I will dispense with false modesty and tell you that my garden grows some fantastic potatoes. This success hasn’t come without effort. I’ve worked on my soil for years, mulching it with compost, leafmould and grass clippings, and growing green manures to protect the soil and add nutrients. Over time I’ve transformed my heavy-to-work clay into the rich, crumbly loam that supports the production of big, beautiful potatoes. There’s only one thing standing between me and a truly spectacular crop of spuds: slugs.
It’s an issue that recurs year after year. Most tubers look okay apart from a small hole or two, but cut them open and you’ll find tunnels bored through. In some cases the potato is virtually hollow – and teeming with slugs! The most annoying thing is that slugs seem especially attracted to the larger potatoes, so what initially looks like a great crop turns out to be a very disappointing one.
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Potatoes can be completely hollowed out by slugs
Wireworm in Potatoes
While slugs are the obvious culprit for holey potatoes, we mustn’t jump to conclusions. Slug entry holes look very much like those made by wireworm, the larvae of the click beetle. The slugs that are known to tunnel into potatoes belong to the keeled slug fraternity (Milax gagates, Tandonia budapestensis and Tandonia sowerbyi). However, the only slugs I’ve ever found in my potatoes are striped specimens that to my eye appear to be dusky slugs (Arion subfuscus). This leads me to suspect that the holes are being made by wireworms and slugs are simply taking advantage of the ready-made access.
I created my vegetable garden out of a patch of rough grass – prime egg-laying habitat for click beetles. Wireworms feed in the soil for four years before transforming into click beetles. They’ll then head over to grassy areas to lay their eggs, so if your vegetable garden is surrounded by lawn, as mine is, there’s a good chance that wireworm will be a perennial problem.
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Wireworm often cause small holes that slugs then use to gain access to potato tubers
There is a biological control that can be used for wireworm in the form of Heterorhabditis bacteriophora nematodes, but it’s pricey. A more frugal method of control is to set wireworm traps. Dig several holes, 10cm (4in) deep, then plant half a potato or a chunk of carrot. Backfill the holes with soil and mark their locations with sticks. Before planting your potatoes, dig up the traps and pop any wireworm you find onto a bird table.
Before planting it’s also worth forking over your soil to expose wireworms and slug eggs to birds, or if you keep chickens allow them to sweep the area for grubs.
Slug-Resistant Potato Varieties
If you’re at your wits’ end, it may be best to focus on fast-maturing varieties in very sluggy soils. Early potatoes are hardly bothered by slugs at all. Most casualties are found in maincrop varieties. The longer they are in the ground, the more likely they are to be munched by slugs. Always game for a garden experiment, this year I tried to outsmart the slugs by harvesting my maincrop potatoes several weeks sooner than normal, in the hope that any reduced yield would be offset by perfect tubers.
I grew two maincrop varieties in the same row: ‘Maris Piper’ and ‘Pentland Dell’. It’s not putting it too strongly to say that ‘Maris Piper’ was a disaster. About half the crop was ruined. Some tubers were completely hollowed out, leaving only a blackened, papery shell.
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Some potato varieties, such as 'Maris Piper', are particularly prone to slug damage
Things looked bleak, but when I started to dig up ‘Pentland Dell’ my fortunes turned. As I dropped tuber after unblemished tuber into my harvesting bucket, my spirits lifted. Only the tiniest fraction of the harvest showed any wireworm or slug damage, and even that was minor.
I noticed that ‘Maris Piper’ produced its tubers close to the surface, while ‘Pentland Dell’ tubers grow further down, so perhaps they were just deep enough to escape the attentions of surface-roaming slugs.
Other Ways to Stop Slugs Ruining Your Potatoes
Slugs like damp conditions, so are more of a problem in wet summers. Watering can be tricky to get right because potatoes do need plenty of the wet stuff to produce a good crop. Watering in the morning is best, to let the soil dry out a little before slugs become active again in the evening.
The nematode Phasmarhabditis hermaphrodita is sold as a biological control for slugs. It’s said to be effective, but as with the wireworm nematode control this is a very expensive option. Beer traps are a cheaper method and slugs die happy, or at least insensible. Set them up in good time, before slugs start feeding on your potatoes.
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Beer traps are inexpensive and effective
Organic slug pellets should only be used as an absolute last resort. If keeled slugs are attacking your potatoes, pellets are unlikely to resolve the problem anyway, since these slugs primarily feed underground. As with beer traps, to be effective pellets must be scattered before slugs become a problem.
Growing in containers is another option, if you don’t mind the price of potting soil. Do you notice a theme here? Slug-less spuds are expensive spuds! Only the most determined slugs are likely to work their way into a container of potatoes, and it all but guarantees wireworm-free tatties too.
As for me, I’ll be growing ‘Pentland Dell’ again next year and looking out for other slug-resistant varieties. Any suggestions?
