Black soap against flea beetles and aphids: how to use it?

Aphids, flea beetles and other small beasts that are greedy for plant resources are useful in our gardens, but there's no question of letting them proliferate either. When they become a little too invasive, good old-fashioned black soap can be an effective and environmentally safe natural ally. Why does it work and how do you use it?

Black soap: a multi-millennial product

Made from potash, water and linseed oil, black soap is a simple product that has been used since ancient times to clean our homes, for washing and in the garden. Totally biodegradable, it is perfectly compatible with organic and permaculture farming. What's more, it's easy to find: nowadays almost every shop stocks several brands of it.

Two versions: one pasty and the other liquid. The liquid version is simpler to use because it's easier to dilute. Above all, be sure to check the labels and the composition of the product, which must contain no additives, perfumes or preservatives.

A natural ally against aphids and flea beetles

The proliferation of aphids endangers the life of colonised plants. They feed on sap, which they transform into a syrupy, sugary substance: honeydew, pumping the lifeblood out of your young plants day after day under the close guard of a cohort of conscientious ants.

The same applies to flea beetles, small black beetles measuring just a few millimetres with blue or green highlights. The larvae are born in the roots of young plants, then colonise the leaves, which they nibble from the inside. Once adult, the flea beetles devour the foliage, leaving behind small holes with characteristic irregular edges. Unfortunately, these holes give off a scent that attracts other flea beetles, and while adult plants may resist, young shoots are in real danger. All the more so as flea beetles know how to jump when they are disturbed and so can quickly conquer neighbouring plants.

How to use black soap in the vegetable garden

Black soap is a simple product to handle. To use it in the garden, you'll need a spray bottle, water and a tablespoon. Its liquid version is dosed at 5 to 10% depending on the degree of infestation, i.e. around 3 to 6 tablespoons of soap for 1 litre of water, rainwater if possible. Spray on and under the leaves, except for solanaceous plants (tomatoes, aubergines) where the foliage must be kept away from moisture. Try to target the main stems and the base of the plant. Carry out an initial spray then repeat the operation 24 hours later, preferably in the evening.

Black soap literally engulfs small insects, which die of asphyxiation. Its effectiveness on aphids is quickly apparent. On the other hand, the product doesn't always reach flea beetle larvae, which develop well under cover, but it does get rid of the adults and reduces the pressure. For my part, after using black soap last year, many ladybirds arrived to eat the green aphid larvae present on the apple tree.