The war on slugs starts at home
This DIY slug killer taps into the power of a natural soil predator, says Toby Buckland.
Article reproduced from the Telegraph, Saturday 30 July 2011
Thanks to the phenomenally dry April, gardeners haven’t had much of a problem with slugs this year.
It’s only now, with the wet weather of July, that numbers are on the rise. In summer,
it takes about 10 days for the clusters of tiny, pearl-like eggs to hatch, so more slugs are definitely on the way.
There are 29 species of slug in Britain, but just four give the rest a bad name: the common garden slug, a leathery gunmetal
grey and the length of your little finger; the fat, sickly grey field slug; the black slug, the biggest (and sometimes rust coloured)
and keeled slugs, which have a ridge along their backs and a taste for potatoes.
Snails do their fair share of damage too, but, while snails chew the edges of foliage and open up the holes already made by slugs,
it’s the slugs’ rasping mouth parts that scrape away and puncture the surface of foliage and turn your hostas into doilies.
Their other calling card is slime. The sparkling silver trails they leave behind aren’t just a way of getting around.
Slugs read them like Braille, to find a mate and new chomping grounds.
When they encounter few trails — as in our gardens following the dry spring – the slugs take it as a signal that there is
virgin territory to be plundered. Where the matrix of slime is dense, however, it’s a warning to disperse and lay eggs
where there is less competition for food and less risk of being attacked by predators. Slugs are prey to not just frogs,
hedgehogs and birds but microscopic bacteria and nematodes that live in soils.
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.
Collect as many slugs as you can find in a jar that has a few small air holes punched in the lid with a hammer and nail –
and a few weed leaves for them to eat. 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. 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.
Prevent the weed and slug mixture from falling into the can with a filter of chicken wire folded over the can so it stays put while you pour.
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.