to be diluted in 1 liter of rainwater with a few drops of liquid soap.
Spray preventively in the spring, or as soon as cochineals appear. It can also be used in case of serious invasion, but always respecting the low concentrations of essential oils.

The best prevention remains the presence of predatory insects.
The mealybug's natural enemies are lacewing larvae, equipped to pierce shells and capable of real massacres. They can be found in specialized shops (for example: https://www.insectosphere.fr)

Of course, many synthetic products against cochineals are also available on the market. Their effect on beneficial insects is not always neutral.

Various species and colors: green, gray or black. They will concentrate massively on the young shoots and tender leaves. Green aphids in particular are protected from predators by ants, which feed on their honeydew. Whatever the species of aphids, the damage can be relevant. The leaves will be deprived of sap and will deform and mend. Aphids appear with the heat, their optimum being between 20 and 25 degrees. In the Mediterranean areas the black aphid is specialized in citrus fruits that it will attack as soon as spring, sometimes seriously compromising the flowering.

A simple spray of cold water is enough to make them fall and kill them, but there will always be some left in the corners. Soapy water spray works best.
Liquid Marseille soap (potassium soap), 5 ml to 10 ml per liter, with a little edible oil to help stick to branches and leaves.
Can be complemented by a repellent: nettle purin as a spray.
Never apply undiluted, but dilute in rainwater at a rate of 1 liter of purin for 10 liters of water (10%). It is smelly but not dangerous.

Alternatively, Neem oil, 5 ml per liter of water, and Marseille black soap, same proportion.

To prevent the arrival of aphids, you can intervene as early as winter: by applying anti-ants glue on the trunk, by applying whiote lime with a brush on the trunk and the low branches (often enriched with trace elements and essential oils). In spring, the best prevention is to let auxiliary predatory insects such as ladybugs, bugs and lacewings proliferate.

As for mealybugs, the essential oils recommended against aphids are peppermint and geranium, with the addition of bitter orange oil (harder to find). Pour 5 to 10 drops of each essential oil, add a spoon of edible oil. Mix in a small bottle, then dilute in 1 liter of rainwater, add a little liquid black soap or dishwashing liquid.
To be sprayed preventively or curatively. Even in the case of a massive attack, never increase the concentration of essential oils. Spray preferably in the morning or evening, and a second time the next day if necessary. Alternate with nettle manure if necessary.

A variety of synthetic and certified organic aphid control products are available, ready-to-use and easy to apply.
Try to avoid those that are also harmful to aphid predators, and could leave the plant without natural defenders the next time it is attacked.

The citrus leafminer is a very small moth (4mm) that will lay eggs in summer and fall on the youngest leaves, often depositing the eggs near the midrib. When they hatch, the larvae penetrate the leaves and young shoots and dig galleries inside. The leaves will curl, turn yellow and dry out.

Since the leafminer only attacks the most tender leaves and shoots, one method of preventing its spread is simply to cut off the damaged leaves or tips. Do not discard these leaves in the compost that will be used for citrus the following year.

The leafminer rarely spreads widely, and the damage it can cause is generally fairly limited.
If, however, it has caused you too much trouble during the summer and autumn, you can anticipate pruning the following winter (January) and cut back the branches more severely. This will accelerate spring regrowth and the leaves will already be too tough in July, when the moth arrives, for the larvae to penetrate.

Systemic insecticides are sometimes used against leafminers in large citrus operations. This scorched earth approach is motivated by important economic issues

It is only rarely observed in Switzerland where it does not succeed in settling permanently because of the too rigorous winters. The flies, probably introduced sporadically by the importation of Mediterranean fruits, are versatile and attack seasonal fruits. In autumn they arrive on persimmons and citrus. Their larvae spoil and render unusable the fruit they attack, sometimes in massive quantities.
In the event of alerts, which are very rare for the moment, specific pheromone traps are very effective and inexpensive if the number of trees is limited (one trap per two young trees, one per adult tree).

Systemic insecticides are used against it in areas of commercial mass cultivation with the usual collateral effects on beneficial insects.

Specialized in citrus fruits, of all varieties, the mite (Eutetranychus orientalis) settles especially on the upper face of the outer leaves, avoiding the new shoots. But it will also feed and sting the fruits and sometimes the young branches. It will spread easily from tree to tree on silky filaments carried by the wind. It can appear in all seasons (except winter).
Affected leaves will turn silvery and then yellow. This can lead to a significant loss of leaves in case of outbreak and compromise the harvest.

It can be eliminated and its propagation stopped by spraying vegetable oil (sunflower, colza, cotton) or mineral oil of parrafine. In the spring you can simply water with cold water to slow it down. This is generally not very effective in summer, unless you can use fairly cold water (5 degrees).
Research carried out in Morocco and published in 2021 tested the effectiveness of thyme essential oils applied to the leaves, with positive results on adults, larvae and eggs(https://www.tandfonline.com/doi/abs/10.1080/0972060X.2021.1927854).
There is also a specific antagonistic mite (Amblyselus californicus), which is readily available in Switzerland and appears to be particularly effective.

The synthetic miticides most commonly applied on large farms are not approved for use in home gardens. Some are authorized , however, on the basis of their lower toxicity to humans. They eliminate all mites without distinction

It is a small gray butterfly (Prays citri) of one centimeter, present all year long in the infested areas, which will reproduce as soon as the temperature reaches 25 degrees. Its larvae are white and tiny when just hatched, reaching a maximum of 5 to 6 mm. They are voracious and attack the flower buds but also the fruits at all stages. Each moth can lay more than 100 eggs.
Affected flowers will drop, immature fruit will abort and mature fruit will be deformed and unpalatable.
In Switzerland, the moth is not widespread, but it may appear in summer, except in the case of a very hot spring or autumn.
The moth shows a clear preference for lemon and lime trees.

Its predators are insects, in particular the earwig, but also birds such as chickadees and other passerines. Encouraging their presence is a preventive measure, for example by installing nesting boxes for small birds, and mulching at the foot of the tree for insects.
There are also specific pheromone traps that will eliminate the males, available from specialists.

Commercial systemic insecticides used on professional farms are generally not allowed in home gardens.

It is a large butterfly (Zeuzera pyrina) of 5 cm, white and hairy and with black-blue spots. It can attack many fruit trees, such as cherry, apple, pear, including citrus. Its larva is a large caterpillar, yellow with black spots, which can reach 4 to 5 cm and appear in summer, from June to August. It will fovud on the main branches and dig galleries that go up into the wood. Damage can besignificant, especially to a young tree.

The tiny caterpillars can spread from tree to tree carried by the wind on a silk thread. The adult caterpillar can live and burrow its way into the tree for two years.

If you see galleries, remove the larvae by pushing a wire to the bottom of the gallery.

To fight against the infestation use specific pheromone traps, which can be obtained from specialists.