i suspect that is is the small soil dwelling slug rather than scarid flies considering the scale of the damage.
have you used and slug bait? Beer traps/ halved skins of oranges/grapefruit/ crushed eggshells/old coffee grounds etc]

Hi there, thank you for your help again :-)

The compost was fresh when I planted the marigolds, so the slugs must have been in the bag I suspect, because I had copper tape around the container and this usually works very well? There isn't much room in the container for traps, but I could try coffee grounds. When you say soil dwelling slugs, does this mean they do not leave a slime trail? Are they smaller than the normal slugs, which I do have a problem with?. When I watered yesterday, the gnats all flew up out of the soil despite the insecticide I applied the day before. I believe these or the larvae chew on the delicate roots, however, the actual flowers heads had also been chewed. It is so dis-heartening to lose the flowers this way

Please tell me how the coffee grounds work, does it kill them or just deter them please, and should I fork this into the soil or just put around the base of the plant?

The easiest thing to do is to give up planting French marigolds. It worked for me! Sorry to be negative but I don't know what else works. I have used them in the greenhouse to protect the tomatoes as they obviously taste nicer (if you are a slug)

the small slugs are usually no bigger than 1" and they do travel up through the drainage holes in the containers and up the side between the pot and compost.
Coffee grounds laid as a mulch on the surface of the pot or use a sharp grit as a top dressing to make it more uncomfortable for them. Slime trails vary from individual to individual and their visibility is also affected by the moisture in the air.

Oh, of course, I hadn't taken into account the drainage holes, how silly of me :-( In that case, what good is copper tape, I wonder. It all makes sense now, because the base of the container is on legs, making it SO easy for the damned slugs to access the base.

I am now going to dig up the marigolds, I refuse to let the slugs get the better of me ! Can you suggest sunny yellow bedding plants for a dark corner please, which the slugs won't like?

I wish there were YELLOW geraniums, I have always found geraniums to stay safe.

Thank you both.

I'm in the US but I use 3% hydrogen peroxide to get rid of those little boogers. It works and won't harm the plants.

yellow pot marigolds also called the English marigold Callendula officinalis. They are an annual and grow easily from seed. They come in a range of yellows and oranges too.
The flowers are edible. if you a few petals to boiling rice it turns yellow as if you had added saffron.

Oh wow Eileen. I didn't know that. Does it taste anything like saffron? I'll have to try it.

I've no idea Paul, don't think I have had anything flavoured with saffron, but it certainly looks pretty.

We have terrible trouble with slugs here. My work colleague suggested lining the bottom of pots with a few layers of old newspaper to prevent them from tunnelling through the holes in the bottom of the pots. I haven’t tried this yet but thought I would pass on his suggestion. Hope it helps x

Thank you all for such interesting ideas. So should I dilute the 3% peroxide and simply water into the container as normal, or only around the base of the plant, please?

Saffron is expensive, so this alternative is such a good suggestion.
I will check out the Callendula.

So marigolds are gourmet food for slugs !

I would use a bucket. Fill with water - add some HP, then submerge the whole plant for a couple minutes. It may bubble and fizz, but that's normal. HP has a very short life and breaks down to water & oxygen - both are good for plants.

Thank you, do you keep fancy goldfish by any chance? :-)

I have in the past, but not now. I'm planning to start up a marine aquarium - doing my research and exploring options.

The peroxide was a give-away :-)

Many years ago, as an amateur, I joined an American Fancy Goldfish list and I learned so much about the right way of taking care of them (more difficult than tropical, I believe) Among the treatments was the peroxide dip.!
The thought of scooping the fish from the tank, and then dipping it in peroxide for several seconds terrified me, but after a while, I overcame my nerves and was able to do this treatment regularly. It certainly got rid of the parasites which plague the poor little creatures. When you mentioned the peroxide, it brought it all back to me, and I suspect you will have used this method also?.

Wow you are brave. I never used the peroxide treatment on my goldfish but I'm glad to hear you've done it with success. I've used that stuff that turns the water blue. It's effectiveness is questionable.

That's the same treatment you would do with your plants. Give your plants the 'peroxide dip' and the bugs will be gone.

We used to treat the fungus on goldfish with potassium permanganate. It turned the water purple but it killed the fungus...(not that this has anything to do with marigolds..)

Yorklass, perhaps not, but interesting to me, nevertheless.
I am assuming by your name you are from Yorkshire? So am I !

I didn't know that there were different kinds of slugs, only the larger disgusting ones, so I have learned from these posts, thank you all for your advice.

the little 1" ones are the biggest pest as they hide in the soil.

there are over 40 different slug/snail species in the UK apparently.

Ugh!

I have two roof patios, and I DO have a hard time with the bigger slugs, why they would want to climb up three storeys is beyond me !

I remember there used to be a liquid slug killer which worked well, however, it was taken off the market and never replaced with an alternative? I have many wild birds who visit, so pellets are not an option.