Answer: For reasons I don't understand, Nasturtiums are magnets for black bean aphids. Aphids are one of those curious insects that give birth to live young. So what may have started as a single aphid has now become a huge population, with multiple generations feeding on your plant. Insecticidal soap can control the aphid population (spray thoroughly, especially on the undersides of the leaves). You might try pruning off the worst of the damage so your plant can grow healthy new vining shoots. If you get the aphids under control, the new growth should come in full and thick.
Best wishes with your nasturtium plant!
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