Slugs Snails and Science

Non - scientist here.
Why don't they analyse what makes slugs and snails avoid certain plants even if, like penstemons, wallflowers, antirrhinums etc, they have fairly soft leaves? They could then develop a concoction of chemicals or whatever is not to their taste.
I'm sure insecticide manufacturers have thought of this already. Why doesn't it work?

 

Would that it was that simple. Hairy leaves, strong smells, nasty taste can deter molluscs, but environmental conditions have an effect as well; it's been dry this year and I've had a lot fewer mollusc problems, last year was wet and problems were horrible. Plants grown under these dry conditions will probably have fewer problems even if it turns damper due to them being tougher to eat having grown slowly during the dry spell.
Also plants will not automatically produce lots of deterrent structures or chemicals unless they have to, uneconomic for the plant.
I've certainly had plants from a nursery planted them and they've gone within a week, the next year they resprout and have fewer problems. Why? nursery grown plants tend to be softer, but also having been munched the first year the second year they are ready to go with the defences.
Throw in the fact that whatever you make must be safe to use, pet and wildlife friendly biodegradable. Then you end up with a product that takes a long time to develop, license and bring to the customers. You also have to have a customer friendly price so how much would you be prepared to pay to to protect pansies for 10 weeks.
I find once past seedling stage antirrhinums and wallflowers are left alone.
Penstemons are on my irrational dislike list so not allowed in my garden.

 

What an interesting question...

I don't know, but could take a few guesses

The compounds found in certain plants that naturally deter slugs may act as a herbicide on other plants.

Many chemical compounds are degraded by sunlight, so it may be that once the stuff has been sprayed on, sunlight degrades it quickly.

Many chemicals will oxidize when exposed to oxygen in the air and so change their composition.

I asked Google and it generated this response-

AI Overview

Slugs avoid certain leaves due to factors like toxicity, tough textures, or strong scents, making them less palatable or even harmful.
Here's a more detailed explanation:

• Toxicity: Some plants, like foxgloves and euphorbias, contain compounds that are poisonous to slugs, deterring them from consuming the leaves.
• Unpleasant Taste: Plants with bitter or strong-tasting leaves, such as some weeds and aromatic herbs like rosemary and lavender, are less appealing to slugs.
• Physical Barriers: Thick, waxy, hairy, or leathery leaves can be difficult for slugs to chew and digest, making them less desirable as food sources.
• Other factors: Some plants produce chemicals that deter herbivores, including slugs
• Examples of slug-resistant plants: Plants with felty, hairy, dusty or tough leaves are less likely to be eaten by slugs, such as lavender, santolina, helichrysum and teucrium.
• Examples of plants that slugs avoid: Euphorbias have milky sap that slugs can't stomach.

 

@Pete8 I got the same answer out of google, however I have lost a tray of foxglove seedlings to molluscs before now, I've also had succulents in the greenhouse including cacti munched.
Last year a grower told me that he couldn't grow North American euphorbias outside without protection form deer, but European euphorbias were left alone. So it can be difficult to generalise about what will eat what.

 

I think seedlings of most plants are fair game to such molluscs.
The new, soft, tender leaves haven't yet developed any defences.
I've noticed a few bits munched out of some of my young tomato plants this morning.

I've also had succulents munched in the greenhouse and slugs/snails have ideal mouths and teeth to munch through the fat juicy leaves.

Completely agree that it is difficult to generalise and probably depends on how hungry they are.
If there's plenty of soft non-toxic leaves to munch through then they can eat their preferred foods.
If supply is short, then they'll likely be less fussy about what they eat.

I've given up growing almost all veg due to slugs/snails and now grow soft fruits in my raised veg beds.

 

I do find tomato plants to be pretty much left alone, tomatoes are different thing altogether.
I object to the molluscs hiding where I can't get at them on cacti, behind the spines.

 

Vampire molluscs

 

Well there are cannibal slugs.

 

They say a Leopard Slug never changes its spots!!!

Anyway when do snails start to reproduce? Handy for when I go out on my snail round up, where I collect them in a bucket and they get rereleased on the railway embankment on the back of the house, so they get a further life or become a good food source for the magpies, jays or blackbirds. So handy or do a round up before they start to breed!

 

Slugs ate our garlic and onion leaves last year.

I think the problem is the in balance in nature we have as well as the weather. Last year was good for slugs but terrible for insects and you get boom and bust years where predators and pray suffer. It's dry this year and I've seen more life out and about but a fraction of what we would have seen decades ago. Birds, frogs and hedgehogs are good sluggers and I think it's better to encourage them. There was something I read about a permaculture farm where they actually call it a beetle farm because they do the heavily lifting with slugs. It's easy to attract beetles by being a little less tidy in the garden.

 

Different species probably have different metabolisms, some can eat stuff that is poisonous to others. In garlic the sulphur compounds are poisonous to all animals just we are strong enough to eat quite a bit without getting sick, certain molluscs might metabolise it better than others.

Ive also noticed they ignore tomatoes, and found they ignore cucumbers when they are big enough, but the small ones still get eaten. Perhaps the tomatoes just smell nasty, or perhaps they cant digest tomatine/solanine well. I think cucumbers might be too spiny?
Do they eat potato plants?
Toms are very pungent plants. Perhaps co planting them with other stuff might dissuade slugs?

 

I lifted the lids of our water butts - snails by the dozen, just under the rim.
Slugs I've not seen this year so far. My seedlings are protected for now. But when I plant them out, it could be a different matter. I imagine there are plenty in the compost bins.

 

I have lots of green cellar slugs in my garden