The virtues of nettle manure under the microscope

Nettle manure, used by our grandparents when pesticides did not exist, is back in fashion. Its scent is supposed to deceive pests, a grail that our grandparents would have underestimated when they discovered the usefulness of pesticides.

Nettle manure is part of a list of products known as "Natural Preparations of Little Concern". These products are said to have no phytopharmaceutical action, but to have bio-stimulant properties. In July 2014, these preparations were the subject of a simplified approval regime for their use and marketing following the action of several associations. An annex to the order of 30 April 2016 establishes a list of plants (1) including chamomile, mint, lemongrass, clove, tarragon, nettle ... which "may be sold by persons other than pharmacists"; a godsend for all those who wish to exploit this economic niche. As for their real effectiveness! Where are the serious scientific studies validating their properties in agriculture? If these products are really effective, why don't farmers use them when they have been known for a long time, rather than investing in very expensive phytosanitary products?

In organic farming, nettle manure is often recommended to prevent all kinds of pest invasions on both vegetable crops and fruit trees. It is a repellent that avoids the use of synthetic pesticides.

I have sometimes used nettle in the form of a decoction or herbal tea. The results were always disappointing. One of these trials ended with a surprising finding. A trial of nettle manure on shallots to prevent the onion maggot did not have the desired effect. However, I was surprised to find adult aphids on some of the shallots a few days later, which were not present before the nettle manure treatment. The possible reasons for this aphid invasion are simple to understand. Either the nettle manure is unable to keep out an already established pest that I would not have noticed, or it attracts them. Like all other plants, nettles are victims of polyphagous pests, including the shallot aphid, which I did not know at the time (2). It is strange to claim that nettle manure can keep pests away when in nature, nettles attract them!

The shallot aphid can indeed find other hosts when it does not find its favourite food like other nettle pests: this is the case of the Arctia caja caterpillar (Scalille Martre) which is fond of nettles and is known to cause damage in vegetable gardens. If you grow sorrel, you should know that it can be attacked by the amptogramma bilineata (golden brocatella), a butterfly that also likes nettles. Several species of common wireworms that are regularly found as adults on nettle foliage are polyphagous and are often harmful to crops (2).

As far as I know, there is no scientific study that has established that the attractive powers of nettle towards all these pests would have disappeared after maceration in water to produce a slurry that would have an opposite effect. However, pests have a keen sense of how to find their hosts from substances emitted by them in very small quantities.

So if you think it is worth signing a contract with the devil to cure your plants, use nettle manure.

Nettle manure is also known to fortify plants and strengthen their natural defences, as it contains a lot of nitrates. It is a kind of foliar fertiliser. But this is also true of all highly fermentable, nitrogen-rich plant products that can be macerated in the same way, such as lawn clippings.