Going through one of my old gardening books, came across this.

Thoroughly boil together 5lb of soft soap and 1/4 pound of shag tobacco in 5gal water. Store in a corked jar, and add 1gall of this mixture to 3 gall water. Ah those were the days.

If you think about it, you could still do this and no-one would be any the wiser — the ingredients are still available. It's probably just as effective without the tobacco, and indeed soft soap is still an approved insecticide (Savona). I remember the days when nicotine was still in use as an insecticide. It was never particularly effective.

I remember buying nicotine shreds and having to sign the poison book at the hardware store - about 50 years ago. I used to pile them up on an old saucer in the greenhouse, set light to them and rush out!

This doesn't look a million miles away from a treatment that JB has recommended a few times (maybe without the soap, but that will definitely be of assistance in this context).

We used Savona - soft soap last year to control whitefly on brassicas. I'm not sure how effective it was as we didn't have as many whitefly earlier in the year anyway so I don't know whether the Savona worked or the improvement was just down to less whitefly!

We'll use it again this year on our brassicas