Are they insects? if so try washing the leaves with a bit of soap. if not try spraying with a bug killer if this is what they are.
Surely a 'bug killer' should always be a last resort to anything, and only if really necessary, certainly not a first suggestion.
@wrightt I see in your profile you say you are 'Winner of Dorset large wildlife garden featured in the Mail on Sunday'. I would have thought 'bug killer' was not particularly wildlife friendly.
My nasturtiums are also ravaged by blackfly more than usual this year. But as others have said, nasturtiums are great for the 'good' insects like bees, and act as sacrificial plants for the 'bad' insects. And if you don't use 'bug sprays' on the blackfly, they in turn also become food for the 'good' insects like ladybirds and hoverflies.
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@wrightt I see in your profile you say you are 'Winner of Dorset large wildlife garden featured in the Mail on Sunday'. I would have thought 'bug killer' was not particularly wildlife friendly.
My nasturtiums are also ravaged by blackfly more than usual this year. But as others have said, nasturtiums are great for the 'good' insects like bees, and act as sacrificial plants for the 'bad' insects. And if you don't use 'bug sprays' on the blackfly, they in turn also become food for the 'good' insects like ladybirds and hoverflies.