[boybeck]

I've just seen white fly on my newly planted spring cabbages.  Will they cause any damage? Do I need to do anything about it?

[pushrod]

At this time of year i would suspect that the oncoming colder weather might do for them.Depends how bad they are, numbers wise.

[boybeck]

There are small clouds of them when I shake the plants.  Will they cause much damage?

[pushrod]

There are small clouds of them when I shake the plants.  Will they cause much damage?

They feed on the plant's phloem - sucking out the sugars that the plant has made and is transporting. The stuff they excrete can also lead to fungal attack - so yes they will weaken plants. Might be worth looking for a spray. Stores like Wilkinsons start discounting all their gardening stuff at this time of year so you might get a bargain. Alternatively you can mist them with soapy water (that usually does for the adults).

[MOLUSC]

A table spoon of a  good quality washing up liquid (one that makes your plates squeak) mixed into a full watering can of water and sprinkled over the leaves should do the trick.
Try again after a week if they persist.
The washing up liquid  breaks down their (the white fly and the scales) waxy defensive exterior.
The scales (immature white fly)suck out nutrients from the leaf -this is evident by the yellow/pale green type blisters formed on the leaf.

[Paul Plots]

A table spoon of a  good quality washing up liquid (one that makes your plates squeak) mixed into a full watering can of water and sprinkled over the leaves should do the trick.
Try again after a week if they persist.
The washing up liquid  breaks down their (the white fly and the scales) waxy defensive exterior.
The scales (immature white fly)suck out nutrients from the leaf -this is evident by the yellow/pale green type blisters formed on the leaf.

The little s*ds.... makes it hard to see sometimes when there's so many - like an early snow storm!

A spray of soapy water, as you say, helps keep them at bay. Not cold enough here most years to wipe them out though.