The thrips get right into the growing centre & the thrip larvae damage the minute, developing leaves. As these leaves grow, they are distorted & the damaged part of the leaf becomes a hole – which gets bigger as the leaf grows.
This gives the impression that something is eating the leaf now – but the hole was made when the thrip was damaging the growing centre where the leaf was forming. Sometimes the leaves around the growing centre have black bits round the edges, almost like they have been burnt.
Thrips normally damage more than one plant.
Blackfly will start a colony on one broad bean plant, but blackfly do not make holes & they would be easy to see on the growing tip of the broad bean plant.
So, holes on broad bean leaves are usually due to thrips.