Are red ants harmful to our crops?

I just cleared a patch which husband had planted with lawn grass. I need the space and was going to put leek plantlets there, but when i removed the grass the soil was teeming with red ants. I asked the doyenne of the allotment site, Patrick, if they could be harmful and he said they'd eat the roots. Is this correct? Are my leeks doomed? I could also put purple sprouting brocoli in, would that be better? Thanks

Thanks head gardener. In the meantime I have read the RHS's info page on red ants an they indicate that they are not usually harmful to plants. I'll go ahead and ant the leks, as I have no over available space, and see what happens. If there is a problem I ll use that Nippon bait mentioned in a previous post.