Cabbage white butterflies and the blue-grey cabbage aphids are the only pests which regularly infest my brassica crops. I grow so many that there is no chance of controlling them by hand. In any case, aphids nearly always make for the tender growing points, easily damaged when rubbing out the aphids digitally. I can often see 50 white butterflies at a time and hundreds of eggs can be laid in one day. This systemic insecticide is the best I have used for these pests and even the buttons on my brussels sprouts are free of aphids.
Aphids seem to thrive under dry conditions.
In England I used to wonder why farmers who grew root crops for animals used mangolds in the dry warmer south and more swedes and turnips in the north where aphids were less common. This is probably why Southland claims to grow the best swedes in the country and why I have seen so many more aphids this year. One other problem, just arisen, is that some of my big Atlas outside tomatoes are splitting. I usually associate splitting with a sudden moisture supply after a dry spell. We are still enduring day-long drizzle as I write but my irrigated soil was not that dry and the drizzle has hardly wet the soil.