Definite Tussock and White-marked Tussock are flying now (September) in our area. Here's how to tell them apart:
The moth on the left is Orgyia definata and the one on the right is O. leucostigma. Because the MPG site had no useful info and the photos in Beadle and Leckie leave something to be desired, IMHO. I dug out Covell and read his descriptions of the two species. Here’s what I found:
In both species, females are wingless. The descriptions are of males.
O. definata: “FW dark grayish brown, with relatively sharp black markings in the upper st. area. Note thin, black costal patch with black and gray streaks below it, near FW apex. St. line usually sharp, accented by white spot near bottom.”
O. leucostigma: “Similar to Definite Tussock Moth. . . Wings usually grayer in [male] of this species; median area usually paler. Blackish patch near apex smaller, with no light streaks below it. White spot accentuates lower st. line as in [male] Definite Tussock Moth.”
I would add that Covell did his descriptions from pinned specimens. Therefore, the forewing costa (leading edge) is “up.”