It’s these nematodes (microscopic eelworms) that gardeners have been buying as a form of biological control since the early Nineties. They really work on those slugs that you don’t tend to see, but which do a lot of damage to underground shoots and potatoes. In a garden, micro-predators live in symbiosis with their slug hosts and only significantly dent the population when slug numbers become disproportionately high.
The mail-order sachets of nematodes infected with deadly mollusc-killing bacteria temporarily raise the proportion of nematodes and brings down the slug population. I’ve been an advocate for years.
However, there is also an allotment-owner’s trick for making your own slug-killing nematode potion, using nothing more than a bucket, some weeds, tap water and the slugs from your own garden. If you are already used to killing slugs by drowning them in a bucket, you’ll find this method right up your street.
How to make your own slug killer;
In any average garden some slugs will be carrying bacterial diseases or be infected by nematodes, but their low density means that they won’t devastate the rest of the population.
But, catch and confine the slugs and, if the disease or nematodes are present, you can concentrate these micro-predators and harness their natural slug-killing power.
If stumbling around with a torch is a bridge too far, look for slugs during the day in the drainage holes of pots, beneath stones and hunkered in long grass. If they evade your efforts, set traps. A classic that works brilliantly for hard-to-find small ground-dwelling slugs is to place the scooped out half-shells of grapefruits near the crowns of vulnerable plants.
Come dawn, the slugs make for the damp yellow domes, as they love to chew the pith inside. Slugs also make a beeline for cardboard. Lay a sheet on the ground among long grass. Check your traps daily and gather your slimy harvest into a jar.
Once you have caught around 10 to 20 slugs – the more you have the better it works – decant them into a bucket with an inch or so of water in the bottom for humidity and a few more handfuls of leaves to make an edible floating island for your catch.
With the slugs safely inside, place a concrete slab (or any firm cover) over the top to seal them in. The bucket is the perfect environment for the nematodes and bacteria to breed. Nematodes spread in water, so check regularly, giving the slugs a stir with a stick. The idea isn’t to drown them but to keep them moist so the nematodes can hunt them out.
Top tip: This is cheating a bit, but you can use a bought pack of nematodes to “seed” the brew. Tap about a teaspoon of powder into the bucket to help it along.
After a fortnight a high level of nematodes will have built up inside the bucket and the slugs will have died from infection. Now, you can dilute the brew: fill the bucket to the top from the tap and decant into a watering can fitted with a rose.
Water the sieved brew around vulnerable plants – the raised nematode population will seek out resident ground-dwelling slugs and see them off.
Like the shop-bought version, this slug killer gives up to six weeks of protection. Save the contents of the chicken wire sieve (uurrgh!) to start off your next nematode brew.
Once Nematodes find the Slug they suck the life out of it.