Bring on the Garden Gnomes

What is the opposite of green-fingered? I had some flourishing nasturtiums growing in pots outside our door. They'd just got to the abundant flowering stage and were looking really bright and cheerful, when I noticed they were covered in blackfly, which were killing off all the new buds. My sister-in-law suggested spraying with soapy water.I think I overdid it. Or else dishwashing liquid is the