Why use polythene

Like many things there are pros and cons with polythene. On the plus side it warms the soil and if it is black it suppresses weeds. Clear polythene keeps the frost off tender plants. Polythene has the advantage over cold frames and greenhouses that it is light to carry around and can easily be rolled up and stored when not in use.

On the downside, anything that lies on the soil tends to harbour slugs and snails. If the polythene is down for any length of time watering becomes an issue - weed suppressant material should have adequate drainage to overcome this problem, but perforated polythene will probably not let much water through. Clear polythene also traps condensation underneath it as there is a temperature difference, so if there is plenty of moisture in the soil any plants underneath will tend to be wet and this can lead to diseases particularly fungal ones like botrytis.

There are probably loads of other pros and cons, these are just my initial thoughts on the issue.

Penellype, here is something you could try, make your own slug killer. Some slugs will be infected by nematodes or carrying bacterial diseases, but their low density means that they won’t devastate the rest of the population, so collect as many slugs as you can find and once you have caught around 15 to 20 slugs (the more you have the better it works) put them into a bucket with an inch or so of water in the bottom for humidity and a few handfuls of leaves to make an edible floating island for your catch, place a cover over the top of the bucket, and, if the disease or nematodes are present, you can concentrate these micro-predators and harness their natural slug-killing power. The bucket is the perfect environment for the nematodes and bacteria to breed. Nematodes spread in water, so check regularly, giving the water a stir with a stick. After two weeks a high level of nematodes and bacterial disease has hopefully built up inside the bucket and the slugs will have died from infection. Now filter the brew into a watering can, top up the bucket from the tap to dilute the brew and decant via a filter to prevent the weed and slug mixture from falling into the watering can, you can make a filter of chicken wire folded over the bucket so it stays put while you pour. Water the sieved brew over the ground, the raised nematode population will seek out resident ground-dwelling slugs and see them off.

Rary!

Likeing this Idea!

Would it not work even better applying a nemetode solution to the bucket of collected "Little Critters"?

it may well, but you would need buy that and why spend for something you can have free.