Photo of a butterfly emailed to me by Brent Tibbatts.

"Three individuals feeding on a Stachytarpheta. Maybe Colias croceus?"

To be precise, these are adults taking nectar from Strachytarpheta flowers. Larval hosts are Leguminosae (HOSTS database).

According to Butterflies of Micronesia
"Palau, Guam, Saipan, Tinian, Rota
Lemon migrant. Range: adagascar to Solomon Islands, with several subspecies known. The larvae feed on various species of Cassia including golden shower trees and candle flower. The butterfly is frequently found in moist spots on the road, and is known to engage in migratory flights. "