There was a scare last weekend that gardeners would be prosecuted if slugs can be shown to feel pain. As far as most gardeners are concerned, the problem with slugs is that they don’t feel enough pain: compared to the exquisite torment suffered by the cauliflower which is eaten cell by cell, dissolved in acid slug drool over a period of weeks, the brief horror of sprinkling salt is merciful. But this is not the time to ask who in this discredited government will speak up for the cauliflower. The question is whether there is any truly humane way to deal with the animals that want to eat the plants which we would also like to eat, or even to admire.
If we are not to use man-made poisons, the answer favoured by progressive gardeners is some kind of natural remedy. The Royal Horticultural Society recommends “nematodes” or eelworms, which sounds wonderful to people who don’t know much biology. In fact, the methods by which nematodes make their living would make Saddam Hussein blench, and this applies with special force to the gardeners’ friends, “Heterorhabditis bacteriophora”:google and “Phasmarhabditis hermaphrodita.”:google Many nematodes — like most living things — are parasites. There is no nook or cranny of the ecosystem that has not been exploited by nematodes: three different species have evolved to live in the rectum of [“American cockroaches.”:google: periplaneta.americana]
The parasites cause all sorts of unpleasant diseases in humans, as well as animals. But the gardeners’ friends, like “c.elegans,”:http://www.thewormbook.com the scientists’ favourite, eat only bacteria. What makes them sinister is that they are gardeners of a sort themselves. The bacteria these worms like to eat must themselves be nourished, and what they like to be nourished on is insect flesh.
All the creatures involved in these stories are minute. They have, quite literally, no brains at all. You can count each one of their 300 or so neurons. Yet they work to a plan of astonishing subtlety. _H.Bacteriophora_ has a pouch in its gut, like a larder, where it stores the spores of its favourite food, _Xenorhabdus_. This is a bacterium of antisocial habits: it secretes an antibiotic which poisons other bacteria. So, you might say, it deserves to die. But to _H.Bacteriophora_ it is as dear as a carrot is to a gardener; and wherever the tiny creature crawls, it takes a supply in its pouch.
Sooner or later, one of these infinitesimal worms will crawl into a caterpillar, usually through the breathing slits. Once there, it is sick, expelling the bacteria from its pouch, and they set to work, eating the caterpillar from the inside. While the caterpillar is being transformed into mass of succulent bacteria _H.Bacteriophora_ eats greedily, pausing only to fertilise itself and give birth to several hundred children. A few bacterial spores are stored again in the pouch, and the nematodes crawl out in search of new victims.
The same system is used by the nematodes which eat slugs, as encouraged by the Royal Horticultural Society. The RHS web site speaks coyly of “a disease” consuming the slugs, but it is really the bacteria which nematodes carry around with them that do the consumption. If anyone really cared about the welfare of slugs and other creepy-crawlies, they would make the use of poisons mandatory, and forbid gardeners to keep nematode worms in their gardens. Ah, but then we would need a human way to kill all the nematodes – perhaps a human way to kill all the bacteria. It begins to look as if the only way to produce a garden with no cruelty at all in it would be to cover it entirely in sterile concrete.