[1] Synanthedon rhododendri is one of the smallest of North American Sesiidae, with a length of both sexes between 10 and 15 mm.

[1] Twigs and small branches are preferably attacked, the larvae subsisting chiefly on the soft pith, digging long tunnels, which are filled with small, reddish pellets and serve for wintering and for pupation in spring.

On larger and older parts of the shrubs the larvae bore under the bark, which peels off, exposing shallow grooves on the hard wood.

A heavy infestation observed in the Brooklyn Botanic Garden, in 1918, killed many shrubs in new plantings and furnished ample material for rearing.

Moths, exceeded in numbers by ichneumonoid parasites, emerged late in May and during June.