Nectars of 11 hawkmoths-visited taxa of Oenothera were studied. Qualitatively the sugars of all taxa were identical. The amount of nectar produced per night among the taxa was quite variable. Potential energy available from nectar of an average flower of O. caespitosa was determined to be 42.1 calories.
Stockhouse, Robert E. II
"Nectar composition of hawkmoth-visited species of Oenothera (Onagraceae),"
Great Basin Naturalist: Vol. 35
, Article 3.
Available at: https://scholarsarchive.byu.edu/gbn/vol35/iss3/3