The caterpillars of some hawk-moth species mainly feed during night. When the sun sets in the evening, they start climbing up their food plants and start their feast. Like many other caterpillars they particularly like feeding on the flowers of their host plants.
Willowherb is the food plant of the Elephant hawk-moth (Deilephila elpenor) and the Bedstraw hawk-moth (Hyles gallii). I’ve mainly spotted the large hornworms of the latter so far. The full-grown caterpillars of the Elephant hawk-moth will follow a bit later.