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Beetle banks are particularly good at harbouring predatory beneficial insects such as ground beetles, ladybirds and spiders. When in close proximity to crops, the invertebrates will eat nearby pests.
Broad- and fine-leaved grasses can provide different functions within beetle banks. For example, tussock-forming grasses are important as overwintering sites for invertebrates, and herbs can be added for greater diversity.
I'd never heard of beetle banks before. Interesting article, definitely worth a read.