What is nibbling holes in our pond liner?

[Greaterwaterparsnip]

We made our pond 4 years ago. It started to leak a year ago.
Looking carefully now, there are many tiny, perfectly round holes in the liner. I don't think they are bird peck holes as they are too round. It is a wildlife pond so no fish but many other species.
I've tried googling but there doesn't seem to be any answers.
We used a thick layer of smooth sand and a liner so I don't think it's coming from underneath. The liner came with a 40 year guarantee but what on earth has gone wrong?
I will attempt to attach a picture if I can work out how!

Maybe...it was about £70 from Amazon with a 40 year guarantee so not the best quality one.
I priced up a new rubber one at about £300.🙁
I wonder what the guarantee would cover. 🤷

Cadis fly larvae made holes in ours

This looks likely...did you solve the problem?

No to be honest it went on for a while then I shut the pond down

Hi, resurrecting this one because I have exactly the same experience. It’s definitely caddisfly larvae like mada says. Scores of tiny holes in my liner, really annoying and near impossible to fix them all. All the white dots are where the larvae have attached themselves, and I’ve plucked them away to reveal holes underneath :(. I’ll probably try to find a cheap moulded pond and get that in in the winter, but won’t use a cheap liner again.