Here is 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.
Collect as many slugs as you can find. The best time to hunt for slugs is after dark. In the gloom, slugs become quite brazen and eat on top of leaves as opposed to holing up in cool, dark and damp places as by day.
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.
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 several handfuls of leaves to make an edible floating island for your catch.
With the slugs safely inside, place 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. Keep the sludge and leaves to start your next batch.
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.