Named after the food source. This is actually a fairly common way of naming animals that are closely associated with a particular host.
This is often reflected in the epitheton as well as the common name:
- true bugs, e.g.: Eurydema oleracea (cabbages), Platypax salviae (sage), Dictyla echii (viper’s bugloss), Heterogaster urticae (stinging nettle), Campylomma verbasci (mulleins)
- moths and butterflies, e.g.: Pieris brassicae, Pieris napi, Pieris rapae (various cabbages), Aglais urticae (stinging nettle), Cucullia verbasci (mulleins)
- bees, e.g.: Colletes hederae (ivy), Chelostoma rapunculi (bellflowers, also called rampion/rapunzel), Tetralonia malvae (mallows)
- beetles, e.g.: Pyrrhalta viburni (viburnum), Chrysomela populi (poplars), Anthonomus ulmi (elms)
Occasionally insects are named after a plant even when there is no close association, e.g. Halictus scabiosae (not closely bound to scabious, though frequently found on it), Mimas tillae (lindens are only one of several host plants), Thecla betulae (birches are not a host plant) etc.