Aphids

although I netted my raised bed when I planted my brussels sprouts when I walked past the bed the other day it was like snow coming of the crop aphids rampant obviously the netting wasn't fine enough should have used environment netting 

 last year I sprayed the brussels with a mixture of washing up liquid and cooking oil never had an aphid problem and used the same netting maybe I planted the crop to close this year and I never sprayed because of unforeseen problems

how come you never see any aphids in field crops all grown and  picked to supermarket spec in a 40 acre field when my raised bed is only 8/10  aphids having a ball 

all be it gallons of chemicals how many brussels sprouts will be sold in the supermarkets and market stalls now and at Christmas?