CHECK your greenhouse, conservatory or coldframe for any overwintering pests.
It’s easy to overlook them at this time of year and insects, especially aphids, will infest many plants, especially on soft, weak winter growth.
This is one of my climbing pelargoniums (geraniums), which has a colony of greenfly living on it.
They’ve only attacked the smooth-leaved plants, not the furry-leaved common varieties. Obviously, they’re much more to their taste.
The leaves are sticky with the honeydew they excrete and the small white flecks are their exoskeletons that have been shed.
Try to isolate them from the rest of your plants and spray infected plants with horticultural soap again, taking care to avoid any seedlings you may have.
Or you can cut off all the weak foliage – I confess, that’s what I’ve done.