Nasturtiums are one of my favourite flowers and also a favourite of white butterflies.  I noticed several leaves with clusters of tiny skittle shaped yellow eggs which have been deposited by Large White butterflies and within a couple of weeks, caterpillars coloured grey, black and yellow will emerge and feed on the leaves.  The Small White butterflies lay eggs singly which become green caterpillars so perhaps there are some in the garden too.  They are often called Cabbage Whites because they love brassicas which they can quickly devastate plants so are a huge problem.