How to Prevent Slug-Eaten Potatoes

, written by gb flag

Slug eating a potato

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.

“Slug-infested
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.

“Wireworm”
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.

“Slug-damaged
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.

“Slug
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?

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Comments

 
"I dug out this year's potato bed and laid black ground cloth down with the sides bent up along the raised bed walls. Then I placed the seed potatoes where I wanted them, added some compost on the potatoes and covered them continually in semi-dried grass clippings. It wasn't a big crop, but the plants produced and yielded nice spuds, only a few were a little scabby. No wireworms! Maybe next year I'll try something else to give the plants a little more soil, if that will help produce more potatoes."
Louie Strano on Friday 6 September 2019
"I catch a hundred slugs every night at 11pm for the first couple of months and then it tapers off to about 6 or 7 arround July, then I can have a rest. I only ever water in the morning. I have come to the conclusion I have a slushy veg plot and that's that. I try to leave the soil free of rubbish. It's strange I have a good sized greenhouse with a soil base that I grow direct into and only catch half a dozen all season. North france"
Melanie on Saturday 7 September 2019
"I've heard that slugs are repelled by coffee grounds worked into the soil - anyone else have any experience of this?"
Denise Fukuda on Saturday 7 September 2019
"Thanks for sharing your experience Louie - that's very interesting! Potatoes do need plenty of nutrition, so adding more compost on top next time if you can should improve yields."
Ann Marie Hendry on Saturday 21 September 2019
"Sounds like you have a good slug-control system going there Melanie. I too find that my greenhouse sees far fewer slugs than outdoors - presumably it gets too warm and dry in there for them."
Ann Marie Hendry on Saturday 21 September 2019
"Hi Denise. While researching an article a couple of years ago I discovered that coffee grounds are not an effective slug repellent. If you type A Common-Sense Guide to Using Coffee Grounds in the Garden into the search box at the top of the page you'll find the article, which contains a link to the original experiment that proved this (I can't include links in comments, sorry!)."
Ann Marie Hendry on Saturday 21 September 2019
"These varieties of potatoes are slug resistant Arran Pilot, Desiree, Sarpo Mira, Lady Rosetta, Golden Wonder, Romano, Pentland, Charlotte, Kestrel and Estima. Or if you prefer to feed the slugs then try Maris Piper, Roosters or Records. A pity because I love Records. Information lifted from howtogarden.ie."
William Ryan on Monday 4 May 2020
"Great list. I can certainly testify to Maris Piper being a slug magnet - it was a potato massacre! I've grown Record before and it was fairly badly affected too. Pentland Dell was almost untouched, however I grew Pentland Crown a few years ago and it was almost as bad as Maris Piper, so beware - not all Pentland tatties are the same. Arran Pilot (probably my favourite potato variety) almost never gets touched - it's a very early potato, so it's ready before slugs really get going. Most early potatoes tend to be OK in my experience."
Ann Marie Hendry on Wednesday 6 May 2020
"Coffee grounds certainly keep the slugs and snails off my hostas! Spraying food crops with a very strong coffee solution also works well. Slugs can cross coffee grounds, they just don’t like it."
Angela Frith on Monday 25 May 2020
"I've put "coffee ground barriers" in my greenhouse, there are a lot more slugs on the oneside than the other with a preponderance hiding at the barriers. Whatever the reasons, I've become a fan of coffee grounds ......"
Bob on Tuesday 30 June 2020
"Thanks for the great article. We use Muscovy ducks for slug and insect control. They're kind of a mixed blessing, but they love to look for things to eat. The vegetables have to be fenced off, but the ducks get in before and after. "
Neal on Saturday 8 August 2020
"It sounds like you have a dedicated and very effective pest control team at work Neal! "
Ann Marie Hendry on Tuesday 11 August 2020
"I agree on every point with the author and my soil conditions are identical to his too. This year however, I have found one of my favourite varieties, Kestrel, has been devastated by slug. AND I invested in nematode treatment...useless on this occasion! My Sarpo Mira seem fine though. I shall be lifting the lot of them shortly. (with bated breath) "
Straker on Friday 14 August 2020
"That's a shame Straker - and surprising, since Kestrel is said by many gardeners to be fairly slug resistant! You win some, you lose some, I guess."
Ann Marie Hendry on Friday 21 August 2020
"After 3 years of trying, even with the help of nematodes, to grow Maris Pipers, I have given up. I now grow them in potato bags, resting on a porous membrane, on the plot where I used to plant out my tatties. No slug damage - I just make sure that not a drop of soil from my garden enters into those bags. Two wee problems - the tatties are a bit on the small side, and I have loads of used soil to dispose of. In the meantime I have planted carrots in the used soil, and they are doing very well."
Hugh Bowman on Sunday 6 September 2020
"My potatoes were all eaten by slugs this year and very scaby. But my biggest shock was when I dug up my beets, they too had been eaten,some completely! Would pouring beer into my soil now,help for next year? I have a case of old beer to get rid of."
Mary Ann Helgeson on Thursday 10 September 2020
"Hugh, I haven't tried growing potatoes in bags personally but from what others have told me they tend not to grow as large as those grown in the open ground. Potatoes do need a lot of water and they don't like to be overheated, so watering them copiously and keeping the bags - though not the foliage - in shade or semi-shade might help. I sympathise with the problem of growing Maris Pipers in the ground - they seem to be irresistible to slugs! I grew Pentland Dell again this year and have had a fantastic crop with very little slug damage. It doesn't make such excellent chips (fries) as Maris Piper, but it's a good general purpose potato. Great idea to grow your carrots in the used soil! When you're done with it, the soil will make a great mulch for your vegetable (or other garden) beds."
Ann Marie Hendry on Tuesday 15 September 2020
"Hi Mary Ann. A case of old beer to get rid of? I'm tempted to say that I could help you with that :D Beer is used in slug traps, but pouring it on your soil won't help (and may be detrimental to other soil life). To be useful for killing slugs, you need to pour it into a shallow container and nestle it into the soil. Keep the lip just above the soil surface to prevent beneficial bugs like ground beetles accidentally falling in. The slugs will be attracted to the beer, get completely hammered, and drown. My beets often have the odd chunk taken out of them by slugs, but only above the soil surface so I wonder if something else is eating them? Rodents like mice or voles could be a possibility. "
Ann Marie Hendry on Tuesday 15 September 2020

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