sprouts wilting, small white grubs eating roots??

Yes, it's cabbage fly unfortunately.

To protect your plants you need to rotate the bed each year, and put a collar around the bottom of the stem when you put them out.  The collars can be made out of cardboard, underlay etc.,  just so long as the flies can't reach the soil around the base of the plant to lay their eggs.

I don't think there's anything you can do once the plants are wilting, because by then the roots and bottom of the stem have suffered too much damage. I'd dig them up and let the birds eat the grubs.

G&R

Hi Lowermeadow,

It is sad when you lose your carefully nurtured plants to something like this. I've suffered the same as you with cabbage fly in the past; this year for me it's woodpigeons which haven't previously been a problem, I'm on my 3rd sowing of peas, now behind metal fencing!

The good news is that the collars do work (search online for "brassica collars"), and it's worth putting them around the remaining healthy plants. They won't work if the grubs are already there underground on the healthy looking plants, but they will protect against further attacks. Cabbage fly comes in 2 or 3 waves over the season, and it's this first wave that normally does the most damage because the plants are smaller.

In terms of understanding what's happened, it's often the case that the first time you grow something new you get good results, and then subsequent crops aren't quite as good. It's partly because the first time you grow something the particular nutrients that crop needs haven't been depleted, and partly because any pests specific to that crop are small in number and you don't notice any damage. In second and subsequent years however, the pests have built up in the previous year. Rotation and manuring help with nutrient levels, and rotation also helps with soil borne diseases but can only help a bit with a fly which can still move around your garden and find host plants nearby.

G&R

 

Hi Lowermeadow,

No, I haven't tried nematodes, I do like the idea of them but they've always seemed quite expensive and need particular soil conditions to work; my soil is an acid sand and in some years I'd struggle even to get them to soak in, let alone reach the roots.

G&R

Don't be too hard on yourself, I planted sprouts on an allotment that had dormant for years, I used cabbage collars and when planted out they when straight under Enviromesh and yet mine are also being destroyed by the root fly.

Like you I have cabbage and kale next to these and it would be pretty gutting to lose all that.

Will debris netting keep them out?
My brassicas are at the top terrace high winds....good or bad??