- Larvae are almost certainly in the crown of Fabaceae host plants. Almost all of their relatives live the there - e.g., black witches are in the crowns of hosts at 10-20m.
- Larvae certainly cryptic, and nocturnally feeding.
- For above reasons, discovery is unlikely, unless by chance.
Daniel Jantzen probably has more field experience with moths of the New World tropics than anyone. His influence in Costa Rica has probably generated many of the white witch records for that country. What he says about the white witch question:
Tropical Lepidoptera often exhibit dramatic changes in abundance - a population boom is called an irruption. Consider a 2014 study, OBSERVATIONS ON AN IRRUPTION EVENT OF THE MOTH ACHAEA CATOCALOIDES AT KAKAMEGA FOREST, KENYA. In 2012, A. catocaloides (Noctuidae: Erebidae) abundance reached an estimated 6.8 moths/m2 in a 12,000 ha. area. Predation on moths was observed for 14 bird species, monkeys, and squirrels. If the moth species was rarely observed in normal years (the authors do not state a typical density), it would be incredibly conspicuous in an outbreak year.
There is some evidence for irruption events of white witches. If one were present for such a phenomenon, and if it occurred where moths were emerging, the chances for locating larval stages would increase in proportion to the size of the population boom.
I contacted Jacqueline Miller, an author of An annotated list of the Lepidoptera of Honduras, which referenced a white witch collected in May of 2010, near Parque National Pico Bonito on the Atlantic coast of Honduras. She mentioned other observations in the same area in Aug and July. She also noted that "one of our former research Associates, Dale Jenkins, who was working at USAID in Mexico City, collected more than 50 specimens of this species in Mexico." Tantalizing.
A 3 Sep article states that the record white witch is in the collection of John Powers, an Ontario collector:
Powers holds the world record for the largest moth for his thysania agrippina, otherwise known as the ghost moth or the birdwing moth. The insect was found in 1934 in Rio De Jenario and spent many years with another collector before Powers took possession. The record has held up since 1984.