Broccoli And Lettuce. Photo. Whats the problem?

My first post on the forum, so here goes

I've planted a dozen purple sprouting broccoli. I've also planted some lettuce (Little Gems) in between them.

Most of the plants are growing well, except one rectangular patch about 3 feet by 3 feet. The plants in this patch, both 4 broccoli and 4 lettuce are limp and discoloured. This patch is on the left of the photo.

To start with all the dozen broccoli started to wilt and the lower leaves went yellow and died off. Some recovered and are OK but these four within this patch are limp and struggling.

I wouldn't be all that bothered about this except that the lettuce in the same 3 x 3 area have also started to wilt and the outer leaves are yellow. The rest of the lettuce plants are OK.

The whole bed has been fed and watered the same. There isn't anything I know of below the surface such as a manhole.

Can anyone suggest what the underlying problem may be?

Thanks for the suggestion David. I'll find out what these little critters look like, and then whether I can see any on the plants.

Ive dug up a couple of the affected plants.

There is no sign of any damage or grubs / maggots on the Broccoli roots.

However on the lettuce roots there are a lot of grubs. They seem to fit part of the standard description of the vine weevil grubs on the RHS site, as they are pale yellow and up to 10mm long. However  they are quite thin (about 1mm thick) and move about quite a lot. They are not the plump grubs shown on the RHS site. I cant decide whether they are definitely the vine weevil grubs or something else.

Do you think they are vine weevil grubs or something else?

The part of the plot where this is happening is where we used to tip old and spent compost, which seems to tie in with what Ive read about vine weevils. Because its quite a small area about 4 foot square I'm tempted to dig out the soil and get rid rather than treat the soil with nematodes. Do nematodes work in a vegetable plot?

glad you seem to have established the "root" cause of the problem the only other thing i would have suggested (wrong in this case but useful for future reference) is red ants that can cause similar problems.  As it is a relatively small area i would be tempted just to dig it out.

When I started growing some veg I didn't realise how many pests and bugs there are in the soil, and that they all seem to make a beeline for your veg plot.

Ive had another look at the grubs. The colour is somewhere between cream and pale yellow. The grubs are quite active and thin and if you dig a root up and expose them, they wriggle and disappear back into the earth quite quickly in 10 to 20 seconds or so.

There are plenty of grubs in the rootball of the wilting lettuces.

There are non in the roots of the brocolli. Only the stump of the broccolli is left, no fibrous root ball.

From the advice and comments I think I've got cabbage root fly rather than vine weevils. From the various comments I'm thinking that this might be the lesser of two evils!