whitefly on brassicas

Hi I had a shocking amount of whitefly on the PSB last year, absolute clouds of them. They were under the brassica netting so quite contained, and lifting the net was like unleashing something from a sci-fi movie. So I get that it is probably the butterflies I should deter in the first instance, but if there are whitefly already on my next door allotment plot for example, can they come over to my brassicas as well? In which case should I use a much finer mesh so they cant get in? But then if they're in, am I not just keeping them safe under the net so that natural predators like birds and wasps can't get to them? I'm a bit confused .... help if you can! And thanks

 

I think a net fine enough to keep white fly out would be so fine as to not even let much light through.
If you dont spray I dont think you can do much about them.

Not sure they have many natural predators.

 

Put fleece over them?

 

Yes you are correct in the 'protection from predators' meme. If you wish to deter them, or just spoil their day, you can try spraying them with weak soapy water (ideally soft horticultural soap) or such as Lux, with salt added 10grams per litre. Brassicae are resistant to salt in low concentrations, white fly and caterpillars, not so much. Do not expect total clearance; but as they say, every little helps. Another approach is to remove the mesh on a windy day and move through the crop. The white fly will be carried down wind to another plot , where they may remain!

 

You could use Pyrethrum which is a contact insecticide.
It is not absorbed by the plant, so it's safe to use on edibles and anything sprayed is fine to eat after 7 days.
It is made from Chrysanthemums.

I grew sprouts once and they were so heavily infested with whitefly the sprouts began to rot and looked disgusting. I pulled them and have not grown brassicas since.
I now grow soft fruits on my veg patch as a combination of slugs, butterflies and aphids were waging a war, and I lost.