Castilleja, Erigeron, and Lupinus species and any Braconidae
There are a few kinds of similar species which are difficult to distinguish:
Intermediate or difficult - possible to learn.
Significantly difficult - almost unlearnable (e.g. many Dialictus bee species).
Impossible to distinguish - impossible to learn, unless based on genetic specimen identification.
I agree that species IDs are the ideal goal for community ID when possible. But for certain taxa groups, the community will learn the finest level of ID may be genus, subgenus, or species complex. So, I’d consider the goal the most specific ID possible.
I agree, many parasitic wasp groups, especially minutely small ones. Telling apart species of Dialictus bees is also very difficult.
My least-favorite ID challenge would be gulls. In my neck of the woods (Northwestern Ontario, Canada), there are really only four expected gull species: Ring-billed, Herring, Franklin’s, and Bonaparte’s. I’m good with that, as the plumage patterns of gulls are so complex that even those four can be a lot of work…and I’ve nearly gone blind scanning flocks of gulls trying to pick out that one stray rarity among 156 Ring-billeds.
There are six more species on the District checklist (Sabine’s, Little, California, Iceland, Glaucous, and Lesser Black-backed)…but, after nearly 13 years of hard slog in the field, I can confirm the return-on-investment searching for these is negligible. Seriously, when was the last time a Sabine’s Gull turned up here? It’s tantamount to false advertizing.
I’m content with the annual late-summer appearance of the Franklin’s Gulls as they migrate south. While the little Franklin’s - nicknamed “Prairie Doves” - are common in Western Canada, they are extremely rare in Ontario.
Moths are impossible. Moths and lichens.
The pattern in moths may even be very well described, but if I try to compare the pictures I have taken (of a species I do not know) based on these various descriptions in words… Well, it doesn’t work. These descriptions may be perfectly logical and accurate when I look at a picture of the species being described, even more so if compared with a similar species, and I see the differences. But not having the faintest idea to begin with, I am afraid I can’t even begin to mentally picture the descriptions, let alone decide if the description fits my observation.
My latest failure: https://www.inaturalist.org/observations/92343151
I am not even sure the ID is correct.
The only reason I even tried my luck in this case is because this moth came in from outside and chose to sleep (?) on my SD-card and battery case.
Moths and lichens.
I will disagree softly on the lichens, since I started I’ve been surprised at how much is actually possible in the field compared to their reputation, especially with some experience, a good handlens and a couple chemicals in the bag.
Cladonia though is still often very much a headscratcher, especially the scyphose ones. There are just so many of them, and the subtle differences in squamules/granules/farinose soredia seem really quite indistinct at times, though I have been making some progress recently. This is somewhat of a shame too since they’re so nicely large an attractive for lichens; often I feel I have have more luck with the brown and grey crusts next to them.